The fuel system is the life blood of a vehicle. Fuel systems have become more complicated, but along with that, they have become more efficient, more environmentally safe, and easier to troubleshoot through electronic diagnostics.

In this manual, we are going to discuss fuel systems overhaul. The topics to be covered will include gasoline fuel injection systems, small engine carburetors, diesel fuel injection systems, air induction systems, cold starting devices for gasoline and diesel fuel injection systems, and emission systems.

When you have completed this manual, you will be able to do the following:

  1. Understand how to troubleshoot and overhaul gasoline fuel injection systems.
  2. Understand how to troubleshoot and overhaul small engine carburetors.
  3. Understand how to troubleshoot and overhaul diesel fuel injection systems.
  4. Understand how to troubleshoot the air induction system.
  5. Understand the principles of cold starting devices for gasoline and diesel fuel injection systems.
  6. Understand how to troubleshoot and overhaul emission systems.


1.0.0 Gasoline Fuel Injection Systems

2.0.0 Diesel Fuel Injection Systems

3.0.0 Air Induction Systems

4.0.0 Diesel Engine Cold Starting Devices

5.0.0 Emission Systems

Review Questions


A typical gasoline injection system uses pressure from an electric fuel pump to spray fuel into the engine's intake manifold. Like a carburetor, the gasoline injection system must provide the engine with the correct air-fuel mixture for specific engine operating conditions. Unlike a carburetor, however, the injection system uses pressure, not engine vacuum, to feed fuel into the engine. This makes a gasoline injection system very efficient.

A gasoline injection system has several advantages over a carburetor-type fuel system. A few of these include:

1.1.0 Timed Fuel Injection System

In gasoline engines, the timed fuel injection system injects a measured amount of fuel in timed bursts synchronized to the intake strokes of the engine. Timed injection is the most precise form of fuel injection; it is also the most complex.

1.1.1 Electronic-Timed Fuel Injection

The electronic fuel injection (EFI) system can be divided into four subsystems:

  1. Fuel delivery system
  2. Air induction system
  3. Sensor system
  4. Computer control system Fuel delivery System

The fuel delivery system of an EFI system as shown in Figure 1 includes:

Figure 1 - Electronic-timed fuel injection system.

The electric fuel pump draws fuel out of the tank and forces it into the pressure regulator. The fuel pressure regulator controls the amount of pressure entering the injector valves. When sufficient pressure is attained, the regulator returns excess fuel to the tank. This maintains a preset amount of fuel pressure for injector operation. Engine vacuum is ported into the fuel pressure regulator. This allows the pressure regulator to lower fuel pressure slightly at idle speed (low engine load) and increase with higher engine speed (higher engine load).

Figure 2 - Cutaway shows the major parts of an electronic fuel injector.

A fuel injector for an EFI system, Figure 2 is simply a coil- or solenoid-operated fuel valve. When not energized, the spring pressure holds the injector closed, keeping fuel from entering the engine. When current flows through the injector armature and the injector valve opens, fuel then squirts into the intake manifold under pressure. Figure 3A illustrates how current through the injector coil builds a magnetic field. The field attracts and pulls up on the armature to open the injector. Fuel then sprays out of the injector. Figure 3B illustrates when the control module breaks the circuit, the spring pushes the injector valve closed to stop the fuel.

Figure 3 - EFI injector operation. Air Induction System

An air induction system for an EFI system typically consists of a/an:

Air flow enters the inlet duct and flows through the air filter. The air filter traps dust and debris and prevents it from entering the engine. Plastic ducts then route the clean air into the throttle body assembly. The throttle body assembly in multiport injection systems simply contains the throttle valve and idle air control device. After leaving the throttle body, air flows into the engine intake manifold. The manifold is divided into runners (passages) that route the air into each cylinder head intake port. Sensor System

The EFI sensor system monitors engine operating conditions and sends information about these conditions to the control module. An engine sensor is an electrical device that changes its electrical output (resistance, voltage, or current) with a change in a condition, such as temperature, pressure, or position. For example, a temperature sensor's resistance may decrease as temperature increases. The control module analyzes the increased current flow through the sensor to determine if a change in injector valve opening is needed.

Oxygen sensor, which is also called an exhaust gas sensor or lambda sensor, shown in Figure 4, measures the oxygen in the engine's exhaust gases as a means of checking combustion efficiency. The exhaust gas oxygen sensor is one of the most important sensors controlling engine operation. The signals from the exhaust gas oxygen sensor are used by the powertrain control module to monitor air/fuel mixture. On-board diagnostics generation one (OBD-I) is a computer control system installed on vehicles built prior to 1996. These vehicles use one oxygen sensor located in the exhaust manifold or exhaust pipe before the catalytic converter.

Figure 4 - Oxygen sensor along with cutaway view showing the components of an oxygen sensor.

Vehicles with on-board diagnostics generation two (OBD-II), (enhanced diagnostic system required on 1996 and newer vehicles and also called for the standardization of codes, data link connectors, and terminology), these vehicles use at least two oxygen sensors-one before the catalytic converter and one after the catalytic converter. The primary sensor is located before the catalytic converter and is used to monitor the oxygen content of the exhaust gases entering the converter. The secondary oxygen sensor, or catalyst monitor, is located after the catalytic converter. It monitors the oxygen content of the gases leaving the converter to determine how well the catalyst elements are working.

The voltage output of the oxygen sensor varies with changes in the oxygen exhaust. If an increase in oxygen from a lean mixture makes the sensor output voltage decrease, a decrease in oxygen form a rich mixture causes the sensor output to increase. In this way, the sensor supplies data on oxygen content to the computer. The computer can then alter the opening and closing of the injectors to adjust the air-fuel ratio for maximum efficiency.

Open and Closed Loop. The term open and closed loop refers to the operating mode of a computerized engine control system. See Figure 5.

Figure 5 - The basic flow of information in an EFI system A- In open loop. B-In closed loop.

When the engine is cold, the computer operates open loop (no feedback from the sensors). After the engine warms to operating temperature, the system changes to closed loop (uses feedback from sensors to control system) operation.

An oxygen sensor must heat up to several hundred degrees before it will function properly. This is the main reason computer systems have an open loop mode. The computer has preprogrammed information (injector pulse width, engine timing, idle speed motor rpm, etc.) that will keep the engine running satisfactorily while the engine sensor is warming up.

When the engine and oxygen sensors are cold, no information flows to the computer. The computer ignores any signals from the oxygen sensor. The "loop of information" is open.

After the sensor and engine are warm, the oxygen sensor, and other sensors begin to feed data to the computer. This forms an "imaginary loop" (closed loop) as electrical data flow from the engine exhaust to the engine, the oxygen sensor, the computer, the injectors, and back to the oxygen sensor. Normally, the computer system functions closed loop to analyze the fuel mixture provided to the engine. This lets the computer "doublecheck itself."

Manifold absolute pressure (MAP) sensor measures pressure, or vacuum, inside the engine intake manifold. Manifold pressure is an excellent indicator of engine load. High pressure (low intake vacuum) indicates a high load, requiring a rich mixture. Low manifold pressure (high intake vacuum) indicates very little load, requiring a leaner mixture. The manifold pressure sensor varies resistance with changes in engine load. This data is used by the computer to alter the fuel mixture, Figure 6.

Figure 6 - Engine manifold absolute pressure (MAP) sensor.

Throttle position sensor (TPS) is a variable resistor, potentiometer, or multi-position switch connected to the throttle valve shaft. It provides data input in the power output of the engine, Figure .7.

Figure 7 - Throttle position sensor.

When the operator presses on the accelerator pedal for more power, the throttle shaft and sensor are rotated. This changes the internal resistance of the sensor. The resistance change is proportional to angle change in degrees. The resulting change in current signals the computer to alter outputs to actuators, as needed.

A throttle position sensor is very important in determining computer outputs. Depending upon the make of vehicle, it can affect air- fuel ration, spark advance, emission control system operation, turbocharger boost, transaxle torque converter lockup, and air conditioner compressor engagement.

Engine coolant temperature sensor monitors the operating temperature of the engine. It is mounted so that it is exposed to the engine coolant. When the engine is cold, the sensor might provide a low resistance (high current flow). The computer would then richen the air-fuel mixture for the cold engine operation. When the engine warms, the computer knows to make the mixture leaner.

Airflow sensor is used in EFI systems to measure the amount of outside air entering the engine. This helps the computer determine how much fuel is needed. The airflow sensor usually contains an air flap or door that operates a variable resistor. Increased airflow opens the flap more, changing the position of the variable resistor. Information is then sent to the computer indicating air inlet volume.

Intake air temperature sensor measures the temperature of the air entering the engine. Cold air is denser than warm air, requiring more fuel for the proper ratio. The intake air temperature sensor helps the computer compensate for changes outside air temperature and maintain an almost perfect air-fuel mixture ratio.

Crankshaft position sensor is used to detect engine speed. It allows the computer to change injector timing and duration with changes in engine rpm. Higher engine speeds generally require more fuel.

Fuel pressure sensor mounts on the fuel rail and sends an electronic signal proportional to the pressure inside the rail to the electronic control unit (ECU). The ECU can then control fuel pump speed and/or fuel injector pulse width to compensate for variations in fuel system pressure. A fuel pressure sensor is often used in returnless fuel injection systems.

Fuel temperature sensor monitors the temperature of the fuel rail. Fuel temperature has a slight effect on a fuel density and how much air and fuel must be atomized together to achieve a stoichiometric mixture and efficient combustion. The ECU, upon the signal from the fuel temperature sensor, can fine tune fuel metering, ignition timing, boost pressure, and other engine operating parameters. In some vehicles, a fuel pressure sensor and a fuel temperature sensor are housed in a single unit. Computer Control System

The computer control system uses electrical data from the sensors to control the operation of the fuel injectors and other engine performance-related devices. A wiring harness connects the sensors to the input of the computer (control module). Another wiring harness connects the output of the computer to the fuel injectors.

The engine control module is the "brain" of an electronic fuel injection system. It is a preprogrammed microcomputer. The control module uses sensor inputs to calculate when and how long to open the fuel injectors. To open an injector, the control module completes, or closes, the circuit between the battery and the injector coil. To close the injector, the module disconnects, or opens, the circuit between the battery and the injector coil.

1.2.0 Throttle Body Injection (TBI) Systems

Throttle body injection, as shown in Figure 8, is a form of continuous injection-one or two injectors delivering gasoline to the engine from one central point in the intake manifold. Though throttle body injection does not provide the precise fuel distribution of the direct port injection, it is cheaper to produce and to provide a degree of precision fuel metering. The throttle body injection unit is usually an integral one and contains all of the major system components. The unit mounts on the intake manifold in the same manner as a carburetor. Airflow sensors and electronic computers usually are mounted in the air cleaner body.

Figure 8 - Throttle body injection has sensors and a control module- operated injector mounted inside the throttle body.

A throttle body injection system (TBI) uses one or two injector valves mounted in a throttle body assembly. A diagram of an injector and its control circuits is shown in Figure 9. The injector(s) spray fuel into the top of the throttle body air horn. The fuel spray mixes with the air flowing through the air horn. The mixture is then pulled into the engine by intake manifold vacuum.

Figure 9 - Throttle-Body Injection Assembly with cutaway view of the TBI injector.


Figure 10 - Throttle body.

The TBI assembly, as shown in Figure 10, typically consists of:

Throttle plates are mounted in the lower section of the housing. A linkage mechanism or cable connects the throttle plates with the accelerator. An inlet fuel line connects to one fitting on the throttle body housing. An outlet return line to the tank connects to another fitting on the housing.

Throttle Body Injector Service. A quick check of the operation of a throttle body injection system can be done by watching the fuel spray pattern in the throttle body. Remove the air cleaner. With the engine cranking or running, each injector should form a rapidly pulsing spray of fuel.

Testing TBI Injectors. If fuel system pressure is within specifications but the TBI injector does not spray fuel, follow service manual procedures to test for power to the injector solenoid.

No power (current) to the injector indicates a problem with the wiring harness or electronic control module.

If you have power but no fuel spray pattern, the injector may be bad. Make sure to have adequate fuel pressure before condemning a fuel injector.

Replacing TBI Injectors. Although exact procedures vary with throttle body design, there are a few rules to follow when replacing a TBU injector. These rules include:

  1. Relieve system fuel pressure before removing any throttle body component.
  2. Disconnect the negative battery cable.
  3. Label all hoses and wires before removal.
  4. Do not damage the throttle body housing when pulling or prying out the injector.
  5. Install new rubber O-rings. It may be required to lubricate the O-ring with approved lubricant to aid new injector installation.
  6. Double-check that you have installed all injector washers and O-rings correctly.
  7. Push the injector into the throttle body by hand. Avoid using a hammer or pliers. You may damage the new injectors.
  8. Make sure the injector is fully seated in the throttle body. Normally, a tab must fit into a notch, ensuring correct alignment.
  9. Check for fuel leaks after the TBI injector installation. Test drive the vehicle.

Throttle Body Rebuild. A throttle body rebuild, like a carburetor rebuild, involves replacing all gaskets, seals, and other worn parts. Remove and disassemble the throttle body, making sure all plastic, rubber, and electrical parts are removed.

The metal parts are soaked in carburetor cleaner, washed in cold soak cleaner, and blown dry.



Carburetor cleaner is a very powerful solvent. Wear eye protection and r ubber gloves when using this type of cleaner. Follow the directions on the solvent label in case of an accident.

Inspect each part carefully for signs of wear or damage. Then, reassemble the TBI unit following the instructions in a manual.

Install the throttle body on the engine the same as a carburetor. Use a new base plate gasket. Tighten the holddown fasteners equally and to the proper torque. Make sure all vacuum hoses are connected to the correct vacuum port on the throttle body. Start the engine. Check for leaks and smooth system operation.

1.3.0 Servicing and Precautions

When a vehicle equipped with a gasoline type of fuel injection system has a problem, check all other systems first, such as ignition, air intake, charging, exhaust systems, and so forth, before you work on the fuel injection system. The fuel injection system is usually the last (least problematic) system to cause trouble. There are servicing precautions you should observe before you work on gasoline fuel injection systems:

  1. Do not jump the battery to start the vehicle.
  2. Do not disconnect the battery cables from the battery with the engine running.
  3. When charging a battery in the vehicle, disconnect the negative (grounded) terminal.
  4. Do not remove or attach the wiring harness plug to the electronic control unit (computer) with the ignition on.
  5. Before performing a compression test, check the manufacturer's repair manual for special instructions.
  6. Always make sure all other systems are in good working order before you adjust or troubleshoot the gasoline fuel injection system.

These precautions are general and apply to most systems. Nevertheless, use good judgment, and always check your manufacturer's repair manual for proper specifications and procedures.

Preventive maintenance is the most frequent type of servicing you will perform on a gasoline fuel injection system. Preventive maintenance consists of periodic visual checks and scheduled fuel filter service. Fuel filters are the cartridge type, in line type, and disposable type.

All fuel injection system control sensors, such as temperature, oxygen, manifold absolute pressure, and so forth, are electrically connected to the electric control module (ECM). Some of these sensors, such as the oxygen sensor, have a regular maintenance cycle. Check your manufacturer's repair manual for special instructions pertaining to these sensors.

Be sure the air intake system is sealed properly. Early detection will save fuel and prevent engine damage. Air leaks are a problem to gasoline fuel injection systems. If the air leak is after the air filter, dirt will be ingested into the engine, causing internal damage to the engine. Air leaks that bypass temperature sensors can cause false readings to be delivered to the ECM, changing injection quantity. Un-metered air leaks in the intake manifold can cause a lean fuel-air mixture to be delivered to the combustion chambers.

During regular maintenance and always after reassembly, you should check for fuel leaks. Gasoline leaks, however small, are extremely dangerous. They must be dealt with immediately. Clean around all areas to be disassembled. Heavy layers of dirt and grime may make some leaks hard to find. Install new seals on leaking connections and replace cracked or leaking hoses.


Gasoline fuel injection systems operate with fuel pressures up to five times greater than that of standard gasoline fuel systems. Any replacement fuel lines used should be approved for higher pressures. Failure to do so will result in an unsafe fuel system on the vehicle with the danger of possible explosion and fire.

Clean around all areas before reassembly. When you tighten injector line nuts (injector head), use new seals and proper torque specifications. When you tighten fuel lines, use flare nut type of wrenches because regular open-end wrenches may damage these fuel line fittings.

1.4.0 Carburetors (Small Engine)

Small gas engines serve us in a number of ways. They power chain saws, pumps, generators, air compressors, welders, compactor/tamper (jumping jack), vibratory plate compactor, numerous types of lawn care equipment, pressure washers, paint sprayers, cement/mortar mixers, concrete saws.and the list goes on. Many of these gas- powered tools and equipment are contained in the Naval Construction Force Table of Allowance.

Small gas engines are made up of individual systems that work together. In this topic, we are only going to discuss small engine carburetors, their theory of operation, basic components, types and application. There are several manufacturers of small engine carburetors and we will only include a few in our topic. They are all generally similar and contain most of the same components and characteristics.

1.4.1 Theory of Operation

Carburetion. The carburetor is an essential part of internal combustion engines. Gasoline engines cannot run on liquid gasoline. The main function of the carburetor is to mix the fuel and air and feed it into the engine, where it is ignited and used to thrust the pistons downward inside the engine block. The fuel must be vaporized and mixed with air in the proper proportions for varying conditions. The basic physics behind the function of a carburetor is called the Bernoulli Principle and the venturi effect. The Bernoulli Principle states that speed of the air is inversely proportional to the pressure. It is the throttle plate or butterfly of the carburetor that manages the amount of air that is delivered to the engine.

The carburetor must create an air fuel mixture that is correct for different circumstances such as:

Air enters the top of the carburetor and is mixed with liquid fuel. The air fuel mixture is forced into the intake manifold by atmospheric pressure and burned in the combustion chamber of the engine. The mixture will vary, depending on conditions. The proportion is given as the number of pounds of air compared to the number of gallons of gasoline.

At normal operating speed, a small engine will use an air-fuel mixture of about 14.7 pounds of air to 1 pound of gasoline.

Carburetors work on the principle of air pressure differences. When discussing pressure differences, we will talk about the following:

  1. Vacuum. An absolute vacuum is an area completely free of air or atmospheric pressure. Although an absolute vacuum is not reached in a small engine, any pressure less than atmospheric pressure is generally referred to as a vacuum.
  2. Atmospheric pressure. Atmospheric pressure is the pressure produced by the weight of air molecules above the earth. A partial vacuum is produced by the piston on the intake stroke. When the intake valve opens, atmospheric pressure forces air through the carburetor to fill it.
  3. Venturi effect. Carburetion would not be possible if not for the venturi effect. A venturi is a restriction in an air passage that increases air speed or velocity. This increase in velocity reduces pressure, causing fuel to be drawn into the air stream. The venturi effect is an example of the Bernouli Principle. Fluid velocity must increase through the constriction in a tube. The kinetic energy is created by the drop in pressure. Carburetors use tubes called a venturi to achieve this effect. Particles of fuel are vaporized by air rushing through the venturi, as illustrated in Figure 11.

Figure 11 - Venturi effect.

Vaporization. Although the venturi breaks the fuel into fine particles, it is further vaporized by the heat of the engine in the intake manifold and by the engine's swirling action of the air in the combustion chamber.

Combustion. Cold fuel is difficult to vaporize, this is why we choke or prime a cold engine to help get it started. Over choking or priming can cause raw fuel to be pulled into the combustion chamber, resulting in bypass or the condition of flooding.

1.4.2 Basic Carburetor Components

A carburetor is a metering device for mixing fuel and air, as illustrated in Figure 12. The correct mixture in the combustion chamber is essential for the engine to run properly. Two conditions must be met for proper carburetion: the fuel must be introduced to the incoming air stream and it must be vaporized.

Figure 12 - Carburetor components.

Venturi. The venturi is the narrowed portion of the carburetor tube where the suction is created and the velocity of the incoming air is increased. Below the venturi there is a valve called a throttle plate, which can be opened and closed by the throttle control.

This valve controls the engine speed by restricting the air flow to the engine and subsequently, the amount of air and fuel mixture that is delivered.

When the gas pedal is depressed, the throttle plate is opened allowing fuel to be drawn into the air stream. These tiny holes are on the smallest section of the venturi are called jets. When the throttle is opened, the vacuum in the intake manifold is decreased. As the velocity of air increases, the low pressure raises the air speed to draw additional fuel into the stream through the nozzle located at the center area of the venturi.

Main Discharge Tube. The main discharge tube is the part through which the fuel travels from the fuel container to the air stream in the venturi during high speed operation.

The main discharge tube of the carburetor is a tube, one end of which is connected to the venturi, and the other end is in the fuel container found below. While air is flowing through the venturi, the effect will be the same as putting a straw in your mouth and sucking on it. If one end of the straw is placed into the liquid, the fluid will be drawn up the straw. The fuel container can be the float bowl, part of the fuel tank, or the entire fuel tank. The fuel is actually pushed through the main jet tube by the difference in pressure between the atmosphere and the venturi throat, as shown in Figure 13. Normal atmospheric pressure pushes on the fuel in the reservoir and moves the fuel to the low pressure area in the venturi.

Figure 13 - Fuel is pushed through the main jet tube.

The greater the volume of air passing through the venturi, the higher the vacuum and the larger the amount of fuel that will be sprayed into the air stream at the main discharge tube.

Fuel Container. The fuel container holds the fuel for use by the different metering circuits in the carburetor.

Fuel Supply Inlet. The fuel inlet is where fuel enters the fuel container form the engine's fuel block

Float. The float is used to control the level of fuel in the fuel container. An essentially unchanging level of fuel must be maintained. Proper metering of fuel-to-air ratios is dependent on a constant distance from the venturi to the surface of the fuel in the container.

When the engine speed or load increases, fuel is rapidly pulled out of the fuel bowl and into the venturi. This makes the fuel level and float drop in the bowl. Fuel enters the bowl, and the float rises.

Bowl Vent. The bowl vent allows atmospheric air pressure to enter the carburetor system. The difference in atmospheric pressure (relatively high) and the venturi pressure (relatively low) pushes the fuel from the fuel container into the venturi while the engine is operating. The vent tends to maintain the air pressure above the surface of the fuel in the bowl at atmospheric levels.

The bowl vent may be an external or internal type. An external vent can be found on the outside of the carburetor body. An internal vent is commonly found in the carburetor air horn near the choke side of the carburetor. An advantage of an internal vent is that a larger venturi can be used to maximize the horsepower of the engine. The pressure supplied to the surface of the fuel in the fuel container is higher than the atmospheric pressure, since air moving through the carburetor is forced into the vent opening.

High-Speed Mixture Adjustment Needle. The high-speed mixture adjustment needle is used to control the amount of fuel entering the air stream at high speed. It can be turned in to decrease the amount of fuel, which makes the air-fuel mixture lean, or turned out for a rich air-fuel mixture.

High-Speed Air Bleed. The high-speed air bleed allows air to break up the fuel before entering the air stream in the venturi. When the air enters the carburetor, it forms a slight pressure near the venturi as the molecules are backed up while waiting to enter the venturi. This backup of air molecules increases the pressure slightly on the choke side of the venturi. With the high-speed air bleed located in this high-pressure area, some of the air moves through this channel to mix with the fuel in the main discharge tube.

Throttle Plate. The throttle plate controls the air flow through the venturi, thereby controlling the fuel flow to the engine. When the throttle plate is closed, all the air flow to the engine ceases.

Choke Plate. The choke plate partially blocks off the air flow, creating low pressure throughout the carburetor to provide a rich in-fuel mixture for cold starting. The fuel is drawn into the limited air flow from the main discharge tube and idle passages.

Idle Passage. The idle passage connects the carburetor's bowl to the engine side of the throttle plate. Fuel is forced through this passage when the throttle plate moves to the idle position.

Low-Speed Mixture Adjustment Screw. The low-speed mixture adjustment screw is used to meter the precise amount of fuel for engine operation at idle.

Idle Air Bleed. The idle air bleed allows air to atomize the fuel entering the air stream while the engine is idling. This premixing of the fuel and air increases the efficiency of engine combustion. When the throttle plate is in the idle position, the transitional fuel passage also allows air to bubble into the idle passages.

Transitional Fuel Passages. The transitional fuel passage provides a temporary fuel supply to the engine during the transition from idle to high-speed operation. As the throttle plate begins to open, both the transitional fuel passages and the idle passage provide the air-fuel mixture.

Idle Speed Screw. The throttle plate must be held open slightly by the needle speed screw. When the throttle plate is opened fully, the air flow into the engine is limited by the size of the venturi.

Inlet Needle. The inlet needle opens as the float level drops and fuel is allowed to enter the bowl area. As the fuel rises, the float pushes the inlet needle back and shuts off the incoming fuel. When the engine is operating, the float pushes the inlet needle back and shuts off the incoming fuel. When the engine is operating, the float and inlet needle regulate the incoming fuel flow to maintain the proper fuel level in the fuel container.

Carburetor Systems. When the air-fuel mixture is drawn into the cylinder on the intake stroke of the piston and the engine is cold, the gasoline vapors tend to condense into large drops on their way to the cylinder. Because all the gasoline supplied to the cylinders does not vaporize, it becomes necessary to supply a rich mixture to have enough vapor for combustion to occur. This is accomplished by a choke system, enrichment system, or primer system.

Choke System. When the choke is tilted in the carburetor to restrict the amount of air entering the air horn, greater suction is created and a larger amount of fuel is drawn into the combustion chamber from the idle passages and the main discharge tube, as shown in Figure 14.

Figure 14- Choke system.

Enrichment System. An enrichment system is an air-fuel metering circuit separate from other carburetor circuits. Fuel for the circuit is drawn from the float bowl through an enrichment jet. The enrichment jet is activated by the user through a special lever.

When the plunger is down, the enrichment circuit is blocked and does not operate. When the plunger is raised, a rich air-fuel mixture is discharged from the enrichment port, which is located directly behind the venturi.

Primer System. The primer system provides a rich mixture by increasing the pressure in the top of the fuel bowl to force extra fuel down on the float to force a higher than normal float bowl level and create a rich condition. In a diaphragm carburetor, the diaphragm may be lifted mechanically or pneumatically.

1.4.3 Types and application Briggs & Stratton

Briggs & Stratton Manufacturers three different basic type of carburetor:

  1. Vacu-Jet
  2. Pulsa-Jet
  3. Flo-Jet

Because environmental safety demands stricter emission and performance controls, many manufacturers realize that it is more cost-effective to contract another manufacturer that specializes in carburetors to design and build them. Briggs & Stratton has entered into partnership with Walbro carburetors, and together they are producing a Flo-Jet type of carburetor with both of their names on it. Vacu-Jet Carburetor

The Vacu-Jet is a simple carburetor that is used in many small displacement Briggs & Stratton engines and functions adequately. It is adjusted differently from other carburetors because of its unique features.

Vacu-Jet Identification. The easiest way to identify the Vacu-Jet design is to check the engine's model number, as shown in Figure 15. Another method is to recognize its appearance, but this could be difficult due to the design changes if you do not work on them often, as indicated in Figure 16. Figure 16A is a Vacu-Jet all-temperature carburetor with automatic choke. Figure 16B is a Vacu-Jet Choke-A-Matic. The most reliable method of identification is when the carburetor is separated from the fuel tank and only one pickup tube is present.

Figure 15 - Example of Briggs & Stratton Vacu-Jet identification by using the model number.


Figure 16 - Example of two Vacu-Jet models.

Vacu-Jet design. Fuel has to be lifted to the venturi. The atmospheric pressure enters the vent in the fuel cap and pushes on the mass of fuel. A single pickup tube extends into the fuel through the bottom of the carburetor. When air passes through the venturi of the carburetor, the pressure in this area is reduced.

Figure  17 - Limiting orifice.

Initial Carburetor Vacu-Jet Adjustment. The air cleaner must be assembled to the carburetor before running the engine. The best carburetor adjustment is obtained with the following steps:

  1. Make sure that the level of fuel in the tank is approximately ¼ full.

Originally, Briggs & Stratton recommended that the fuel tank be ½ full or full when adjusting. Many warm restart problems have prompted a change in this initial fuel tank level. The distance the fuel has to be lifted will affect the mixture adjustment. At ¼ full, we have an average operating condition, and the adjustment will be satisfactory even if the engine is run with the tank full or nearly empty.

  1. Gently turn the needle valve clockwise until it seats. The valve may be damaged by turning it too far.
  2. Open the needle valve 1½ turns counterclockwise. This initial adjustment will permit the engine to be started.
  3. Next, with a warmed-up engine for 3 to 5 minutes operating time, make the final adjustment.

Final Carburetor Vacu-Jet Adjustment.

  1. Place the speed control lever in the FAST position.
  2. Turn the needle valve in clockwise until the engine speed just starts to slow. Now open the needle valve ¾-turn counterclockwise.
  3. With your fingers, rotate the throttle plate counterclockwise and hold it against the throttle plate stop while adjusting the rpm. Turn the idle speed adjustment screw to obtain 1750 rpm, as recommended by the factory specifications. Use a tachometer to set the idle speed. If the idle speed is lower than recommended, the engine may not perform properly.
  4. Release the throttle plate. The engine should accelerate smoothly. If the engine does not accelerate properly, the carburetor should be readjusted, usually to a slightly richer mixture, by opening the needle valve an additional ¼-turn counterclockwise.
  5. Check the adjustment by moving the engine control from SLOW to FAST speed. The engine should accelerate smoothly. If the engine tends to stall or die out, increase the idle speed or re-adjust the carburetor, usually to a slightly richer mixture.

Previously, Briggs & Stratton recommended that the carburetor be adjusted to the "midpoint," between too rich and too lean. The adjustment worked well with an engine being started for the first time, but not in the case of a hot engine. The midpoint proved to be too rich and resulted in an engine that was hard to restart. Based on this new information, the company adopted new procedures.

Starting a flooded Vacu-Jet. Flooding can occur if:

  1. The engine is tipped at an angle for a prolonged period of time.
  2. The engine is cranked repeatedly when the spark plug wire is disconnected.
  3. The carburetor mixture is adjusted too rich.

If flooding occurs in a Vacu-Jet with an automatic choke, move the governor control to STOP and pull the starter rope at least six times; in this situation, the governor spring holds the throttle plate in a closed (idle) position. Thus, cranking the engine with a closed throttle creates a higher vacuum, which opens the choke rapidly, permitting the engine to clear itself of excess fuel. Then, move the control to the FAST position and start the engine. If the engine continues to flood, lean the carburetor needle valve ¼- turn clockwise. Pulsa-Jet Carburetor

The Pulsa-Jet is a carburetor that incorporates a diaphragm fuel pump and a constant level fuel chamber, as illustrated in Figure 18. The fuel tank, fuel pump, and constant level fuel chamber serve the same functions as the gravity feed tank, the float, and the float chamber of the conventional "float type" carburetors.

Figure  18 - Pulsa-Jet carburetor.

With this carburetor, the fuel level stays constant in the small chamber, no matter what fuel level exists in the main tank. Very little "lift" is required to draw the gasoline into the venturi. The venturi can be made larger, as shown in Figure 19, permitting a greater volume of air-fuel mixture to flow into the engine, enhancing the volumetric efficiency of the engine and increasing the horsepower.

Figure  19 - Pulsa-Jet uses a larger venturi than the Vacu-Jet.

Pulsa-Jet construction. When the carburetor is removed from the fuel tank, notice that there are two differentlysized fuel pipes. The long one transfers fuel from the large tank to the small fuel cup by means of a fuel pump, while the short tube transfers the fuel from the fuel cup to the venturi.

Pulsa-Jet identification. Just as with the Vacu-Jet carburetor, identification of the Pulsa-Jet is by using the model number. Using the example in Figure 15 for Pulsa-Jet, the center number (5) will change to either a 2 for horizontal crankshaft or 9 for vertical crankshaft. The other method again is by visual recognition if you are that familiar with the various models.

Pulsa-Jet fuel pump. The built-in fuel pump is actuated by the changes in pressure in the carburetor air horn or throat. When the piston is moving downward on the intake stroke, a low-pressure area builds on the engine side of the venturi, as illustrated in Figure 20. The low pressure is transferred to the pump diaphragm through a hole in the bottom of the air horn. The diaphragm is raised upward against the tension of the spring, and the area below the diaphragm develops a lower pressure or suction as it attempts to expand. The space increases as fuel is sucked through valve, which is a one-way flap that permits fuel to flow into the chamber, but not through valve #2, which is also a one-direction flexible flap that allows fuel to exit the compartment only.

Figure 20 - The vacuum from the piston movement moves the pump diaphragm upward.

When the piston is not on the intake stroke, the engine's intake valve is closed, as shown in Figure 21, and the engine vacuum in the venturi area disappears. The spring that is located in the upper pump area pushes the diaphragm downward, causing an increase in the pressure in the lower chamber. The added pressure attempts to drive the fuel out of the area, but only valve #2 is allowed to open, as valve #1 is held shut by pressure. Fuel is pushed into the small fuel cup of the carburetor.

Figure 21 - The spring pushes the pump diaphragm downward when no vacuum is present.

The vacuum/no vacuum condition creates an action of the fuel pump's diaphragm, which along with the two pump valves, moves the fuel along from the pick-up tube to the small fuel cup. Keep in mind that the diaphragm separates the air compartment and spring from the fuel compartment.

A common mistake in re- assembling the carburetor is the placement of the fuel pump spring and spring cover. The inexperienced person will place it into the fuel tank side of the diaphragm. The correct position, as shown in Figure 22, is in the recess on the carburetor side of the diaphragm. This is a calibrated spring and should never be stretched. If the spring is bad, it should be replaced.

Figure 22 - Correct placement of the fuel pump spring.

Pulsa-Jet variations. The newer Pulsa-Jet aluminum carburetor has a fixed main fuel jet with an adjustable idle fuel circuit, as shown in Figure 23. With the carburetor removed from the tank, Pulsa-Jet carburetors can be seen to have two fuel pickups.

Figure 23 - Pulsa-Jet carburetors have two fuel pickup tubes.

Figure 23A shows that the shorter pickup tube is part of the carburetor casting in this fixed-jet version. (In the adjustable-jet version, the shorter pickup tube is plastic and screws into place.) For this particular carburetor, the main jet is located on the side of the shorter fuel pickup tube and is fixed (not adjustable). Figure 23B shows a version of the Pulsa-Jet carburetor that has a fixed jet at the bottom of the pedestal. A venturi is cast as an integral part of the carburetor body, improving the engine's starting, idling, acceleration, and response to load. The fuel tank is vented through a passage within the carburetor. These aluminum body carburetors are equipped with a throttle plate shaft dust seal.

A 0.046-inch diameter hole has been placed in the fuel tank's reservoir cup. When an engine runs out of fuel, both the tank and reservoir cup are nearly empty of fuel. When the fuel tank is filled to a level above this hole, fuel will transfer into the reservoir cup. This reduces the number of pulls required to "prime" the reservoir cup, allowing for quicker and easier starting.

Pulsa-Prime Carburetor. Another Pulsa-Jet family carburetor is the "Pulsa-Primer" carburetor, as shown in Figure 24. The main body of the carburetor is injection-molded, glass-reinforced nylon polymers. By using plastic for the body construction, no machining is required. The Pulsa-Prime carburetor has no idle system and no air or fuel adjustments. The governor system will allow for a 600 rpm decrease in speed from the top no-load speed, but the engine will never reach a true idle speed. The carburetor has high-speed jets that will provide fuel throughout the operating speed of the engine.

Figure 24 - "Pulsa-Prime" carburetor.

Primer. A wet bulb primer has been added to the carburetor. This eliminates the need for the automatic choke. The wet bulb primer will inject fuel directly into the throat of the carburetor for quick start with a cold engine through the high-speed nozzle. The primer bulb should be firmly pressed three times before starting the engine. Priming is usually unnecessary when restarting a warm engine. The primer bulb will also assist when starting after refueling.

Pressing on the primer bulb compresses air inside the carburetor body seating and the check valve in the fuel pipe. Air is pushed past the primer check valve. When the primer bulb is released, a vacuum now exists inside the primer fuel pipe. Air pressure inside the fuel tank pushes the fuel past the fuel pipe check valve and up the fuel pipe, past the check valve to begin filling the fuel pump cavity. Fuel also moves through the primer passages to the primer bulb. Pushing the primer bulb only one time on a dry fuel system will not fill all passages. On the third push, fuel will fill the fuel pump, pushing fuel past the outlet valve into the fuel well in the top tank, through the jet screen and up the fixed jet.

Cleaning the Pulsa-Prime Carburetor. The Pulsa-Prime carburetor should be cleaned on a regular basis to keep the carburetor operating properly. Debris and contaminants should be removed and the filter screens should be cleaned. The carburetor is removed from the fuel tank when the fuel tank is cleaned. Remove any debris from the filter screens on the fuel pump pickup tube (long tube) and the main jet assembly, as shown in Figure 25. An easy way to remove the fuel from a Pulsa-Prime fuel tank is to utilize a clear or opaque turkey baster. The fuel can be suctioned out and examined or discarded.

Figure 25 - Clean the debris from filter screens.

Fuel Pump Spring. The fuel pump spring must be installed for the fuel pump to operate properly. Install the spring as shown in Figure 26.

Figure 26 - Diaphragm spring.

High-Altitude Compensation. When the Pulsa-Prime carburetor is used at high altitudes and performance is poor, remove the main air jet air bleed, as shown in Figure 27. By removing the main air jet bleed, more air is allowed to mix with the fuel and lean the mixture, which is necessary for proper performance at higher altitudes.

Figure 27 - Main jet air bleed.

Initial Carburetor Pulsa-Jet Adjustment. The air cleaner must be assembled to the carburetor before running the engine:

  1. Gently turn the needle valve clockwise until it just seats. The valve may be damaged by turning it in too far.
  2. Open the needle valve 1½ turns counterclockwise. This initial adjustment will permit the engine to be started.
  3. Next, with a warmed-up engine (3 t0 5 minutes operating time), make the final adjustments.

Final Carburetor Pulsa-Jet Adjustment

  1. Place the speed control lever in the FAST position.
  2. Turn the needle valve inward (clockwise), until the engine speed just starts to slow. Now open the needle valve ¾-turn counterclockwise.
  3. With your fingers, rotate the throttle plate counterclockwise and hold it against the throttle plate. Stop while adjusting idle rpm. Turn the idle speed adjusting screw to obtain 1750 rpm, as recommended by the specifications. Use a tachometer to set the idle speed. If the idle speed is lower than recommended, the engine may not perform properly.
  4. Release the throttle plate. The engine should accelerate smoothly. If the engine does not accelerate properly, the carburetor should be readjusted, usually to a slightly richer mixture, by opening the needle valve an additional ¼-turn counterclockwise.
  5. Check the adjustment by moving the engine control from SLOW to FAST speed. The engine should accelerate smoothly. If the engine tends to stall or die out, increase the idle speed or readjust the carburetor, usually to a slightly richer mixture.

Previously, Briggs & Stratton recommended that the carburetor be adjusted to the "midpoint," between too rich and too lean. The adjustment worked well with an engine being started for the first time, but not in the case of a hot engine. The midpoint proved to be too rich and resulted in an engine that was hard to restart. Based on this new information, the company adopted new procedures.

Automatic Choke (Vacu-Jet and Pulsa-Jet). Theory of operation. The automatic choke provides a rich mixture condition when starting the engine. A diaphragm under the carburetor is connected to the choke shaft by a rigid link. A calibrated spring under the diaphragm holds the choke valve closed, as shown in Figure 28, View A, when the engine is not running. When the engine starts, the choke opens as the vacuum created in front of the venturi is transferred via a calibrated passage to the area below with a choke diaphragm. As the diaphragm and choke link move downward, as shown in Figure 28, View B, the choke is pulled open.

Figure 28 - Automatic choke.

This system has the ability to respond in a fashion similar to an acceleration pump. As the speed decreases during heavy loads, the choke valve partially closes, resulting in a richer mixture and improved acceleration performance.

Preloading diaphragm. Before tightening the carburetor mounting screws in a staggered sequence, move the choke plate to an over-center position and hold it there. This pushes the choke link downward to the bottom of its travel and pre-stretches the diaphragm. Tighten the screws in a crossing sequence to 35 inch-pounds.

Automatic choke inspection.

  1. Remove the air cleaner and replace the stud. Observe the position of the choke valve. It should be fully closed.
  2. Move the speed control to the stop position. The governor spring should be holding the throttle plate in a closed position. Pull the starter rope rapidly. The choke valve should alternately open and close.
  3. If the engine can be started, run for 3 to 5 minutes at normal operating speed. Close the needle valve by turning clockwise enough to make the mixture lean, and then adjust the needle valve ¾-turn open counterclockwise.
  4. Allow the engine to run at idle speed for 3 to 5 minutes. Again, close the adjusting speed needle; the mixture should become so lean that the engine will stop. If the engine continues to run at idle with the needle valve closed, a fuel leak is occurring at the diaphragm.

Bimetal choke. An engine equipped with a safety stopping mechanism can be expected to be stopped, and then restarted, on a frequent basis. This means that the engine must start easily each time. The bimetal automatic choke carburetor is used in these situations.

The diaphragm springs that are used in the automatic choke carburetors may look alike but will vary in function, depending on usage. Therefore, utilize only the spring recommended in the parts manual for the engine being worked on. The springs have been color-coded according to their strength. The stronger the spring, the longer the engine will be kept in the choke condition. The springs from the strongest to the weakest are: no color, red, blue, and green.

The operation of the automatic choke remains the same at moderate engine starting temperatures. However, at "hot" or "cold" starting temperatures, the bimetal choke carburetor, as shown in Figure 29, automatically compensates for temperature changes by influencing the choke opening time.

Figure 29 - Bimetal choke compensates for temperature changes.

At higher engine starting temperatures, the "hot" crankcase air passing through the breather tube causes the bimetal spring to expand and curl outward. This action causes the inner end of the bimetal spring to "pull" the choke shaft/choke plate open, but since the diaphragm spring is opposing this force, the choke plate continues to remain closed. This action does, however, make it easier for starting vacuum to open the choke plate quickly and improve "hot" starting problems.

At lower engine starting temperatures, the "cold" ambient air causes the bimetal spring to contract and curl inward. This action causes the inner end of the bimetal spring to "push" the choke shaft/choke plate closed, which assists the diaphragm spring in holding the choke plate closed longer. The result is a slightly longer period of a rich air- fuel mixture, assuring improved "cold" temperature starting.

Automatic choke inspection areas. When a problem is suspected in the automatic choke system, certain areas should be inspected to determine whether:

  1. The carburetor is adjusted too lean to too rich.
  2. The fuel pipe check valve is inoperative (Vacu-Jet only).
  3. There is a bent air cleaner stud.
  4. There is a sticking choke shaft due to dirt.
  5. The choke spring is damaged or too short.
  6. The diaphragm is not preloaded.
  7. The diaphragm is ruptured or the vacuum passage is restricted.
  8. Gasoline or oil is in the vacuum chamber.
  9. There is a leak between link and diaphragm.
  10. The diaphragm folded during assembly.

Flo-Jet Carburetor. The Flo-Jet resembles the internal workings of the carburetors generally found on vehicles. The fuel tank is mounted higher than the carburetor, and gasoline flows into it by the pull of gravity. A float inside the carburetor bowl regulates the flow of gasoline, similar to the float inside your toilet tank that regulates the level and flow of water.

Flo-Jet identification. The easiest way to identify the Flo-Jet design is to check the engine's model number. If you refer back to Figure 15, the #5 in the model number would change to 3 for horizontal Flo-Jet pneumatic governor or 4 for horizontal Flo-Jet mechanical governor. You can identify by recognition of the carburetor, but only the most experienced should rely on that method. Figures 5-30, View A - 5-30, View C illustrates three models of Flo-Jet carburetors. Notice their differences in appearance.

Figure 30 - Flo-Jet carburetors.

Flow-Jet carburetor adjustment. Prior to the separation of the upper and lower half of the carburetor, unscrew the packing nut and remove the high-speed needle valve, as shown in Figure 31.

Figure 31 - Packing nut and high-speed needle valve.

Next, remove the nozzle (emulsion tube), as shown in Figure 32, View A. The nozzle must be removed by using a screwdriver without a taper (Briggs & Stratton tool #19280), as shown in Figure 32, View B, since the nozzle is made of brass, a soft metal, and is easily stripped or damaged.

See Figure 32, View C. Failure to remove the nozzle will result in a carburetor that may leak when the engine is not operating. The nozzle has holes along its length that must be cleared in addition to the hole down the center (Figure 32, View D).

Figure 32 - Remove and clean nozzle.

Flo-Jet carburetor cleaning. Particles of dirt and debris in the fuel system will create erratic engine operation and affect carburetor adjustments. If this problem is suspected, clean the entire fuel system, including the carburetor. Install a fuel filter in the fuel line and adjust the float level.

Flo-Jet float adjustment. With the body gasket in place on the upper body and the float valve and float installed, the float should be parallel to the body mounting surface. If not, bend the tang on the float with needle nose pliers until it is parallel. DO NOT PRESS ON THE FLOAT.

Initial Flo-Jet adjustments. Before starting the engine, the air cleaner should be clean and assembled to the carburetor. If the mixture valves are adjusted without the air cleaner in place, difficulties will be encountered after the air cleaner is attached. Some resistance to the air intake is caused by the cleaner, and this will result in added suction in the air horn (which was not present when the adjustments were made).

  1. Gently turn the maximum valves clockwise until they just seat. Valves may be damaged by turning them too far and too tightly.
  2. Open the high-speed needle valve 1½-turns counter clockwise and the idle valve one turn. This initial adjustment will permit the engine to be started.
  3. Start engine and allow it to warm up for approximately 5 minutes prior to final adjustment.

Final Flo-Jet adjustment.

  1. When the engine is warm and running, place the speed control lever in the FAST position.
  2. Turn the high-speed needle valve inward until the engine slows (clockwise-lean mixture). Then turn it outward, past smooth operating point (counterclockwise- lean mixture). Now turn the needle valve to midpoint between rich and lean.
  3. Adjust the idle to 1750 rpm according to the specifications by rotating the throttle plate counterclockwise and holding it against the stop and turning the idle mixture valve screw.
  4. While holding the throttle plate against the idle stop, turn the idle mixture needle valve clockwise until the engine slows. Then turn it counterclockwise, past the smooth operating point. Set it at a midpoint between rich and lean.
  5. Recheck the idle rpm and release the throttle plate. If the engine does not accelerate properly, readjust the high-speed mixture valve approximately ¼-turn counterclockwise (rich).

Flo-Jet problems.

Leakage. Carburetor leakage is not only irritating, but also a safety concern. The first step toward finding the cause of the problem is to identify when leak occurs.

Float bounce is one cause for the Flo-Jet carburetor to leak. Float bounce is a condition that typically occurs when the engine/equipment is transported. The use of a fuel shut- off valve is recommended for the use on all float-style carburetor systems.

The fuel supply (fuel tank) may be located too far above the carburetor, resulting in excessive pressure at the fuel tank inlet needle and a leak. The maximum tank height recommended for gravity feed duel systems is 45 inches. The use of an inline filter is recommended for all float-style carburetor systems.

If the carburetor leaks shortly after the engine is turned OFF, it may due to a long coast- down period, which is a prolonged spinning of the engine. A long coast-down period will cause accumulation of unburned fuel. Whenever possible, slow the engine's speed to an idle before shutting OFF the engine. Using the choke as a means to shut off the engine will only aggravate the leakage condition and is not recommended.

The remaining causes for Flo-Jet carburetion leakage involve parts that are loose, missing, assembled/adjusted incorrectly, damaged, or affected by contaminants in the fuel system, such as dirt, water, or additives. Understanding the design of the carburetor is necessary to efficiently isolate and identify the actual cause of the problem.

Common causes. A common cause of fuel leakage in a two-piece Flo-Jet carburetor is an improper seal between the main nozzle and the lower half of the carburetor. There are three ways to correct this problem:

Option #1

  1. Take an old nozzle and grind all the threads off the outside. Do not leave any sharp edges that could damage the threads inside the carburetor
  2. Put a small amount of fine lapping compound on the shoulder of the nozzle. Note: Lapping is precision mating of metal parts by placing an abrasive paste for cutting, smoothing, and finishing of metal surfaces.
  3. Using a screwdriver as a lapping tool, lap the nozzle into the carburetor before reassembly. This will remove corrosion and restore a sealing surface. Be sure to thoroughly clean the carburetor before reassembly.

Option #2. Use a part from the Briggs & Stratton carburetor repair kit #391413 for servicing a Pulsa-Jet carburetor. Using a teflon washer from the kit, force the washer over the end of the nozzle. The washer will act as a gasket, stopping any leakage between the nozzle and carburetor body.

Option #3. Replace the lower half of the carburetor or the entire carburetor assembly.

Damage at the float valve seat/bushing may be the result of abrasives, corrosion, or careless use of tools. If the condition is limited to the seat/bushing, the problem can be corrected quickly and easily. Pressed-in float valve seat/bushing are replaceable on all float-style carburetors.

Hard to restart. When the warm engine is difficult to start, the primary cause is a rich or flooded condition. If an engine has a hard hot restart symptom, first check the engine's spark plug to determine if a flooded condition exists. This is accomplished by removing the plug and observing the tip to see if it is covered with fuel. Hard hot restart can be caused by an improperly adjusted engine and/or equipment controls, or by a partially restricted air filter. Perform all initial adjustments and recheck for a hard restart condition before attempting more repairs.

A damaged adjusting needle, O-ring, or loose needle/seat will cause a rich condition, which contributes to hard hot restart problems. Inspect for missing, damaged, or loose parts.

Another critical factor is the engine's idle speed. When the engine's idle rpm is set too low, hesitation occurs during acceleration. To eliminate this situation, some mechanics make the air-fuel mixture richer, which will cure the hesitation, but also will cause a hot restart condition.

Winter-grade fuel used in warm weather may vaporize too rapidly and cause a flooding (hot restart) condition. Small Briggs/Walbro

One-Piece Flo-Jet Carburetor Diaphragm carburetor with fuel pump. The diaphragm carburetor is widely used, especially in chainsaws and cutoff saws or other tools/equipment where the engine may be used in many different positions, as shown in Figure 33. Most diaphragm carburetors have a built-in fuel pump, as illustrated in Figure  34, to ensure that the fuel supply is available to the carburetor, regardless of the position of the fuel tank.

Figure  33 - The cutoff saw and chainsaw can be used in various positions due to the diaphragm carburetor.

Figure 34 - Diaphragm carburetor with fuel pump.

With small variations to the basic overhaul technique, you can easily and correctly tune adjustable carburetors not only on the various chainsaws, but also on the other equipment within the TOA with two-stroke engines.

Fuel pump operation. The fuel pump side of the carburetor is identified by the presence of a fuel inlet fitting. When the pump cover is removed, a fuel gasket and diaphragm are found. Each revolution of the engine produces two changes in crankcase air pressure. The downward movement of the piston creates a positive pressure in the crankcase, while the upward movement creates a negative pressure (vacuum). These impulses are channeled to the fuel pump diaphragm through an impulse channel hole. The impulses actuate the diaphragm just above the fuel reservoir in the pump chamber by moving up and down to pump fuel from the tank. The pump's "one-way flap valves" work in conjunction with the crankcase pressure variations to keep the fuel moving in one direction. A negative impulse, as shown in Figure 35, brings fuel from the fuel line through valve #1 and closes valve #2. A positive impulse, as shown in Figure 36, closes valve #1 and pushes the fuel through valve #2 into the metering side of the carburetor.

Figure 35 - Negative impulse.

Figure 36 - Positive impulse.

The impulse channel hole in the carburetor may be external or internal. The external channel is connected atop the fuel pump cover, while the internal connects through the mounting against the crankcase. This hole may be plugged with foreign material or from improper gasket installation.

A surge protector is installed in many diaphragm carburetors. When the demand for fuel is low and the pressure in the fuel tank is great, the stress is relieved by the flexible part of the diaphragm expanding so that the excess pressure is relieved.

Diaphragm carburetor operation. Fuel flows from the fuel pump to the inlet needle valve. The valve opens and closes according to the movement of the metering diaphragm. The dry side (lower) is exposed to atmospheric pressure through an atmospheric vent in the bottom cover. The fuel side is influenced by the degree of vacuum in the venturi, as shown in Figure 37.

Figure 37 - Diaphragm moves upward as fuel is removed.

As the fuel is elevated into the venturi through either the main discharge tube or the idle ports, atmospheric pressure moves the diaphragm upward against the calibrated metering spring. This depresses the inlet control lever, allowing the inlet needle valve to open. Fuel can then flow from the pump side into the metering chamber and through the idle and high speed channels. The inlet needle may have a viton tip, which not only resists the effects of exotic fuels, but also is more resistant to wear. When the discharge of fuel decreases or ceases from the idle or main ports, the incoming fuel pushes the diaphragm downward against the inlet lever, and the flow of fuel is halted.

Diaphragm Engine Operation. Starting a cold engine requires a rich fuel mixture. The choke shutter is in a closed position, as shown in Figure 38, which exaggerates the vacuum in the venturi. A larger quantity of fuel is drawn in both the main discharge and the idle circuits.

When the engine is started, the choke shutter is opened to allow additional air to mix with the fuel. Fuel is drawn up the main discharge port, and the volume of fuel is controlled by the high-speed adjusting needle. Some carburetors are assembled with a fixed jet, the jet hole size is calibrated to supply the correct amount of fuel required by the engine at high-speed operation. This orifice size is critical and should not be altered in any manner. Clean only with compressed air.

When the engine is idling, the throttle plate is narrowly open. Engine suction is now permitted only through the low-speed discharge ports, and the volume is controlled by the low-speed mixture adjusting screw.

Figure 38 - Diaphragm carburetor operation.

Diaphragm Carburetor Service. To ensure long life and top performance of your tools and equipment, a regular general overhaul of the carburetor is advised. Before beginning disassembly, remove all excess dirt and debris from the carburetor. Do not use a cloth, as tiny lint particles are likely to adhere to the components and cause malfunction. Always keep a clean work surface.

Remove pump cover. Beneath the fuel pump cover is the pump gasket. Around the pump gasket there should be clean, well- defined imprints indicating properly sealed-off areas of the pump surface, such as the fuel intake chamber, pulse chamber, and the inlet and outlet valve areas, as shown in Figure 39. Any cross-leaking between these areas may cause starting and high-speed problems.

Figure 39 - Well-defined imprints indicate proper sealing.

Inspect pump valves. Check the flaps of the valves for any excessive wear, peeling, rupture, or distortion. Make certain that they rest flat against the pump surface. If one is curled, the diaphragm should be replaced.

Inspect inlet screen. The inlet screen is located in the chamber above the inlet needle on the fuel pump side of the carburetor, as shown in Figure 40. This screen filters fuel to the metering chamber. The screen mesh is very small and difficult to identify. Clogging of the screen will restrict fuel flow, affect acceleration, and impair high- speed performance. Remove this delicate mesh carefully with a sharp object and clean it thoroughly.

Figure 40 - Remove and clean inlet screen.

Metering cover and diaphragm. The vent hole is exposed to the atmosphere, where it may become clogged with dirt. Check for tearing or peeling of the gasket in this area. The diaphragm must be flexible and show no signs of deterioration.

Lever height. Lever height must be adjusted properly and the lever must move easily, as shown in Figure 41.

Figure 41 - Lever height must be adjusted.

Disassemble the inlet seat components. Hold the inlet lever down with your index finger when removing the fulcrum pin retaining screw, as there is a tension spring directly underneath. Check the lever for wear at the point of contact.

Keeping the inlet needle/fulcrum spring in place during assembly can be very tricky. A very small amount of clean, lithium-based grease can be placed in the well where the spring seats. This will hold the spring erect and lessen the chance of it dropping out.

High- and low-speed needles.

Remove the high- (Figure 42, View A) and low speed (Figure 42, View B) mixture adjustment needles and check for rusting or damaged threads. Roll them on a flat surface to inspect for a bend. A bent needle point can damage the precision-machined orifice.

Figure 42- Check needles for damage.

Remove Welch plug. This can be accomplished by using a pointed punch, as shown in Figure 43, or by drilling a hole in the center using a ¼-inch drill. When a new plug is put in, expand it with a punch.

Figure 43- Remove Welch plug to check clogged passages.

Rinsing and air cleaning. With all pump and metering components and mixture needles removed, clean the parts in a solvent. After rinsing, blow thoroughly through all the channels. Do not use drills or any hard metal objects to clean away obstructions. Soft tag wire may be used carefully.

Pressure test. When the carburetor is fully assembled, pressure test the inlet seat to detect any leaks that may remain. Connect the pressure inflator to the fuel inlet fitting and apply 5 psi. If there is no leakage around the inlet needle, this should hold steady for about 4 seconds. Depress the metering diaphragm with a pointed instrument, such as a pencil, and repeat this test.

A bow-off pressure test can also be done. Continue to pump until the inlet needle is unseated. This will usually occur between 15 to 25 psi. The dial needle will be seen to drop and should stabilize above 5 psi.

Final inspection. Check the throttle plate and choke shafts. They should open and close freely, When closed, the throttle plate should completely seal the throttle bore.

Diaphragm Carburetor Adjustments.

There are only three basic carburetor adjustments:

  1. Low-speed (L) fuel mixture needle
  2. High-speed (H) fuel mixture needle
  3. Screw for setting idle speed

Make initial adjustments before starting the engine.

Initial adjustments.

  1. Gently close the high-low speed mixture needles by turning them clockwise until they are lightly seated. The points of the needles are delicate and rest in a metered orifice. Do not over torque or damage to the needles may occur.
  2. Turn the screws outward about 1½-turns. This is usually a rich condition because most carburetors are designed for best fuel flow at about one turn open. Start turning the engine from the rich side of the adjustments.
  3. Turn the idle speed screw outward until it does not touch the idle stop. Then screw it back inward until it barely touches the throttle plate lever. At this point you can pre-adjust the screw by turning it an additional 1½-turns.

Final adjustments.

  1. Start the engine and flick the trigger a few times. If necessary, reset the idle speed so that the clutch is not engaging. Most small two-stroke cycle engines idle between 2500 and 3000 rpm.
  2. Turn the low-speed mixture needle inward about ¼-turn from the initial setting. The engine speed will increase slightly because you are going from a rich condition to a less rich, or lean, condition. If the clutch starts to engage, back off slightly on the idle speed screw. Accelerate the engine to flush out excess fuel from the crankcase and repeat, alternating from low-speed needle to idle speed screw and back again. About the third time you turn the low-speed needle, engine speed will decrease slightly. Stop at this point. You are just on the lean edge of the best idle mixture. If you squeeze the trigger, the engine will probably stumble a little during acceleration. Richen (turn counterclockwise) the low-speed mixture needle just enough to get quick, smooth acceleration. Use the idle speed screw one last time to achieve correct idle speed.
  3. The high-speed mixture needle is adjusted at wide-open throttle plate with a load on the engine. Close the needle clockwise just far enough to hear the engine increase in speed and break into the fringe of a clean, or "bumblebee," sound. This adjustment is too lean to do work. Open the needle counterclockwise far enough to hear again the rich sound of four-cycling as the speed decreases slightly. What you are doing here is tuning by ear, like a piano tuner. Remember, the fuel is the only source of lubrication for the two-stroke engine, and that rich four-cycle sound is good assurance. If you set it too rich, you will lose power and invite faster carbon buildup. The engine should break into a clean sound when you put it to work. Take caution to avoid running the engine too long at wide- open, no-load situation. You should be able to set the adjustment in about 10 to 15 seconds.


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2.1.0 Caterpillar Fuel Injection Systems

2.1.1 Sleeve Meter Fuel System

The sleeve metering system uses a sliding collar sleeve through which the stroking plunger moves. The sleeve and plunger are lapped together to make a matched set. The position of the sleeve collar controls the length of the plunger's effective stroke, thereby determining the amount of fuel delivered. A control unit or lever connected to the governor is used to control the position of the sleeve collar. Components

The main components of a fuel injection pump using the sleeve metering fuel system are the barrel, plunger, and sleeve. The plunger moves up and down inside the barrel and sleeve, as shown in Figure 44. The barrel remains stationary, but the sleeve can be moved up and down on the plunger to make a change in the amount of fuel injected.

Figure 44 - Sleeve metering barrel and plunger assembly.

Located in the inlet side of the system is a priming pump. When you open the bleed valve and operate the priming pump, air is removed from the injection pump housing filters and suction lines. Operations

When the engine is running, pressurized fuel from the transfer pump goes in the center of the plunger through the fuel inlet during the plunger's downward stroke. Fuel cannot go through the fuel outlet at this time because the fuel outlet is blocked by the control sleeve, as shown in Figure 45, View A. The action of the cam lobe lifts the plunger from its BDC position. As the plunger moves upward in the barrel, it closes the inlet port, as shown in Figure 45, View B, and injection can begin. This is because the fuel pressure above the plunger increases to the point where it opens the delivery valve and high pressure fuel can flow through the lines to the injection nozzles.

Figure 45 - Components and operation of sleeve metering system.

Injection stops when the fuel outlet is lifted above the edge of the sleeve, as shown in Figure 45, View C. This is the end of the plunger's effective stroke. With the outlet now open, pressure above the pumping plunger is lost and the fuel above and inside the plunger passes through the fuel outlet and returns to the fuel injection housing.

When the sleeve is raised on the plunger, the fuel outlet will be covered for a longer time, increasing the effective stroke and causing more fuel to be injected. If the sleeve is located low on the plunger, the fuel outlet is covered for a shorter period of time and fuel delivery is decreased. The metering sleeve allows infinitely variable amounts of fuel to be delivered to the injection nozzles.

Figure 46 illustrates a sleeve metered multiple plunger inline pump equipped with a mechanical governor. The lever of the governor is connected by linkage and governor springs to the lever sleeve control shafts. Any movement of the lever causes a change in position of the sleeve control shafts. When the lever is moved by governor action to feed more fuel to the engine, the lever acts to compress the governor springs and move the thrust collar forward. As the thrust collar moves forward, the connecting linkage will cause the sleeve control shafts to turn. The turning movement of the control shafts causes the sleeve levers to lift the sleeves, increasing the amount of fuel sent to the cylinders.

Figure 46 - Components of a multiple plunger inline fuel injection pump equipped with sleeve metering and a mechanical governor. Governor

A governor is needed to regulate the amount of fuel feed to the cylinders. The governor ensures that there is sufficient fuel delivered at idle to prevent the engine from stalling. It also cuts the fuel supply when the engine reaches its maximum rated speed. Without a governor, a diesel engine could quickly reach speeds that would destroy it. The governor is included in the design of the fuel injection pump.

The mechanical type governor shaft of the governor for the sleeve metering fuel system controls the position of the sleeve on the plunger, which regulates the amount of fuel injected. The volume of fuel injected is equal to the displacement of the plunger lift into the barrel between the start and end of injection. The start-up control sets the fuel injection pumps at full stroke to aid in starting, regardless of the throttle position. Normal governor operation takes over at low-idle speed, approximately 500 rpm.

Various applications call for different types of engine speed governing. The governing requirements for diesel engines in heavy-duty trucks differ from earthmoving equipment to the type used in stationary generators. The functions designed into a governor depend on the type of load placed on the engine and the degree of control required.

Limiting speed governors have been developed to prevent the engine from stalling at low idle speed and to prevent racing. All speeds in between can be controlled by the operator. This type of governor is generally used in on-highway vehicles.

Variable speed governors are often used on drill rigs, tractors, excavators, locomotives, and marine engines. This type of governor gives the engine automatic speed control and is easily adjusted during operation. It can control flow over a wide range, from low idle to maximum speed. For example a bulldozer may experience many changes in load per minute as it operates over varying terrain and conditions. The operator cannot anticipate these rapid changes, but a variable speed governor can adjust fuel delivery to maintain the proper engine speed during all load levels.

Constant speed governors maintain engine speed at a constant rpm, regardless of the load. These governors are generally used to control engine speed on generators and other stationary applications. Overspeed governors are designed to prevent the engine from exceeding a specified maximum speed. A load-limiting governor limits the load that the engine must handle at various speeds. The purpose is to prevent the engine overloading.

There are some terms that are integral to governors which describe their state or operation: Mechanical governors

Speed sensing is by means of rotating flyweights driven at a speed proportional to engine rpm. In a mechanical governor, centrifugal force generated by the rotating flyweight acts directly on the fuel control mechanism, see Figure 47. The governor spring tension is set to oppose the centrifugal force and will define the top engine limit or high idle speed. The thrust collar, also known as thrust washer or thrust sleeve, acts as an intermediary between the spring force and centrifugal force and is connected to the fuel control mechanism.

Figure 47 - Cutaway view of a mechanical governor.

Governor spring tension is usually designed to be variable and increases with accelerator pedal travel:

2.1.2 Mechanical Electronic Unit Injector (MEUI)

The mechanical electronic unit injector is a common unit injector with an electronic solenoid that is controlled by the ECM. Mechanical pressure is created by the camshaft moving a roller and a pushrod, and a follower pressing on top of the injector unit. The rate and amount of fuel injected into the cylinder is controlled by the opening and closing of the solenoid that is controlled by the ECM.

2.1.3 Hydraulic Electronic Unit Injection (HEUI)

The HEUI is an integral pumping, metering, and atomizing unit controlled by electronic/engine control module (ECM) switching apparatus. The unit is essentially an EUI that is actuated hydraulically rather than by cam profile. While following this description of HEUI, refer to Figure 48 C6- and Figure 49 C7. At the base of the HEUI is a hydraulically actuated, multi-orifii nozzle. When the HEUI pumping element achieves the required nozzle opening pressure value, the valve retracts, permitting fuel to pass around the nozzle and exit the nozzle orifii directly to the engine cylinder. A valve close orifice (VCO) or sacless nozzle design is used.

Figure 48 - The HEUI injection cycle.

The amplifier or intensifier piston is responsible for creating injection pressure values. This component is termed an amplifier piston in International Truck terminology and an intensifier piston in Caterpillar terminology; amplifier piston will be used herein to maintain continuity. When the HEUI is energized, high-pressure oil supplied by a stepper pump acts on the amplifier piston and drives its integral plunger downward into the pumping chamber

Figure 49 - The HEUI injection control and pulse.

A duct connects the pump chamber with the pressure chamber of the injector nozzle valve. The amount the HEUI is de-energized, the oil pressure acting on the amplifier piston collapses, and the amplifier piston return spring plus the high-pressure fuel in the pump chamber retracts the amplifier piston, causing the almost immediate collapse of the pressure holding the nozzle valve open. This collapse results in rapid ending of the injection pulse. In fact, the real time period between the moment the HEUI solenoid is de-energized and the point that droplets cease to exit, the injector nozzle orifii is claimed to be significantly less with HEUI than with equivalent EUI systems.

HEUIs typically have normal operating pressure (NOPs) of 5,000 psi with a potential for peak pressures of up to 28,000 psi attainable, depending on application. The oil pressure acting on the HEUI amplifier piston is "amplified" by seven times in the fuel pump chamber. This amplification is achieved because the sectional area of the amplifier piston is seven times that of the injection plunger, which means that the descent velocity of the injection plunger is variable and dependent on the specific actuation oil pressure value at a given moment of operation. Because the ECM directly controls the actuation oil pressure value, it can therefore control injection pressure. The injection pressure determines the emitted droplet size: the higher the injection pressure, the smaller the droplets, which is what is meant by rate-shaping ability of HEUI. In short, rate shaping allows the ECM to optimize the extent of atomization to suit the combustion conditions at any given moment of operation.

Rapid pressure collapse enabled by HEUIs avoids the injection of larger-sized droplets toward the end of injection that would be difficult to completely oxidize in the "afterburn" phase of combustion. At the completion of the HEUI duty cycle or pulse width (PW), the pressurized oil that actuated the pumping action is spilled to the rocker housing.

HEUI injectors are capable of being driven or switched at high speeds. The latest versions have plunger and barrel geometry that provide pilot injection. Pilot injection is a term used to describe an injection pulse that is broken into two separate phases. In a pilot injection fueling pulse, the initial phase injects a short duration pulse of fuel into the engine cylinder, ceases until the moment of ignition, and at that point, resumes injection, pumping the remainder of the fuel pulse into the engine cylinder. Pilot injection is used as cold-start and warm-up strategy in EUI systems to avoid excess of fuel in the engine cylinder at the point of ignition and as a result, minimize the tendency to cold- start detonation. HEUI systems with the pilot injection feature are designed to produce a pilot pulse for each injection. This has been proven to reduce both hydrocarbon (HC) and oxides of nitrogen (NOx) emissions.

HEUI Oil and Fuel Manifold. Actuation oil and fuel at charging pressure are routed to the HEUI units of an oil/fuel manifold on the cylinder head. The HEUIs are inserted into the cylindrical bores in the cylinder head, and they use a dedicated external annulus separated by O-rings to access the oil and fuel rifles.

Figure 50 shows a cylinder head cross section showing the oil/fuel manifold and the ducts that connect them to individual HEUIs.

Figure 50 - HEUI oil and fuel manifold. Heui Subcomponents

The HEUI injector assembly can be subdivided, as shown in Figure 51.

Figure 51 - International Trucks HEUI internal components and operating phases.

Solenoid. The solenoid is switched by the ECM using a 115v coil-induced voltage. The HEUI electrical terminals connect the solenoid coil with wiring to the ECM injector drivers.

Poppet Valve. The HEUI poppet valve is integral with the solenoid armature. It is machined with an upper and lower seat. For most of the cycle, the poppet valve seat loads the lower seat into a closed position, preventing high pressure oil from entering the HEUI. The upper seat is open, venting the oil actuation spill ducting. When the HEUI solenoid is energized, the poppet valve is drawn into the solenoid, opening the lower seat and admitting high-pressure oil from the injection pressure regulator (IPR). When the poppet valve is fully open, the upper seat seals, preventing the oil from exiting the HEUI through the spill passage.

Intensifier or Amplifier Piston. The intensifier/amplifier piston is designed to actuate the injection plunger, which is located below it. When the poppet control valve is switched by the ECM to admit high-pressure oil into the HEUI, the oil pressure acts on the sectional area of the amplifier piston. The actual oil pressure (managed by the ECM) determines the velocity at which the plunger located below the amplifier piston is driven into the injection pump chamber. The sectional area of the amplifier piston determines how much the actuating oil pressure is multiplied in the injection pump chamber. This value is specified as seven times in current HEUI systems. In other words, an actuating oil pressure of 3,000 psi would produce an injection pressure potential of 21,000 psi. The amplifier piston and injection plunger are loaded into their retracted position by a spring.

Plunger and Barrel. The plunger and barrel form the HEUI pump element. The first versions of the HEUI injectors did not offer the pilot injection feature. This description will use the recently introduced HEUI with the PRIME feature. The injection cycle shown in Figure 48 shows a HEUI with the PRIME feature. The acronym PRIME is produced from the words pre-injection metering. As the injection plunger is driven into pump chamber, fuel is pressurized for a short portion of the stroke, actuating and opening the injector nozzle. The pressure rise is of short duration because when the PRIME recess in the plunger registers with the PRIME spill port in the barrel, the pressure in the pump chamber collapses as fuel spills through the PRIME spill port. This closes the injector nozzle and injection ceases. However, the moment the PRIME recess in the plunger passes beyond the spill port, fuel is once again trapped in the pump chamber and pressure rise resumes. This results in the injector being opened for the delivery of the main portion of the fuel pulse. The fueling pulse continues until the ECM ends the effective stroke by de-energizing the HEUI solenoid. At this point, the poppet control valve is driven onto its lower seat, opening the upper seat and permitting the actuating oil to be vented. With no force acting on the amplifier piston, the plunger is driven upward by the combined force of the high-pressure fuel in the pump chamber and the plunger return spring. This causes an almost immediate collapse of pump chamber pressure and results in almost instantaneous nozzle closure. A feature of the HEUI is its ability to almost instantly effect nozzle closure at the end of the plunger effective stroke.

Injector Nozzle. The HEUI injector nozzle is a multi-orifii injector nozzle of the valve closes orifice (VCO) type that is a little different from any other injector nozzle used in a mechanical unit injector (MUI) or electronic unit injector (EUI) assembly. A duct connects the nozzle pressure chamber with the HEUI pump chamber. A spring loads the injector nozzle valve onto its seat. The spring tension defines the nozzle opening pressure (NOP) value. When the hydraulic pressure acting on the sectional area of the nozzle valve is sufficient to overcome the spring pressure, the nozzle valve unseats, permitting fuel to pass around the nozzle seat and through the nozzle orifice. The nozzle valve functions as a simple hydraulic switch. Because of the nozzle differential ratio, the nozzle closure pressure is always lower than the NOP. For instance, a Caterpillar version of the HEUI with a NOP identified at 4,500 psi will not close until the pressure drops to 4,000 psi. Stages of Injection

When the newer PRIME HEUIs are used, the injection pulse can be divided into five distinct stages. Refer back to Figure 48 and Figure 49 to follow the stages of injection.

Preinjection. The HEUI internal components are all located in their retracted positions, as shown in Figure 52. In fact, they are in the preinjection position for most of the cycle.

Figure 52 - Caterpillar HEUI preinjection cycle: subcomponent identification.

The poppet valve seat is spring loaded into the lower seat, preventing the high-pressure actuating oil from entering the HEUI, and the amplifier piston and plunger are both in their raise position. Fuel enters the HEUI to charge the pump chamber at the charging pressure value, as shown in Figure 53.

Figure 53 - Caterpillar HEUI fill cycle.

Pilot Injection. The pilot injection phase begins when the plunger is first removed into the HEUI pump chamber. The pressure rise created opens the injector nozzle to deliver a short pulse of fuel. The pilot injection phase ends when the PRIME recess in the HEUI plunger is driven downward enough to register with the PRIME spill port, causing the pump chamber pressure to collapse and the nozzle valve to close.

Delay. The delay phase occurs between the ending of the pilot injection phase and restart of the fuel pulse. The objective is to cease fueling the engine cylinder while the prime pulse of the fuel is vaporized and heated to its ignition point. It is important to note that the plunger is still being driven through its stroke during this phase because the HEUI poppet control valve is in the open position, and oil pressure continues to drive the amplifier piston downward.

Main Injection. When the PRIME recess in the plunger passes beyond the PRIME spill port, fuel is once again trapped in the HEUI pump chamber because it can no longer exit through the spill port. The resulting pressure rise opens the injector nozzle a second time to deliver the main volume of fuel to be delivered. In an HEUI injector with no PRIME feature, the plunger has no cross and center drillings and PRIME recess, so main injection begins when the plunger leading edge passes the spill port on its downward stroke, as shown in Figure 54.

Figure 54 - Caterpillar HEUI effective stroke.

End of Injection. The end of injection begins with the de- energizing of the HEUI solenoid. The armature is released by the solenoid coil and a spring drives the poppet valve downward to seat on its lower seat. The instant the poppet valve starts to move downward, the upper seat is exposed, permitting the actuating oil inside the HEUI to spill. When the actuating oil pressure acting on the amplifier piston is relieved, the fuel pressure in the HEUI pump chamber combined with the plunger return spring collapses the fuel pressure almost instantly. Injection ends when there is insufficient pressure to hold the nozzle valve in its position, and the three moving assemblies (poppet valve, amplifier/plunger, and nozzle valve) in the HEUI are all in their return positions outlined in the preinjection phase, as shown in Figure 55.

Figure 55 - Caterpillar HEUI end of injection. ECM Functions

The ECM has four functions:

  1. Reference voltage regulator
  2. Input conditioning
  3. Microcomputer
  4. Outputs

Reference Voltage. Reference voltage (V-Ref) is delivered to system sensors that divide this input and return a percentage of it as a signal to the ECM. Thermistors (temperature sensors) and potentiometers (TPS) are examples of sensors requiring reference voltage. Reference voltage values use area at 5V pressure and the flow is limited by a current-limiting resistor to safeguard against a dead short ground. Reference voltage is also used to power up the circuitry in Hall effect sensors used in the system, such as the camshaft position sensor (CPS).

Input Conditioning. Signal conditioning consists of converting analog signals to digital signals, squaring-up sine wave signals, and amplifying low intensity signals for processing.

Microcomputer. Both Caterpillar and International Trucks HEUI microprocessors function similarly to other vehicle system management computers. It stores operating instructions control strategies and tables of values and calibration parameters. It compares sensor monitoring and command inputs with the logged control strategies and calibration parameters and then computes the appropriate operating strategy for any given set of conditions. The current Caterpillar ECM, the advanced diesel engine management (ADEM IV) uses a 32-bit processor and a clock speed of 24 mHz. As with your home computing system, you can expect the computing power on truck engine management systems to increase as each year passes.

ECM computations occur at two different speeds, referred to as foreground and background calculations.

The difference in foreground and background computations is simply speed at which the microprocessor is required to react to a change in operating characteristics. A change in throttle position requires an immediate adjustment in engine fueling, and therefore, this command input requires almost instant response by the ECM.

However, while an increase in coolant temperature could have serious consequences if ignored, engine overheat conditions occur gradually, so an almost instant reaction by the ECM is not required.

Diagnostic strategies include monitoring input data on a continuous basis and flagging codes when an abnormal operating parameter is detected. Calibration tables and operating strategies are retained in read-only memory (ROM). This data is not lost by opening the ignition circuit or disconnecting the vehicle batteries as it is magnetically retained. Random-access memory (RAM) data is electronically retained and thus is only retained while a circuit is energized.

HEUI system ECM RAM stores information sourced from electronic monitoring and data processing/manipulation, which is volatile and as such, is dumped each time the ignition circuit is opened. When KAM is used, it is nonvolatile RAM and functions to log fault codes. Out-of-normal parameters may also result in adaptive strategies being written into KAM; subsystem failure or component wear are examples. KAM data is retained when the ignition circuit is opened but dumped when the vehicle batteries are disconnected. Both Caterpillar and International Trucks PRIME are described as vehicle personality module (VPM) and are both customer and proprietary data programmable. The function of the VPM is to trim engine management to a specific chassis application and customer requirements. The engine family rating code (EFRC) is located in the VPM calibration list and can be read with an electronic service tool (EST), which identifies the engine power and emission calibration of the engine.

Outputs. The switching apparatus within the ECM can be referred to as actuator control. The ECM controls the system actuators by delivering a signal to the base of the transistor output drivers. These drivers, when switched, ground the various actuators circuits. The actuators may be controlled through a duty cycle (that is, percent time on/off), controlled by modulating pulse width or simply switched on and off, depending on the actuator type. Injection Driver Module (IDM)

The injector driver module is responsible for switching the HEUIs. In older International Trucks, the IDM was housed separately from the ECM housing. Currently, International along with Caterpillar integrates all the engine modules into a single housing.

The IDM has four functions:

  1. Electronic distributor for the HEUIs
  2. Powers the HEUIs
  3. Output driver for the HEUIs
  4. IDM and HEUI diagnostics

Electronic Distributor for the HEUIs

The ECM determines engine position from the camshaft position (CMP) sensor located at the engine front cover. The ECM uses the signal to determine cylinder firing sequence and then delivers this command data to the IDM as a fuel demand command signal (FDCS). FDCS contains injection timing and fuel quantity data. Figure 56 shows the relationship between the IDM and ECM and the FDCS.

Figure 56 - Injection Driver Module (IDM) operation.

Power the HEUIs. The IDM supplies a constant 115v DC supply to each HEUI. This 115v DC supply is created in the IDM by making and breaking a 12v source across a coil using the same principles employed by the ignition coil in a spark ignited (SI) engine. The resultant 115v induced is stored in capacitors until discharged to the HEUIs, as indicated in Figure 57.

Figure 57 - IDM: power source.

Output Driver for the HEUIs. The IDM is responsible for switching the HEUIs. The unit controls the effective stroke of the HEUI by closing the circuit to ground. The direct control of the HEUI is managed by an output driver transistor in the IDM. When the FDCS signal is delivered from ECM processing cycle, the beginning of injection (timing) and fuel quantity is determined. Figure 58 shows the role of the output drivers and Figure 59 shows the role of the timing sensor.

Figure 58 - Injector driver module operation: output driver operation.

Figure 59 - IDM and ECM communications signals and relationship

IDM and HEUI Diagnostics. The ECM software is capable of identifying faults within its electronic circuitry and can determine whether an HEUI solenoid or its wiring circuit is drawing too much or too little current. In the event of such an electronic malfunction, a fault code is logged. The self- diagnostics can also set an ECM code, indicating that the module has failed and requires replacement; see Figure 60.

Figure 60 - IDM diagnostic operation. Injection Actuation Pressure

The ECM is responsible for marinating the correct injection actuation pressure during operation. This means monitoring and adjusting the high-pressure oil circuit responsible for actuating HEUI fueling. It does this by comparing actual injection pressure with desired injection pressure and using the injection actuation pressure solenoid valve to attempt to keep the two values equal. Actual injection pressure is signaled to the ECM by the injection actuation pressure sensor. Desired injection actuation pressure is based on the fueling algorithm effective at any given moment of operation. In processing, the ECM will evaluate any differential between actual and desired actuation pressures and will modulate an output signal to the injection actuation control solenoid valve to keep the values close.

Figure 61 shows a Caterpillar HEUI schematic. Note the location of the high-pressure pump, injection actuation pressure (IAP) valve, and the IAP sensor a little downstream from the IAP control valve solenoid.

Figure 61 - Caterpillar HEUI schematic. Caterpillar Electronic Technician (ET)

Electronic Technician (ET) is the Windows platform software used to read, program, and troubleshoot Caterpillar advanced diesel engine management (ADEM) systems. The software makes full use of Windows graphics, and user-friendly instructions take the technician from step to step in programming and diagnostic procedures. ET will do the following on a Caterpillar HEUI system:

Two ET tests are distinct to HEUI systems as follows:

Injector Cutout Test. Unlike the cylinder cutout test on Caterpillar EUI systems in which the ability to cut between one and five cylinders during the test produces a comprehensive cylinder balance analysis, currently HEUI cylinder cutout is one cylinder at a time. The test sequence must be performed with the engine at operating temperature and with intermittent loads, such as A/C disconnected.

Test sequence:

  1. Governor maintains programmed idle speed.
  2. ET turns off one injector at a time, this means the functioning five HEUIs must increase their duty cycle if the specified engine rpm is to be maintained.
  3. The ADEM ECM measures the average duty cycle of the five functioning HEUIs at each stage of the cutout test.
  4. The ECM assigns a test value for each HEUI tested.
  5. The cutout test cycle is then repeated.

Injection Actuation Pressure Test. The injection actuation pressure (IAP) test checks high-pressure oil pump and IAP valve operation. The test is always performed at low idle. It functions by having ET read desired IAP pressure versus actual IAP pressure using ADEM ECM data. The IAP test sequence uses four desired pressure values:

  1. 870 psi
  2. 1450 psi
  3. 2100 psi
  4. 3300 psi

The IAP test sequence compares these with actual IAP values read by the IAP pressure sensor. International HEUI Diagnosis

The International Trucks electronic engine PC software, known as EZ-Tech, is used to read, diagnose, and program customer data to HEUI engines. Proprietary data programming can be effected by modem to the International Trucks data hub, known as TechCentral and vehicle electronic programming software (VEPS).

International Trucks' Self-Test. International Trucks' HEUI electronics are capable of performing self-test procedures. When the dash-mounted self-test input (STI) button is depressed and the ignition circuit is closed, the ECM begins the self-test cycle. When complete, the oil/water and warn engine dash lights are used to signal fault codes. All International Trucks fault codes are three digits. Following is the sequence:

  1. Oil/water light flashes once, indicating the beginning of the active faults display.
  2. Warn engine light flashes each digit of the active fault code, pausing between each. The oil/water light will flash once between each active code readout when multiple codes are logged. Code 111 indicates no active fault codes.
  3. Oil/water flashes twice to indicate that inactive fault codes will be displayed. Inactive fault codes are then blinked out in the same manner as active fault codes.
  4. When all codes have been displayed, the oil/water light will flash three times.
  5. Test sequence can be repeated by retracing the preceding steps, but all active faults should be repaired before progressing to later tests.

International Trucks' HEUI, like all electronically managed systems, requires that troubleshooting strategies be sequentially and scrupulously followed using the correct instrumentation and tooling as follows:

2.2.0 Stanadyne Fuel Systems

The original distributor injection pump, the Roosa Master, was an opposed inlet metered pump. This type of injection pump used only one metering valve to control the fuel and either two or four opposed plungers to pump the fuel. One component, the distributor rotor, is used to distribute the metered fuel out through the hydraulic head to the injectors. These pumps have a fuel delivery capacity of engines rated between 10 - 40 hp per cylinder.

Over the years, the Seabees have been associated with the "Roosa Master Fuel Injection Pump" on the diesel engine inventory. The original Roosa Master Company is now the Diesel Systems Division of Stanadyne Automotive Corporation. Stanadyne DB4 and DM4 (four-plunger) and DB2 and DM2 (two plunger) distributor pumps are highly refined versions of the Roosa Master pump that reflect almost 40 years of design evolution and improvement.

Construction. The Stanadyne pump, shown in Figure 62 incorporates four pumping plungers. The driveshaft engages the distributor rotor in the hydraulic head. The rotor holds the four pumping plungers. The plungers are actuated simultaneously toward each other by an internal cam ring through rollers and shoes located in slots at the end of the rotor. The number of lobes normally equals the number of engine cylinders.

Figure 62 - The Stanadyne DB4 injection pump uses four opposed plungers.

The transfer pump is also a positive displacement vane type. It is enclosed in the end cap, which also houses the fuel inlet strainer and transfer pump pressure regulator. The distributor rotor incorporates two charging ports and a single axial bore. One discharge port serves all the outlet ports to the injection lines. The hydraulic head contains the bore in which the rotor revolves, the metering valve bore, the charging ports, and the head discharge fittings. The high pressure injection lines to the nozzles are fastened to these discharge fittings.

Stanadyne pumps have their own mechanical governor. The centrifugal force of the weights in their retainer is transmitted through a sleeve to the governor arm and to the metering valve. The metering valve can be closed to shut off fuel by an independently operated shut-off lever. The automatic speed advance is a hydraulic mechanism that advances or retards the beginning of fuel delivery. This can respond to speed alone or to a combination of speed and load changes.

Components. The aluminum alloy pump housing of an opposed plunger distributor injection pump contains the driveshaft, distributor rotor, transfer pump blades, pumping plungers, internal cam ring, hydraulic head, end plate, adjusting plates, transfer pump, pressure regulator assembly, governor, automatic advance, and metering valve.

Driveshaft. The driveshaft connects to the engine drive gear and is supported by a bushing or ball bearing. It supports the governor assembly and drives the distributor rotor and transfer pump. The transfer pump consists of four linear blades. It delivers fuel to the metering valve located in the hydraulic head at low pressure. It also provides a fuel inlet to the pump and contains a pressure regulating valve that controls the transfer pump pressure throughout the speed range.

Hydraulic Head. The hydraulic head is machined with bores and passages that allow fuel to flow from the transfer pump to the metering valve, from the metering valve to the charging ports, and from the discharging ports to the discharging fittings. On the latest designs, hydraulic heads have been fitted with individual delivery valves to maintain residual line pressure and eliminate secondary injection.

Distributor Rotor. The distributor rotor is lapped fitted to the hydraulic head and the governor weight retainer assembly is fastened to its drive end. The plungers are fitted to the rotor and are pushed inward by the rollers and shoes to pump the diesel fuel. The rollers fit into the shoes and contact the cam in a way similar to a cam follower, as shown in Figure 63. Adjusting plates are mounted on the rotor and limit the outward travel of the rollers and shoes to control the fuel delivery.

Figure 63 - Exploded view of a rotor assembly showing the cam rollers and shoes.

Cam Ring and Metering Valve. A circular cam ring surrounds the rotor base and is located over the shoes and rollers. The number of internal cam lobes equals the number of cylinders. The cam ring forces the plungers toward each other, which causes the fuel to be pumped. It can also be rotated back and forth about the rotor to vary the start of injection.

The metering valve contained in the hydraulic head regulates the volume of fuel entering the rotor. A piston valve is used with hydraulic governors. This valve is spring- loaded and controls the fuel according to the valve's axial position. When a mechanical governor is used, the valve is a rotary type, with a slot cut in the periphery. The valve is rotated by the governor arm to regulate fuel injection.

Automatic Advance and Governor. An automatic advance device is located I the bottom of the pump. A hydraulic piston rotates the cam ring against the direction of pump rotation via the cam advance stud. The cam advance stud threads into the cam and connects it to the cam advance mechanism.

The governor weight retainer may be permanently fixed, splined, or bolted to the rotor drive end. Because the fuel metering mechanism can be affected by vibrations and shocks, the retainer often uses a cushioning device to isolate engine vibration and pulsation from the driveshaft. One end of the governor control arm rests against the thrust sleeve, and the other end connects to the governor spring and to the metering valve via a linkage hook. The control lever is connected to the shut-off lever and the fulcrum lever is connected to the governor spring.

Pump Operation and Fuel Flow. The operating principles of an opposed plunger pump can be understood more readily by following the fuel circuit during a complete pump cycle. Figure 64 illustrates the fuel flow for a Stanadyne DB2 two-plunger distributor pump. The fuel flow for the DB4 four-plunger pump is the same with the exception of the charging of two additional plungers. As shown in the diagram, the transfer pump pulls fuel from the fuel tank. The fuel passes through a water separator and secondary fuel filter before reaching the transfer pump. Once through the transfer pump, some of the fuel is bypassed to the transfer pump's suction side through the pressure regulator assembly.

Figure 64 - Fuel flow during the pumping cycle in a Stanadyne DB2 distributor injection pump.

Fuel under pressure flows past the rotor retainers and into an annulus on the distributor pump rotor. Some fuel flows through a connecting passage in the head to the automatic advance mechanism. The remaining fuel moves into the charging passage. This fuel flows around the annulus, through a connecting passage, and to the metering valve.

The radial position of the metering valve regulates the fuel flow into the charging annulus, which holds the charging ports.

Pressure Regulating Valve Operation. The pressure regulating valve is located in the end plate and performs two important functions. When the injection pump is being primed, fuel is forced into the inlet connection through the mesh filter. Fuel enters the regulating sleeve located at the upper port, forcing the regulating piston downward and compressing the priming spring. When the piston has moved down far enough to uncover the lower port in the sleeve, the fuel flows directly into the hydraulic head. The pump is now primed and ready for start-up.

When the engine is running, the pump rotates and fuel is pulled into the end plate by the transfer pump. It passes through the mesh filter and is forced into the hydraulic head and end plate. When the transfer pump builds pressure, it forces the piston upward against the regulating spring (Figure 65). When the correct pressure is reached, the piston uncovers the regulating port. This bypasses a small amount of fuel back to the inlet side of the transfer pump to maintain fuel pressure at the desired level.

Figure 65 - Pressure regulating valve operation.

Transfer pump pressure can be adjusted in one of two ways. On some pumps, the spring guide is replaced with one of a different size. This changes the fuel pressure by altering the amount the regulating spring can be compressed. Other models are equipped with an adjustment device that can be set using a special tool when the pump is running on a test bench.

Charging Cycle. As the rotor revolves, the two inlet passages align with the charging ports in the annulus. Fuel under pressure from the transfer pump and controlled by the metering valve flows into the pumping chamber, forcing the plungers apart.

The plungers move outward a distance proportional to the amount of fuel required for injection on the following stroke. If a small quantity of fuel is admitted into the pumping chamber, the plungers move out a short distance. Maximum fuel delivery is limited by a leaf spring or springs that contact the edge of the roller shoes.

During the charging phase of injection, the angled inlet passages in the rotor are in alignment with the ports in the charging annulus. The rotor discharge port is not in alignment with a head outlet, as shown in Figure 66. The rollers are also off of the cam ring lobes.

Figure 66 -Fuel flow during the opposed plunger pump's charging cycle.

Discharging Cycle. As the rotor continues to revolve, as shown in Figure 67, the inlet passages move out of alignment with the charging ports. The rotor discharge port opens to one of the head outlets. The rollers then contact the cam lobes, and injection begins.

Figure 67 -Cutaway showing the opposed plunger pump's discharge cycle.

Further rotation of the motor moves the rollers up the ramps, pushing the plungers inward.

During this stroke, the fuel trapped between the plungers flows through the rotor's axial passage and discharge port to the injection line. Delivery to the injection line continues until the rollers move past the innermost point on the cam lobe and begin to move outward. The pressure in the axial passage is then reduced, allowing the nozzle to close and ending injection.

Delivery Valve Operation. On some distributor pumps, individual delivery valves (sometimes called pressure valves) are installed in the hydraulic head outlets for each cylinder. In other pump models, such as Stanadyne's, a single delivery valve mounted in a bore in the center of the distributor rotor serves all injection lines. The delivery valve or valves keep the lines full of fuel so that a full charge of fuel can be injected at the next cycle for that cylinder.

In addition, the delivery valve rapidly decreases injection line pressure to lower than nozzle closing pressure. This allows the nozzle to snap shut quickly without nozzle dripping or dribble that could cause excessive exhaust smoke.

Lubrication Circuit. The pump is located with diesel fuel supplied by the transfer pump. A lubrication groove runs from the annular ring to the front of the rotor. From this point, the lubricating fuel flows to the main pump housing. At the top front of this housing, a return fitting allows fuel to return to the fuel tank, as shown in Figure 68. This fuel lubricates all governor components. It also bleeds any air that may have entered the fuel system.

Figure 68 - The oil return circuit allows excess oil in the distributor pump to return to the fuel tank.

Advance Mechanism Operation. The opposed plunger distributor pump design permits the use of a direct- acting hydraulic advance mechanism powered by pressure from the fuel transfer pump. The pressure is used to rotate the cam and vary delivery timing. The advance mechanism advances or retards the start of fuel delivery in response to engine speed changes.

Fuel from the transfer pump enters the hollow hydraulic head locating screw. Fuel then flows to the advance piston, as shown in Figure 69.

Figure 69 - Operation of the opposed plunger pump automatic mechanism.

The piston advances the injection timing by pushing the advance cam screw to the cam ring, forcing the cam ring from retarding by holding the fuel pressure in the chamber. When fuel pressure decreases because of a reduction in engine speed, fuel from the position area drains through the orifice below the ball check valve to allow the cam ring to retard. At engine idle speed, transfer pump fuel pressure is low, and the cam ring is held in the retard position by the spring and roller force. Maximum cam ring movement is limited by the piston length and the size of the piston hole plugs. A trimmer screw is provided to adjust advance spring preload, which controls the start of the cam movement.

Mechanical Governor Operation. The opposed plunger distributor pump can be equipped with either a mechanical or hydraulic governor. The mechanical governor is shown in Figure 70. The flyweights transmit force through the thrust sleeve, causing the governor lever to pivot. This pivoting movement rotates the metering valve, which reduces or increases the amount of fuel fed to the pumping cylinder. The flyweight force is opposed by the main governor spring and the idle spring. These forces balance each other to maintain a set engine speed. When needed, a mechanical shutoff bar rotates the metering valve to the shutoff position, regardless of the engine speed.

Figure 70 - Components of a mechanical governor.

At idle, the flyweights offer little force against the governor lever. However, the main governor spring is slack, so the weights fly outward, causing the metering valve to rotate to the idle position. Within the idling speed range, the idle spring provides sensitive speed control.

At full load speed, the throttle applies maximum spring pressure on the lever. However, because of the load applied to the engine, it can only run fast enough to balance heavy spring load with the flyweight force. The metering valve rotates to allow the maximum amount of fuel to enter the rotor.

If the load is suddenly removed from the engine, the engine speeds up. The increased force of the flyweights overcomes the spring pressure and rotates the metering valve to the closed position. Once the valve closes completely, governor cutoff occurs, limiting maximum engine speed to safe levels. The engine then slows down until the flyweights and spring reach a balance that allows only a small amount of fuel into the rotor. This condition is known as high idle speed.

Electrical Shut-off. The governor may be equipped with an electrical shut-off device housed within the governor control cover. The device is available for 12-, 24-, and 32- volt systems. The device can either be energized-to-shut off (ETSO) or energized-to-run (ETR).

Figure 71 - Electrical solenoid activated fuel shut-off mechanisms. A-Energized-to shut-off. B-Energized-to-run.

Troubleshooting. A field test on an engine is an efficient way to pinpoint the cause of poor engine performance. This test will eliminate unnecessary fuel injection pump removal. Since this field test permits some analysis of engine condition as well as the fuel system, you will quickly see the extent of the difficulty and the required remedies.

Since most tests are more conveniently made under no load conditions, all possible readings are determined at high idle. If the supply pressure is lower than normal, an engine can still operate smoothly at approximately the correct high- idle speed. The governor opens the metering valve further to make up for the lower pressure; therefore, you can take successful readings at high idle.

First, disconnect the throttle linkage. Then, with the engine running, hold the throttle lever all the way to the rear. Adjust the high-idle stop screw until the specified high-idle speed is obtained to test the fuel pressure at high idle. Install the gauge assembly in the pressure trap of the transfer pump, as shown in Figure 72. If this reading does not fall within the prescribed range, the pump will not deliver sufficient fuel to obtain full power under load. The most common causes of low pressure are restricted fuel supply, air leaks on the suction side of the pump, worn transfer pump blades, or a malfunctioning regulator valve.

Figure 72 Gauge installed for checking transfer pump pressure.

To test for excessive pressure, remove the injection fuel pump timing plate, as shown in Figure 73. Be sure you make a small hole in the timing plate gasket as you install the gauge on the pump. This hole allows pump pressure to reach the gauge as you operate the engine at both low and high idle. If the pressure is excessive, a restricted fuel return line is the probable cause. To test for restricted fuel supply on the suction side of the pump, operate the engine at high idle and read the vacuum developed. If the vacuum reading exceeds 10 inches mercury (Hg), check the fuel supply system for dirty filters, pinched or collapsed hoses, or a plugged vent.

Figure 73 - Testing pump housing pressure.

Removal. If you find after field testing that you must remove the injection fuel pump from the engine, be sure to remove all external grease and dirt. Remember that dirt, dust, and other foreign matter are the greatest enemies of the injection fuel pump. As a precaution, keep all openings plugged during removal and disassembly.

Disassembly. The workbench, surrounding area, and tools must be clean. You should have a clean pan available in which to put parts as you disassemble the pump. You also need a pan of clean diesel fuel oil in which the parts can be washed and cleaned. After mounting the pump in a holding fixture, clamp the fixture in a vise. Now you are ready to disassemble the pump. Follow the step-by-step procedure in the manual for the model pump on which you are working.

Cleaning, Inspecting, and Reassembly. Now that you have disassembled the pump and inspected all the parts carefully, replace all O rings, seals, and gaskets, and inspect all springs for wear or distortion. Clean and carefully check all bores, grooves, and seal seats for damage of any kind. Replace damaged parts as necessary.

Also, inspect each part of the injection pump for excessive wear, rust, nicks, chipping, scratches, cracks, or distortion. Replace any defective parts.

When you have finished cleaning and inspecting the pump, reassemble it. Follow the steps specified by the manufacture's maintenance and repair manual.

2.2.1 Electronic Unit Injection (EUI)

Electronic unit injectors have proven to be the most readily adaptable of all the fuel injection systems to electronic control. The spray-in pressures and fine atomization possible with unit injection systems, coupled with the system's compatibility to electronic control, has led many manufacturers to switch to unit injection. To better understand how an electronic unit injector operates, Figure 74 illustrates a unit injection system.

Figure 74 -Example fuel system layout.

The Delphi EUIs used by Caterpillar have changed little since their introduction in the late 1980s. Figure 75 shows a sectional view of the Caterpillar Delphi EUI with the subcomponents labeled.

Figure 75 - Sectional view of a Delphi EUI and internal components.

  1. Terminal. Connects to the injector drivers in the ECM.
  2. Control cartridge. Solenoid consisting of a coil and armature with an integral poppet control valve. A spring loads the armature open. Energizing the solenoid closes the armature/poppet control valve.
  3. EUI tappet spring. Loads the EUI tappet upward. This enables the tappet/plunger actuation train to ride the cam profile and retract the tappet after a mechanical stroke.
  4. Poppet control valve. A valve integral with the solenoid armature. In Figure 75 the control valve is shown closed (solenoid energized): this prevents fuel from exiting through the spill port, trapping it in the EUI circuitry, enabling effective stroke.
  5. Plunger. The reciprocating member of the pump element, the plunger, is lugged to the tappet, so it reciprocates with it. In Figure 75, the plunger is shown in its upward position.
  6. Barrel. The stationary member of the EUI pumping element containing the fill port, pump chamber, and duct connecting the pump chamber with the injection nozzle.
  7. Upper O-ring. Seals the upper fuel charging gallery.
  8. Lower O-ring. Seals the lower fuel charging gallery.
  9. Nozzle spring. Defines the nozzle opening pressure value, typically around 5,000 psi. Nozzle opening pressure is initial and rebuild set by shims acting on the nozzle spring.
  10. Spacer or shims. Define nozzle spring tension and therefore the specific Nozzle opening pressure value.
  11. Upper nozzle assembly body. Machined with the ducting that feeds fuel to the pressure chamber of the nozzle valve.
  12. Nozzle valve. The nozzle assembly is a hydraulically actuated, multi-orifii, valve closes orifice (VCO), Figure 76 and Figure 77.

Figure 76 - External view of a multi-orifice injector nozzle.

Figure 77 - Sectional view of a valve closes orifice (VCO) nozzle. Electronic Unit Injector Operation

Fuel enters the injector through two filter screens. Fuel not used for injection cools and lubricates the injector before exiting through the return port and returning to the supply tank, as shown in Figure 78.

Figure 78 -Fuel inlet and return holes in an electronically controlled unit injector.

Figure 79 - Example of an engine electronically controlled/mechanically actuated unit injector.

Figure 79 illustrates the actuator components of the unit injector. The electronic unit injection system uses mechanical action to create the pressures needed for injection. As in the mechanical unit injection system, the camshaft pivots the rocker arm through its roller follower.

This forces the injector follower down against its external return spring. This action raises the trapped fuel to a pressure sufficient to lift the injector needle valve off its seat. However, fuel metering is electronically controlled by the ECM based on input signals from various sensors.

2.2.2 Fuel Flow

Fuel flow through the electronic unit injector is illustrated in Figure 80 and Figure 81.

Figure 80 - Cutaway view of a fuel flow unit injector.

Figure 81 - Caterpillar EUI injection cycle.

Once fuel enters the electronic unit injector, it passes through the inlet filters and flows through a drilled passage to an electronically controlled poppet valve. At this point in the injection cycle, poppet valve is held open by spring pressure. Fuel flows through the plunger and bushing into the fuel supply chamber. With the poppet valve open, fuel simply fills the injector.

When the piston is approximately 60° BTDC on its compression stroke, the camshaft begins to lift the injector rocker arm roller follower. The EUI internal plunger moves on its bushing, increasing fuel pressure. However, no increase above fuel pump pressure is possible until the ECM sends out a voltage signal to the electronic distributor unit. The electronic distributor unit (EDU) handles the high current needed to activate the injector solenoid.

When the solenoid on the EUI is energized, its armature is pulled upward, closing the poppet valve and descending the plunger. This creates a rapid rise in the pressure within the fuel supply chamber that leads to the spray-tip assembly.

A small check valve is located between the plunger base and the spray tip prevents combustion gas blowby from leaking into the injector. During normal operation, the fuel pressure below the plunger increases until it is powerful enough to lift the needle valve from its seat. The strength of the needle valve spring determines when the valve will lift off its seat. Opening pressures of 2800-3200 psi are common. When the needle valve unseats, fuel flows through the orifices in the injector tip. Forcing the fuel through these small openings increases the pressure to approximately 20,000 psi.

The start and duration of injection are controlled by the pulse width signal from the ECM. The longer the EUI solenoid is energized, the longer the poppet valve remains closed and the greater amount of fuel injected. Holding the poppet valve closed sets the injector plunger effective stroke. The plunger always moves down the same distance on every injection stroke, but the length of time the fuel is pressurized beneath the plunger is controlled by the solenoid.

When the ECM de-energizes the EUI solenoid, spring pressure opens the poppet valve. High-pressure fuel now flows through the small return passage in the injector body. Pressure is lost, and the force of the needle valve return spring forces the needle valve onto its seat. This results in a clean, quick end to injection. Fuel at pump pressure immediately flows into the EUI through the open poppet valve. The plunger continues to the bottom of its stroke, but it is pushing fuel through the injector tip at high pressure. When the plunger completes its downward stroke, the follower return spring pulls it up to its original position. The injector is now in position to begin the injection cycle again.

2.2.3 Stanadyne Electronically Controlled Distribution Pump

The Stanadyne electronically controlled distributor pump shown in Figure 82 regulates fuel quantity with a solenoid valve that controls the amount of low pressure fuel entering the high pressure pumping chamber. The solenoid is not pulsed. It is either fully opened or fully closed. The solenoid driver mounted on the side of the pump housing operates the solenoid on command from the ECM. The driver senses when the solenoid is fully closed to tell the ECM when injection has ended.

Figure 82 - Example of an electronically controlled distributor fuel injection pump.

An acceleration pedal position (APP) sensor supplies data on pedal position and movement to the ECM. It then operates the fuel solenoid accordingly. The pedal position sensor contains three separate potentiometers, each with its own 5v reference and district return signals. This triple redundancy helps ensure a signal is delivered to the ECM. If one or two sensors fail, the engine will run only at limited power. If all three fail, the engine will run only at idle. If an APP sensor fails, the ECM will log a trouble code into memory and turn on the Service Throttle Soon light. Each APP signal can be checked using a scan tool or an oscilloscope.

An optical/temperature sensor mounts on the pump itself. It consists of a thermistor-type fuel temperature sensor and two optical position pick-ups that share a housing and a 5v reference signal. The optical pick-ups read the position of tone wheels rotating with the cam ring inside the pump. One pick-up provides a high resolution signal, generating per cylinder firing stroke.

Combined with fuel temperature and crankshaft position data, this extremely fine position signal makes it possible for the ECM to trim the fuel quantity for each individual combustion stroke.

The sensor's second pick-up has eight slots, and reports pump cam position to locate the start of injection for each cylinder and to index cylinder number1. Combined with the crankshaft position signal, this information is used for pump timing, idle speed, and other powertrain control events. Additional inputs include coolant and intake air temperature, crankshaft position, barometric pressure, vehicle speed, and automatic transmission sensors.

Injection pump timing is controlled by a stepper motor mounted to the side of the distributor pump. By changing the position of the cam ring, first movement of the high pressure plunger (plunger lift) can be varied relative to crankshaft position. With mechanical governors, this is a function of hydraulic pressure in the pump housing, increasing with rpm to advance timing as speed increases. On electronically controlled pumps, the ECM operates the stepper motor. Other outputs include the injection pump driver, timing stepper motor, Service Engine Soon and Service Soon lights, and glow plug relay.

2.3.0 Cummins Fuel Systems

2.3.1 Pressure-Time Fuel Injection System

For many years, Cummins manufactured engines equipped with a mechanically operated pressure-time (PT) fuel injection system that remains unique to the industry. To meet stringent emission control laws, Cummins introduced its Pace system on its PT-equipped engines. PACE provides electronic control of the PT system through the use of sensors, an electronically activated fuel control valve, and a PT control module. Cummins also offers an electronically controlled unit injector fuel injection system, known as Celect-ECI. Some Cummins engines also use inline or distributor injection system, depending on the application.

PT Fuel System Operation. Figure 83 illustrates the basic components of the Cummins PT fuel system. The PT fuel injection system derives its name from two primary variables affecting the amount of injected fuel per cycle. The P refers to the pressure of the fuel at the injector inlets. This pressure is collected by the fuel pump. However, the PT fuel pump is not the same as an inline or distributor injection pump. The T refers to time available for the fuel to flow into the injector cup. The time is controlled by engine speed through the camshaft and injection train. Cummins engines are four-cycle engines, so the camshaft is driven at one-half engine speed, making additional governing of fuel flow necessary. The fuel pump varies pressure to the injectors in proportion to engine rpm.

Figure 83 - Basic components of the Cummins Pressure-Time Fuel injection system.

The final controlling element in the fuel metering process is the size of the injector openings (orifices). The orifice size, also called flow area, is determined by calibration of a complete set of injectors.

With a given flow area, fuel metering is controlled by rail pressure and flow time. The flow time or metering time is controlled by engine speed through a camshaft-actuated plunger. The camshaft rotary motion is changed into reciprocating motion of the injector plunger. The plunger movement opens and closes the injector barrel metering orifice, as illustrated in Figure 84. The period of time the metering orifice is open is the time available for the fuel to flow into the injector cup. The greater the engine speed, the less available time to meter the fuel. Figure 84, View A illustrates the injector metering orifice closed, while Figure 84, View B illustrates the injector metering orifice opened.

Figure 84 -A PT fuel system, the camshaft actuated injector.

At any given speed, only rail pressure in the PT fuel system controls the quantity of fuel metered to the injectors. One of the major advantages of the PT system is that unlike other fuel injection pumps, it is not necessary to time the PT fuel pump to the engine.

The PT pump supplies fuel at a given flow rate and at a specified pressure setting to a common rail supplying all the injectors. In the PT injection system, the injectors themselves are timed to ensure that the start of injection occurs at the correct time for each cylinder.

Pressure-Time Governed Fuel Pump Functions. The pressure-time fuel pump also drives the governor. This assembly is often referred to as the pressure-time governed (PTG) fuel pump. The basic functions of the PTG fuel pump include:

The PTG fuel pump assembly is coupled to the compressor drive and driven from the engine gear train. The fuel pump main shaft drives the gear pump, governor, and tachometer shaft assemblies at engine speed, as illustrated in Figure 85. Fuel from the supply tank enters the gear pump inlet and is carried around outside the two meshing gears to the gear pump outlet. From this point, fuel flows through a wire mesh magnetic filter to the governor inlet passage. In addition to providing idle speed and maximum speed governing, the PT governor also regulates fuel pressure.

Figure 85 - Components of the Pressure-Time Governed Air-Fuel Control (PTG-AFC) fuel pump.

Governor. The PT fuel system governor is illustrated in Figure 86. It is a simple flyweight-operated mechanical governor. The flyweights are driven at engine speed by the fuel pump mainshaft. The governor plunger is held between the flyweight feet and rotates with the flyweights. The rotating flyweights pivot on pins, allowing their feet to exert an axial force on the governor plunger. At any given engine speed, the plunger position is determined by the balance between the flyweight and spring forces. Increasing engine speed also increases the force exerted by the flyweights and forces the plunger to move to the right, as shown in Figure 87.

Figure 86 - Components of a Cummins PT fuel pump mechanical flyweight governor.

Figure 87 - As rotational speed increases, the centrifugal force of the governor flyweights force the governor plunger to the right.

Rail pressure control of the PT fuel system begins by regulating the fuel or supply pressure to the governor assembly. Supply pressure control is accomplished by using a bypass pressure regulator inside the governor. The pressure regulator has a button valve designed to unseat when a designated pressure is reached, as shown in Figure 88. For the regulator to open, an excess supply of fuel must be delivered to the governor. This ensures that some fuel is always being bypassed, which is necessary for the regulator to maintain control of the supply pressure.

Figure 88 - Button valve pressure regulator.

When the fuel pressure exceeds the force holding the button valve and plunger together, fuel is bypassed to the gear pump suction side illustrated in Figure 89. The unseating pressure is determined by dividing the spring force by the button recess area. The recess area, or counterbore, is the area the fuel is pushing against. Increasing the counterbore area will reduce the pressure at which the fuel bypass opens. Decreasing the size of the recessed area will increase the pressure at which the fuel is delivered to the injector.

Figure 89 - Fuel bypass occurs when the button is unseated.

Throttle. The engine's power output can be controlled by the operator, within the established governor limits, through the use of a throttle. The throttle shaft is located between the governor and the fuel pump discharge. It allows the operator to control the rail pressure, and therefore, the engine power. It acts as a variable orifice, controlling the amount of fuel exiting the governor main passage. The throttle shaft's travel is limited by two stop screws located in the throttle shaft housing. A fuel- adjusting screw is located within the throttle shaft. Its setting determines the maximum flow area when the throttle shaft passage is wide open. The screw is used to adjust the rail pressure.

While the throttle is closed, there is a small amount of fuel flowing through the throttle shaft, This small amount of fuel is known as throttle leakage. This controlled fuel leakage is needed in the PT system to keep the injectors cooled and lubricated when the throttle is closed. If throttle leakage is set too high, the engine can experience slow deceleration and excessive injector carbon loading. If the throttle leakage is set too low, engine hesitation can result. The injector plunger may also be damaged.

When the throttle is in its closed position, the small amount of fuel flowing to the injectors is not enough to maintain idle speed. Additional fuel is provided from the governor through an idle passage around the throttle shaft.

PTG Air-Fuel Control (AFC). The PTG air-fuel control (PTG) fuel pump used on turbocharged heavy duty engines has an exhaust smoke control device built into the pump body. The AFC unit restricts fuel flow in direct proportion to intake manifold pressure during engine acceleration, under load, and lug- down conditions.

The main components of the AFC assembly are shown if Figure  90.The AFC senses air pressure in the intake manifold. Changes in intake manifold pressure will change the piston position, which controls the plunger shoulder position over the AFC inlet passage. This determines the amount of fuel delivered to the injectors during acceleration and other engine operational states. Air pressure is applied to the diaphragm and piston through the cover inlet fitting. Increasing air pressure overcomes spring force, causing the plunger to move in the barrel. As the plunger moves, it uncovers the passage, allowing fuel to flow from the throttle shaft through the AFC. As the air pressure increases, the plunger is pushed even further, eliminating the fuel restriction and permitting maximum fuel flow through the AFC.

Figure 90 - Cutaway showing components and fuel flow through the air- fuel control (AFC) assembly.

When there is little or no air pressure applied to the AFC diaphragm, maximum fuel pressure and flow is controlled by the No-air adjusting screw. The plunger is positioned by the return spring to block the main fuel passage around the No-air adjusting screw.

Shutdown Valve. From the AFC assembly, fuel flows to the shutdown valve. Most shutdown valves are controlled by an electrically-operated solenoid. In the shutdown mode, a spring washer seats a disc that prevents fuel flow out of the pump, as shown in Figure 91, View A. When the solenoid is energized, the electro-magnetic force overcomes the spring washer pressure. The disc unseats and flows from the pump to the injectors, as shown in Figure 91, View B. A single low pressure line from the fuel pump serves all injectors. This means the pressure and quantity of fuel metered to each injector are equal.

Figure 91- Operation of PT fuel pump shutdown valve.

Injector Operation. Figure 92 illustrates the pressure-time type D Top Stop injector commonly used in Cummins PT fuel injection systems. The injector plunger is actuated by engine camshaft rotation. When the cam follower roller is on the inner base circle, the injector return spring lifts the plunger, uncovering the metering orifice. When the cam follower roller is on the outer base circle, the injector plunger's downward movement overcomes the injector return spring, closing the metering orifice and injecting fuel into the cylinder. The injector plunger is now seated in the injector cup.

Figure 92 - Components of a PTD Top Stop Injector.

Fuel entering the injector flows through a wire mesh filter screen and an adjustable orifice. The size of the orifice determines the injector flow rate and the pressure at the metering orifice. From the adjustable orifice, fuel flows down an internally drilled passage in the injector adapter and barrel. The fuel seats a check ball, while continuing its flow toward the metering orifice. The check ball prevents the reversal of fuel flow during deceleration and shutdown as the plunger moves downward across the metering orifice, as demonstrated in Figure 93.

Figure 93 - Fuel flow into the PTD Top Stop injector.

When the metering orifice is uncovered, fuel flows into the injector cup. This occurs during the end of the engine's intake stroke and the beginning of the compression stroke. As the cam follower roller travels toward the base circle of the camshaft injector lobe, the injector return spring lifts the plunger, uncovering the metering orifice shown in Figure 94, View A. With the metering orifice open, the plunger also blocks the drain port in the injector.

Figure 94 - Metering of fuel in the PTD Top Stop Injector.

With continued camshaft rotation, the cam roller travels up the injection ramp. The upward movement of the push rod pushes the injector plunger downward. As the injector plunger moves down on the compression stroke, the metering orifice closes. Shortly after the metering orifice is closed, the drain port opens. At this point, the injector cup contains the proper amount of fuel to be injected, as indicated in Figure  94, View B. The point at which injection begins varies with the level of fuel in the injector cup, as illustrated in Figure 95. With an increase in the fuel level, the injector plunger contacts the fuel earlier, advancing the start of injection.

Figure 95 - Injector cup operation.

Fuel is injected when the pressure exerted by the injector plunger on the fuel is greater than the combustion chamber pressures. Injection ends when the plunger bottoms out in the cup, as illustrated in Figure 96, View A. At this point, the drain groove on the injector plunger aligns with drain passages in the injector barrel, permitting fuel to flow out of the drain groove and return to the tank, as demonstrated in Figure 96, View B.

Figure 96 - PTD fuel injector fuel flow. Troubleshooting

Troubleshooting is an organized study of a problem and a planned method or procedure to investigate and correct the difficulty.

Most troubles are simple and easy to correct. For example, excessive fuel oil consumption is caused by leaking gaskets or connections. A complaint of a sticking injector plunger is usually corrected by repairing or replacing the faulty injector; however, something caused the plunger to stick. The cause may be improper injector adjustment or, more often, water in the fuel.

In general, the complaint of low power is hard to correct because it can have many causes. There are many variables in environmental operation and installations, and it is difficult to measure power in the field correctly. With the PT fuel system, you can often eliminate the pump as a source of trouble. Simply check to see that the manifold pressure is within specified limits. The fuel rate of the pump must not be increased to compensate for a fault in other parts of the engine; damage to the engine will result.

When you check the fuel pump on the engine, remove the pipe plug from the pump shutoff valve and connect the pressure gauge. At the governed speed (just before the governor cuts in), maximum manifold pressure should be obtained. If the manifold pressure is NOT within specified limits, adjust for maximum manifold pressure by adding or removing shims from under the nylon fuel adjusting plunger in the bypass valve plunger. Be careful you do not lose the small lock washer that fits between the fuel adjusting plunger and the plunger cap.

To check the suction side of the pump, connect the suction gauge to the inlet side of the gear pump. The valve in the pump, if properly adjusted, should read 8 inches on the gauge. When the inlet restriction reaches 8.5 to 9 inches, change the fuel filter element and remove any other sources of restriction. The engine will lose power when the restriction is greater than 10 to 11 inches.

Always make the above checks on a warm engine. Also, operate the engine for a minimum of 5 minutes between checks to clear the system of air.

If the pump manifold and suction pressures are within specified limits and there is still a loss of power, you should check the injectors.

Carbon in the PT injector metering orifices restricts the fuel flow to the injector cups, which results in engine power loss. Remove the carbon from the metering orifices by reverse flushing; it should be performed on a warm engine. To remove carbon, perform the following steps:

  1. Loosen all injector adjusting screws one turn from the bottom or one and one- eighth turns from the set position. Lock with the jam nut after completing the required turns.
  2. Start the engine and accelerate with maximum throttle from idling to high10 to 15 times.
  3. Readjust the injectors to their standard setting. The engine will be difficult to start with the loose injector setting; it will smoke badly and will be sluggish. If the injector adjusting screws are loosened, the meter orifice will not be closed during injection. Extremely high injection pressure will force some of the fuel to backflow through the orifice and should remove carbon deposits. If this method is not effective, remove the injectors for cleaning.

When working on the PT fuel system of a turbocharged Cummins engine, you may find an aneroid control device. This device creates a lag in the fuel system so that its response is equivalent to that of the turbocharger, thus controlling the engine exhaust emissions (smoke level).


The aneroid is an emissions control device. Removing it or tampering with it is in direct violation of state and federal vehicle exhaust emissions laws.

During troubleshooting of the fuel system, you should check the aneroid according to the manufacturer's specifications. Pump Disassembly

If you determine that the fuel pump (Figure 85) must be removed from the engine, take the following precautions:

Good cleaning practices are essential to good quality fuel pump repair. Take special care when the PT fuel pump, which is made of a lightweight aluminum alloy, is disassembled. Use proper tools to prevent damage to machined aluminum surfaces, which are more easily damaged than parts made of cast iron.

Before disassembling the unit, try to determine what parts need replacement.

After you place the fuel pump on the holding device, place the device in a vise and disassemble the pump. Follow the procedures given in the manufacturer's maintenance and repair manuals. Pump Cleaning and Inspection

Now that the pump has been disassembled, you should clean and inspect all parts. Do not discard parts until they are worn beyond reasonable replacement limits. The PT fuel pump parts will continue to function long after they show some wear. Parts that are worn beyond reasonable replacement limits must not be reused. From experience you know reasonable replacement limits. Reuse all parts that will give another complete period of service without danger of failure.


Take special care when you clean aluminum alloy parts. Some cleaning solvents will attack and corrode aluminum. Mineral spirits is a good neutralizer after using cleaning solvents. Pump Reassembly

After you complete] y clean and inspect the pump and its parts, reassemble the pump as prescribed by the manufacturer's manual. In all assembly operations, be careful to remove burrs and use a good pressure lubricant on the mating surfaces during all pressing operations. A good pressure lubricant aids in pressing and prevents scoring and galling. Use flat steel washers. They go next to the aluminum to prevent goring by the spring steel lock washers. Pump Testing

Mount the PT fuel pump on a test stand and in the test, the pressure from the PT pump is measured and adjusted before the pump is placed on the engine. To test this pump, let pressure develop across the special orifices in the orifice block assembly. The pressure is measured on the gauges provided. All pump tests should be made with the testing fuel oil temperature between 90°F and 100°F. Now you are ready to conduct the test.

Open the fuel shutoff valve and manifold orifice valve. Open the stand throttle and start and run the pump at 500 rpm until the manifold pressure gauge shows the recommended pressure. If the pump does not pick up the specified pressure, check for closed valves in the suction line or an air leak.

If the pump is newly rebuilt, run it at 1500 rpm for 5 minutes to flush the pump and allow the bearings to seat. Continue to run the pump at 1500 rpm and turn the rear throttle stop screw in or out to find the maximum manifold pressure at full throttle.



With a standard governed pump, the throttle screws will be readjusted later. If the pump has a variable speed governor, the throttle shaft is locked in full-throttle position; do not readjust. On a dual or torque converter governor pump, the throttle must be locked in the shutoff position and the converter-driven governor idle-adjusting screw turned in until the spring is compressed. The converter-driven governor must be set on the engine.

The pump idle speed is set by closing the bypass and manifold orifice valves and opening the idle orifice valve. Set the pump throttle to idle and run at 500 rpm. To decrease or raise the idle pressure, add or remove shims from under the idle spring. Remember not to set the idle screw until you have adjusted the throttle screws.

Once the tests and adjustments have been completed according to the specifications recommended by the manufacturer, remove the pump from the test stand. Make sure the suction fitting is not removed or disturbed. Next, loosen the spring pack cover and drain the pump body. Cover all openings and bind fittings with tape until you are ready to install the pump. Injector Maintenance and Testing

In the PT fuel system, fuel is metered by fuel pressure against the metering orifice of the injector. Any change in fuel pressure, metering orifice, or timing will affect the amount of fuel delivered to the combustion chamber. The following two things will interfere with the normal functions of injector orifices:

1. Dirt or carbon in the orifices or in the passages to and from the orifices; and

2. A change in the size or shape of the orifices, particularly caused by improper cleaning of the orifices after soaking dirty injectors in a cleaning solvent to remove the carbon. Be sure to dip the injectors in a neutral rinse, such as mineral spirits, and then dry them.



Never use cleaning wires on PT fuel injector orifices.

Be sure to use a magnifying glass to inspect the injector orifices for damage. When the injector orifices are damaged, they cannot be made to function properly and must be replaced.

Check the injector for a worn plunger or injector body. Worn injectors may cause engine oil dilution from excessive plunger-to-body clearances. Dilution may also result from a cracked injector body or cup or a damaged O ring. To check the injector for leakage, assemble it. Remember to plug the off the injector inlet and drain connection holes; then mount the injector on the injector test stand.

Test the injector at a maximum of 1000 psi with the fuel flowing upward through the cup spray holes. If the counterbore at the top of the injector body falls with fuel in less than 15 seconds, the plunger clearance is excessive and may cause engine oil dilution. During this check, inspect the injector for leaks around the injector cup, body, and plugs. If the injector does not pass the test and checks, remove the damaged parts and replace them with new parts.

Any time you remove an injector plunger, use the lubricant recommended by the manufacturer when you replace the plunger in the injector body.

If the injector plunger does not seat in the injector cup, change the cup rather than trying to lap the plunger and cup together. Lapping changes the relationship between the plunger groove and metering orifice and disturbs fuel metering. Always use a new injector cup gasket when you assemble the cup to the injector body to avoid distortion of the cup. When the cup is tightened to the injector body, the gasket compresses everywhere, except under the milled slot on the end of the injector body. Then, if the gasket is reused, the uncompressed areas may cause the injector cup to cock and prevent the injector plunger from seating properly.


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The purpose of an air intake system is to supply the air needed for combustion of the fuel. In addition, the air intake system of a diesel engine will have to clean the intake air, silence the intake noise, furnish air for supercharging, and supply scavenged air in two- stroke engines.

The three major components of the air induction systems that increase internal combustion engine efficiency are blowers, superchargers, and turbochargers. They may be of the centrifugal or rotary type, gear driven directly from the engine, or driven by the flow of exhaust gases from the engine.

In the following sections, certain abnormal conditions of air induction system components, which sometimes interfere with satisfactory engine operation, are covered. Also, methods of determining causes of such conditions will be covered. Before performing any work on these components, make sure you follow the recommendations given in the manufacturer's service manual.

3.1.0 Superchargers

A supercharger or blower is an air pump that is mechanically driven by the engine. Its job is to force more air into the combustion chamber. More air permits a larger fuel charge to be burned in the cylinder, creating additional power from the same basic engine. Because the blower is driven by the engine, it robs the engine of power. However, this loss is more than offset by the power gained.

Many diesel engines use the process called scavenging and supercharging. In some four-cycle diesel engines, air enters the cylinder due to change in pressure created as the piston moves away from the combustion space during its intake stroke. The air is forced into the cylinder due to greater atmospheric pressure outside the engine. This process is referred to as a naturally aspirated intake.

The blower is used to increase airflow into the cylinders on all two-cycle engines and some four-cycle engines. The blower compresses air and forces it into an air box or manifold connected to the engine cylinders. This provides an increased amount of air under constant pressure to be used during engine operation.

Scavenging principles. The air produced by the blower fills the cylinder with fresh air, clearing it of combustion gases. The process is known as scavenging. The air forced into the cylinder is called scavenged air, and the entrance ports are referred to as scavenge ports.

Scavenging can take up only a small portion of the operating cycle; however, the length of time it takes is different in two-cycle and four-cycle engines. In a two-cycle engine, scavenging takes place during the end of the downstroke (expansion) and the beginning of the upstroke (compression). In a four-cycle engine, scavenging occurs as the piston is passing top dead center during the end of exhaust upstroke and the beginning of the intake downstroke. Intake and exhaust openings are both open during the scavenging process. This allows the air from the intake to pass through the cylinder and into the exhaust manifold, removing exhaust gases from the cylinder and cooling hot engine parts in the process.

Scavenging air must be directed when entering the cylinder of an engine so that waste gases are removed from the remote parts of the cylinder. The two principle methods of accomplishing this are sometimes referred to as port scavenging and valve scavenging. Port scavenging may be of the direct (cross-flow) or uniflow types, as shown in Figure 5-97, View A and Figure 97, View B. Valve scavenging is of the uniflow type, as shown in Figure 97, View C. These scavenging air actions also cool the internal components, such as the piston, liner, and valves, with approximately 30% of engine cooling being provided by its airflow. This leaves the cylinder full of fresh air for combustion purposes when the piston covers the liner ports.

Figure 97 - Methods of scavenging used in diesel engines.

Supercharging principles. Increased airflow to the cylinders can also be used to increase the power output of an engine. Since engine power develops from burning fuel, increasing the power requires more fuel. The increase in fuel, in turn, requires more air in order for combustion to take place. Supercharching is a process that supplies more air to the intake system than is normally taken in under atmospheric pressure.

3.1.1 Removal

When the supercharger has to be removed from the engine, follow the procedures given in the manufacturer's service manual.

3.1.2 Disassembly

If you have to disassemble the supercharger, be careful when you remove the intake and discharge connections. Be sure to cover both openings. To prevent damage to its finished surfaces, which are usually made from aluminum, wash the outside of the supercharger with mineral spirits. Use the correct service tools and follow recommended disassembly procedures in the manufacturer's maintenance and repair manuals.

3.1.3 Cleaning and Inspecting

As the supercharger parts are disassembled, you should clean and dry them thoroughly with filtered, compressed air. Discard all used gaskets, oil seals, recessed washers, roller bearings, and ball bearings. Replace these parts with new ones.

Inspect the rotors, housing, and end plates for cracks, abrasions, wear spots, and buildup of foreign material. With a fine emery cloth, smooth all worn spots found. Discard cracked, broken, or damaged parts. Remember, rotors and shafts we not separable. They must be replaced as a matched set or unit.

Inspect the drive coupling for worn pins, distorted or displaced rubber bushings, and damaged or worn internal splines. Examine the hub surface under the oil seal and replace the coupling if its surface is grooved or worn.

Check the gear fit on the rotor shafts and the gear teeth for evidence of chatter and wear. Replace the rotors and gears if they are not within the required tolerances.

Inspect all dowels, oil plungers, piston ring seals, and gasket surfaces. Replace them as necessary.

3.1.4 Reassembly

After you have inspected, cleaned, and replaced worn or damaged parts, put the supercharger back together, as prescribed in the manufacturer's maintenance and service manuals. Upon complete reassembly and after the supercharger is installed on the engine, add the proper quantity of recommended engine lubricating oil to the gear end plate through the pipe plughold.

3.2.0 Turbochargers

Turbochargers can be used on both two- and four-cycle diesel engines, as shown in Figure 98. They utilize exhaust energy, which is normally wasted, to drive a turbine- powered centrifugal air compressor that converts air velocity into air pressure to increase the flow of air into the engine cylinders. The air pressure drawn into a naturally aspirated engine cylinder is at less than atmospheric pressure. A turbocharger packs the air into the cylinder at more than atmospheric pressure. A turbocharger improves combustion, resulting in decreases exhaust emissions, smoke, and noise. Increased power output and the ability of the turbocharger to maintain nearly constant power at high altitudes are also benefits of a turbocharged engine.

Figure 98 -A turbocharger.

Fuel economy is another very significant reason for using a turbocharger with a diesel engine. The extra air provided by the turbocharger allows increased horsepower output without increasing fuel consumption. Lack of air is one factor limiting the engine horsepower of naturally aspirated engines. As engine speed increases, the length of time the intake valves are open decreases, giving the air less time to fill the cylinders and lowering volumetric efficiency.

Figure 99 - The components of a turbocharger.

The turbocharger normally consists of three components, as shown in Figure 99:

  1. A radial inward flow turbine wheel and shaft
  2. A centrifugal compressor wheel
  3. A center housing, which supports the:

The center housing has connections for oil inlet and oil outlet fittings.

The turbine wheel (hot wheel) is located in the turbine housing and is mounted on one end of the turbine shaft. The compressor wheel, or impeller (cold wheel), is located in the compressor housing and is mounted on the opposite end of the turbine shaft to form an integral rotating assembly, as illustrated in Figure 100.

Figure 100 - A- Compressor wheel and B- Turbine wheel.

Other parts of the rotating assembly include the thrust bearing (or spacer), backplate, and wheel retaining nut. The rotating assembly is supported on two pressure- lubricated bearings that are retained in the center housing by snap rings. Internal oil passages are drilled in the center housing to provide lubrication to the turbine shaft bearings, thrust washer, thrust collar, and thrust space, as shown in Figure  101.

Figure 101 - Oil flow through a turbocharger's center section.

The turbine housing is a heat- resistant alloy casting that encloses the turbine wheel and provides a flanged exhaust gas inlet and an axially-located turbocharger exhaust gas outlet. The compressor housing, which encloses the compressor wheel, provides an ambient air inlet and a compressed air outlet. In a typical installation, the turbocharger is located to one side, usually close to the exhaust manifold. An exhaust pipe runs between the engine exhaust manifold and the turbine housing to carry the exhaust gases to the turbine wheel. Another pipe, called the crossover tube, conducts fresh compressed air to the intake manifold, as shown in Figure 102.

Figure 102 - Typical image showing the location of the crossover pipe or tube.

Figure 103 lists common turbocharger problems and problem causes. If a defective turbocharger is suspected, always make sure the turbocharger is really at fault. Repairs are sometimes performed on the turbocharger when the real source of the problem is a restricted air cleaner, a plugged crankcase breather, or deteriorated oil lines.

Common symptoms that may indicate turbocharger problems include:

A lack of engine power and black smoke can both result from insufficient air reaching the engine and can be caused by restrictions in the air intake or air leaks in the exhaust system or the induction system. The first step in troubleshooting any turbocharger is to start the engine and listen to the sound of the turbocharging system makes. As a mechanic becomes more familiar with this characteristic sound, he/she will be able to identify an air leak between the engine.

Figure 103 - Sample Turbocharger Troubleshooting Guide.

3.2.1 Removal, Disassembly, and Cleaning

The removal of the turbocharger from the engine is not a complicated task when you follow the procedures in the manufacturer's instructions. After removing the turbocharger from the engine, you should make sure the exterior of the turbocharger is cleaned of all loose dirt before disassembly to prevent unnecessary scoring of the rotor shaft. Disassemble it according to the manufacturer's maintenance and repair manuals.

The turbocharger parts accumulate hard-glazed carbon deposits, which are difficult to remove with ordinary solvents. This is especially true if the turbine wheel and shaft, diffuser plate, nozzle ring, and inner heat shield are affected. The cleaner must remove these stubborn deposits without attacking the metal. All parts should be cleaned as follows:

  1. Place all parts in a divided wire basket so parts will not be damaged through contact with each other. Do not pile them in the basket. Avoid mutilating precision ground surfaces.
  2. Immerse the parts in mineral spirits or similar solvents.



Never use a caustic solution or any type solvent that may attack aluminum or nonferrous alloys.

  1. Allow the parts to soak as needed to remove the carbon. A soft bristle brush may be used, if necessary, to remove heavy deposits. Never use wire or other brushes with stiff bristles.
  2. With the oil orifice removed, flush out the oil passages in the main casing from the bearing end to remove dirt loosened by the soaking.
  3. Remove the parts from the tank. Drain and steam clean thoroughly to remove all carbon and grease. Apply steam liberally to the oil passages in the main casing.
  4. Blow off excess water and dry all parts with filtered, compressed air.
  5. Carefully place parts in a clean basket to avoid damaging them before inspection and reassembly.

3.2.2 Parts Inspection

Inspect all turbocharger parts carefully before you rinse them. All parts within the manufacturer's recommended specifications can be used safely for another service period. Damage to the floating bearing may require replacement of the turbocharger main casing with a new part or an exchange main casing.

Inspect the turbine casing. If you find cracks that are too wide for welding, replace the casing.

Do not use the exhaust casing if it is warped or heavily damaged on the inside surface caused by contact with the turbine wheel or a foreign object, or if it is cracked in any way.

Usually, oil seal plates do not wear excessively during service and can be reused if they have not been scored by a seizure of the piston ring.

As you inspect the diffuser plates, look for contact scoring by the rotor assembly on the back of the diffuser plate or broken vanes. This scoring will make the plate unacceptable for reuse.

Inspect the inner heat shield. If it is distorted, replace it.

Dents found on the outer heat shield can usually be removed, allowing its reuse. However, if this shield is cut or split in the bolt circle area, replace it.

Inspect the nozzle rings closely for cracks. If the nozzle rings are cracked or if the vanes are bent, damaged, or burnt thin, replace them.

If you see signs of wear or distortion during the inspection of the piston ring seals, discard and replace them with new ring seals.

Inspect the turbocharger main casing for cracks in the oil passages, cap screw bosses, and so forth. Also, check the casing for bearing bore wear. If it exceeds the limits allowed by the manufacturer, the bearing bore may be reworked to permit oversize, outer diameter bearings.

Check the oil orifice's plug for stripped or distorted threads. Install a new plug if necessary. The rotor assembly, which consists of a turbine wheel, thrust washer, and locknut, is an accurately balanced assembly. Therefore, if any one of the above parts is replaced as a result of your inspection, the assembly must be rebalanced according to the manufacturer's specifications.

When inspecting the semifloating bearing, measure both the outside and inside diameters of the bearing. If either diameter is worn beyond limits allowed by the manufacturer, replace the bearing.

The front covers that are deeply scored from contact with the compressor wheel cannot be reused. Slight scratches or nicks only can be smoothed out with a fine emery cloth and the covers reused. Cracked covers, however, cannot be reused and must be replaced with new ones.

All cap screws, lock washers, and plain washers should be cleaned and reused unless they are damaged.

3.2.3 Reassembly and Installation

After inspection of the turbocharger component parts and replacement of damaged or worn parts, reassemble the turbocharger as prescribed by the manufacturer's maintenance and repair manuals.

Close off all openings in the turbocharger immediately after reassembly to keep out abrasive material before you mount it on the engine.

Turbochargers can be mounted on the engine in many different positions. Always locate the oil outlet at least 45 degrees below the turbocharger horizontal center line when the unit is in the operating position.

After reassembly, prime the turbocharger before engine start-up by removing the oil supply inlet fitting and adding approximately 0.5 pint of clean engine oil to the turbocharger. Operate the engine at low idle for a few minutes before operating it at higher speeds. Finally, check the system for leaks.

Turbocharged engines require proper shutdown procedures to prevent bearing damage. If the engine is shut down from high speed, the turbo will continue to rotate after engine oil pressure has dropped to zero. Always idle the engine for several minutes before shutting it down.

Turbocharger damage can also be caused by oil lag. Oil lag is a lack of lubrication that occurs when oil pressure is not sufficient to deliver oil to the turbocharger bearings.

Before running the engine up to high rpm, operate at low speeds for at least 30 seconds after initial start-up to allow the oil flow to become established. Additional time should be allowed when the outside temperature is below freezing. After replacing a turbocharger, or after an engine has been unused or stored for a significant time, there can be a considerable lag after the engine is started.


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Starting a cold diesel engine can be somewhat frustrating. The heat generated by compression tends to dissipate through the cylinder and head metal. Clearances when the engine is cold may be such that much of the compressed air escapes past the piston rings. Other problems may include the effect of cold on lubricating oil and diesel fuel viscosity. The spray pattern at the injectors coarsens and the drag of heavy metal oil between the engine's moving parts increases friction.

It is important to remember that starting has three distinct phases:

  1. During the initial phase, the breakaway torque needed to start cranking can be substantial, since the engine's rotating parts have settled to the bottom of their journals and are only partially lubricated.
  2. The second phase occurs during the first few revolutions of the crankshaft. In most cases, the first few crankshaft revolutions are free of heavy compressive loads. Cold oil pumped to the journals collects and wedges between the bearings and shafts. As the shaft rotates, this oil is heated by friction. As the oil thins, drag reduces significantly, and cranking speed increases dramatically. Engine compression also begins to build during this phase.
  3. The third phase begins as the engine reaches firing speed. Compression levels within the cylinders are sufficient to initiate combustion. The time between the initial breakaway phase and the firing speed depends on the capacity of the starter and battery, the mechanical condition of the engine, lubricating oil viscosity, ambient air temperature, flywheel inertia, and the number of cylinders in the engine.

To ensure long engine life, if the ambient temperature dropped below 25°F, it is advisable to warm any diesel engine before initial start-up. Warming the engine will allow it to start quickly and reduce wear on starting system components. There are several methods of warming an engine in cold weather. These include coolant heaters, lube oil heaters, intake air heaters, battery heaters, and glow plugs.

Figure 104 - Circulating coolant heater.

Coolant heaters. There are two types of coolant heaters - circulating heaters and immersion heaters:

Lubricating oil heaters. Most lubricating oil heaters are electric-powered immersion heaters that are installed in the oil sump through the drain plug or dipstick opening. A thermostat can also be installed as part of the heating unit. This type of heater is designed to keep the oil pan warm and allow heat to flow up into the block, warming the entire engine.

Battery warmers. Battery warmers use an electric heating pad beneath or around the battery to keep it at a temperature that allows full cranking current to be delivered to the starter motor. Battery warmers can be used alone, but should be coupled with another cold starting aid, such as a coolant or lube oil heating system.

The manifold flame heater system shown in Figure 105 is another type of cold-starting system found on diesel engines. This system is composed of the housing, spark plug, flow control nozzle, and two solenoid control valves. This system is operated as follows:

  1. The spark plug is energized by the flame heater ignition unit.
  2. The nozzle sprays fuel under pressure into the intake manifold assembly.
  3. The fuel vapor is ignited by the spark plug and bums in the intake manifold heating the air before it enters the combustion chamber.

Figure 105 - Manifold flame heater system.

The flame fuel pump assembly is a rotary type, driven by an enclosed electric motor. The fuel pump receives fuel from the vehicle fuel tank through the supply pump of the vehicle and delivers it to the spray nozzle. The pump is energized by the on/off switch located on the instrument panel.

The intake manifold flame heater system has a filter to remove impurities from the fuel before it reaches the nozzle.

The two fuel solenoid valves are energized (open) whenever the flame heater system is activated. The valves ensure that fuel is delivered only when the system is operating. These valves stop the flow of fuel the instant that the engine or the heater is shut down.

When troubleshooting or repairing these units, consult the manufacturer's repair manual.

4.1.0 Glow Plugs

Glow plugs, as illustrated in Figure 106, are heating elements that warm the air in the precombustion chambers to help start a cold diesel engine. The glow plugs are threaded into holes in the cylinder head. The inner tip of the glow plug extends into the precombustion chamber.

Figure 106 - A glow plug.

A glow plug control circuit automatically disconnects the glow plugs after a few seconds of operation. The entire engine coolant temperature sensor checks the temperature of the coolant. It feeds this electrical data to a control unit. Thus, if the engine is already warm, the control will not turn on the glow plugs.

Indicator lights also operated by the control unit inform the operator whether or not the engine is ready to start. The glow plugs need only a few seconds to heat up.

When the engine is cold and the operator turns the ignition switch to run, a large current flows from the battery to the glow plugs. In a few seconds, the glow plug tips will heat to a dull red glow.

When the glow plug indicator light goes out, the operator can start the engine. The compression stroke pressure and heat, along with the heat from the glow plugs, help the engine to start easier.

Glow plugs are not complicated and are easy to test. Disconnect the wire going to the glow plug and use a multimeter to read the ohms resistance of the glow plug.

Specifications for different glow plugs vary according to the manufacturer. Be sure and check the manufacturer's repair manual for the correct ohms resistance value.

4.2.0 Ether


Starting fluids must never be used if the engine is equipped with either glow plugs or an electric coil air intake heater.

For many years, highly combustible ether was used as a starting aid for diesel engines. Mechanics and operators poured ether on a rag and placed it over the air intake, or removed the air filter element and inserted it into the casing. If they guessed the dosage correctly, the engine might start. But, if too much ether was used, severe detonation or an explosion sometimes occurred, resulting in broken pistons and rings and bent connecting rods. The force of these explosions inside the cylinders destroyed many engines, and also caused injury to personnel.



Ether is to be used in extreme emergency. If you must use ether, the engine has to be turning over before you spray it into the air intake.

Use of spray cans of ether is discouraged as they can be dangerous. The only method of safely using ether is with a closed dispensing system. Starting aid systems of this type normally consist of a cylinder of pressurized ether, a metering valve, tubing, and an atomizer installed in the intake manifold, as shown in Figure 107. The valve is tripped only once during each starting attempt to prevent build-up of ether in the intake manifold that could lead to an explosion or hydraulic lock.

Figure 107 - Ether starting aid system.

Diesel engines can also be fitted with another one-shot starting device consisting of a holder and needle. A capsule containing ether is inserted into the device. The needle pierces the capsule, releasing the premeasured dose of ether into the intake manifold. Regardless of the system used, starting fluid should only be introduced into the intake manifold while the engine is cranking, and then very sparingly.



Diesel engines can become "dependent" upon ether as a starting aid, even to the point that an engine will not start without it.


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There are four types of exhaust emissions:

  1. Hydrocarbons
  2. Carbon monoxide
  3. Oxides of nitrogen and
  4. Particulate

Hydrocarbons (HC) are a form of emission resulting from the release of unburned fuel into the atmosphere. All petroleum products are made of hydrocarbons (hydrogen and carbon compounds). This includes gasoline, diesel fuel, LP-gas, and motor oil.

Hydrocarbons are produced by incomplete combustion or by fuel evaporation. For example, hydrocarbons are produced when unburned fuel escapes from the exhaust system of a poorly running engine. They can also be produced by fuel vapors escaping from the vehicle's fuel system. Some of the additives used in gasoline are extremely reactive, which allows the unburned fuel vapors to easily combine photochemically with other elements in the air to form smog.

Hydrocarbon emissions are a hazardous form of air pollution. They can contribute a variety of illnesses, including eye, throat, and lung irritation, and possibly cancer.

Carbon Monoxide (CO) is an extremely toxic emission resulting from the release of partially burned fuel. It is a result of incomplete combustion of a petroleum-based fuel.

Carbon monoxide is a colorless, odorless, and deadly gas. CO prevents human blood cells from carrying oxygen to body tissues. It can cause death if inhaled in large quantities. Symptoms of carbon monoxide poisoning include headaches, nausea, blurred vision, and fatigue.

Any factor that reduces the amount of oxygen present during combustion increases carbon monoxide emissions. For example, a rich air-fuel mixture increases CO.

Oxides of Nitrogen (NOx) are emissions produced by extremely high temperatures during combustion. Air consists of approximately 79% nitrogen and 21% oxygen, With enough heat above approximately 2500°F, nitrogen and oxygen in the air-fuel mixture combine to form NOx emissions. Oxides of nitrogen contribute to the dirty brown color of smog. They also produce ozone in smog, which causes an unpleasant odor and is an eye and respiratory irritant. Oxides of nitrogen are also harmful to many types of plants and rubber products.

An engine with a high compression ratio, lean air-fuel mixture, and high-temperature thermostat will produce high combustion heat, resulting in the formation of NOx. This poses a problem, as these same factors tend to improve gas mileage and reduce HC and CO emissions. As a result, emission control systems must interact to lower each form of pollution.

Particulates are solid particles of carbon soot and fuel additives that blow out a vehicle's tailpipe. Carbon particles make up the largest percentage of these emissions. The rest of the particulates consist of other additives sometimes used to make gasoline and diesel fuel.

While a particulate emission is rarely a problem with gasoline engines, it is a serious problem with diesel engines. Diesel particulates are normally caused by an extremely rich air-fuel mixture or a mechanical problem in the injection system.

About 30% of all particulate emissions are heavy enough to settle out of the air in a relatively short period of time. The other 70%, however, can float in the air for extended periods.

Sources of Vehicle Emissions. The majority of vehicle emissions come from these three basic sources:

  1. Engine crankcase blowby fumes-chemicals that form in the engine bottom end from heating of oil, as well as unburned fuel and combustion by-products that blow past the piston rings and into the crankcase
  2. Fuel vapors-various chemicals that enter the air as fuel evaporates
  3. Engine exhaust gases-harmful chemicals produced and blown out the tailpipe when an engine burns a hydrocarbon-based fuel (or most other fuels)

Vehicle Emission Control Systems. Several different emissions control systems are used to reduce the amount of air pollution produced by vehicles. The major ones found include:

Positive Crankcase Ventilation. A positive crankcase ventilation system uses engine vacuum to draw blowby gases into the intake manifold for reburning in the combustion chambers. Prior to the PCV, crankcase fumes were simply vented into the atmosphere. A road draft tube vented crankcase fumes and blowby gases out the back of the engine and this contributed to air pollution.

Engine blowby is caused by pressure leakage past the piston rings on the blower strokes. A small percentage of combustion gases can flow through the ring end gaps or the piston ring grooves and into the crankcase. If not reburned in the engine, these fumes will contribute to air pollution if vented to the atmosphere. If not vented from the crankcase, the gases will build to a point where engine damage would occur.

Engine blowby gases contain unburned fuel (HC); partially burned fuel (CO); particulates; and small amounts of water, sulfur, and acid. For this reason, blowby gases must be removed from the engine crankcase. Blowby gases can cause:

A PCV system keeps the inside of the engine clean and reduces air pollution. Older engines used an open PCV system. This system was not sealed and gases could leak out when the engine was shut off. These systems have been completely replaced by the closed PCV system.

A closed PCV system uses a sealed oil filler cap, a sealed oil dipstick, ventilation hoses, and either a PCV valve or a flow restrictor. The gases are drawn into the engine and burned. The system stores the gases when the engine is not running.

PCV System Operation. Although designs vary and can use either vacuum or electronic control, the operation of all PCV systems is basically the same, as shown in Figure 108. The PCV system draws vapors out of the crankcase and routes them into the engine to be burned.

Figure 108 -PCV system.

A hose usually connects the intake manifold to the PCV valve. With the engine running, vacuum acts on the engine's crankcase. Air is drawn in through the engine's air cleaner, through a vent hose into a valve cover, then drawn into the crankcase.

After the fresh air mixes with the crankcase gases, the mixture is pulled by vacuum past the PCV valve, through the hose, and into the engine intake manifold. The crankcase gases are then drawn into the combustion chambers for burning.

An electronically controlled PCV system often uses a solenoid valve in the vacuum line leading to the valve. The computer system can energize or de-energize the solenoid to block or pass vacuum. This allows the computer to help control PCV operation.

PCV Valve. A PCV valve is used to control the flow of air through the PCV system. It may be located in a rubber grommet in a valve cover, in a breather opening in the intake manifold of plenum, or on the side of the engine block. The PCV valve varies the flow of air for idle, cruise, acceleration, wide open throttle, and engine-off conditions.

Figure 109 shows the action of a PCV valve under various conditions. At idle, the PCV valve is pulled toward the intake manifold by high vacuum. This restricts the flow of air and prevents a lean air-fuel mixture. When cruising, the lower intake manifold vacuum allows the spring to open the PCV valve. However, enough vacuum is present to keep the PCV valve from completely closing. More air can flow through the system to clean out crankcase fumes. At wide open throttle or with the engine off (low or no intake manifold vacuum), spring pressure closes the PCV valve completely.

Figure 109 -PCV valve operation under operating conditions.

In case of an engine backfire (air-fuel mixture in the intake manifold), the PCV valve plunger is seated against the body of the valve. This keeps the backfire (burning) from entering and igniting the fumes in the engine crankcase.

Electric PCV Valve. Most vehicles now use electronically controlled crankcase ventilation systems. In these systems, an electric PCV valve, which contains a small solenoid and air valve, is controlled by the ECM to regulate engine crankcase ventilation. When energized by the engine ECU, the valve opens to allow the blowby gases in the engine crankcase to be routed back into the engine intake manifold for combustion, as the one shown in Figure 110.

Figure 110 - Electric PCV valve.

Oil/Air Separator. An oil/air separator is a device that makes oil vapors condense and flow into the oil pan. It can be used instead of a PCV system to reduce emissions and prevent oil sludging. The separator simply allows oil mists and vapors to settle out into a liquid so that they do not continue to circulate through the engine.

Evaporative Emissions Control System. The evaporative emissions control (EVAP) system prevents toxic fuel system vapors from entering the atmosphere. Gasoline and many of its additives evaporate easily, especially if exposed to the atmosphere.

Pre-emission-control vehicles used vented gas tank caps. Carburetor bowls were also vented to the atmosphere, which caused a considerable amount of hydrocarbon emissions from unburned fuel. Today, vehicles use an evaporative emissions control system to prevent this source of air pollution

Evaporative Emissions System Components. The components of a typical evaporative emissions system are shown by the diagram in Figure 111.

Figure 111 - Evaporative emissions control.

Figure 112 - Cutaway view of a charcoal canister

Evaporative Emissions Control System Operation. Figure 113 illustrates evaporative emissions system operation. Note that this system contains a vacuum- operated purge valve. When the engine is operating above idle speed, the intake manifold causes the purge valve to open. This allows gases to flow through the purge line and causes fresh air to be drawn through the filter in the bottom of the canister. The incoming fresh air picks up the stored fuel vapors and carries them through the purge line. The vapors enter the intake manifold and are pulled into the combustion chambers for burning. When the engine is shut off, gasoline slowly evaporates, producing unwanted vapors. These vapors flow through the fuel tank vent line and into the charcoal canister. The activated charcoal in the canister absorbs the fuel vapors and holds them until the engine is started again.

Figure 113 - The operation of an evaporative control system and its relayed components.

An evaporative emissions control system contains an electronically operated purge valve, or purge solenoid. The purge solenoid is normally closed and opens when energized by the ECM. The ECM energizes the solenoid only after the following conditions have been met:

Enhanced Evaporative Emissions Control System. As its name implies, an enhanced evaporative emission control system has several components and features not found in conventional EVAP systems. The enhanced system, which is found in OBD II vehicles, not only provides better control of fuel vapors, but it also monitors the condition of the fuel system. In addition to the components found in a conventional evaporative emission system, the enhanced EVAP system contains the following:

The canister used in enhanced EVAP systems does not have a bottom inlet filter. Instead, fresh air is fed to the canister by the vent solenoid. The purge valve, or purge solenoid, in these systems is an electrically operated valve that controls the flow of vapors from the canister to the manifold, as shown in Figure 114.

Figure 14 - The canister evaporative emission system.

Enhanced Evaporative Emission Control System Operation. The enhanced EVAP system often uses a normally closed, pulse-width modulated purge solenoid. The control module can send different length electrical pulses to the solenoid to precisely control vapor flow. When energized, the purge solenoid opens to allow vapor to be pulled into the engine.

The canister vent solenoid is normally open, allowing fresh air into the canister during purge mode. When the system is in the diagnostic mode, the control module closes the vent solenoid, blocking airflow into the canister. The module then opens the purge solenoid, creating a vacuum in the system. When the control module determines that there is enough vacuum in the system (based on the tank pressure sensor signal), it closes the purge solenoid, sealing the system. The module then monitors the pressure sensor signal. If the system is properly sealed, the signal should remain steady until the control module reopens the vent solenoid.

IM 240. IM 240 is an enhanced emissions test that requires the vehicle to be operated on a dynamometer at speeds up to 55 mph for 240 seconds while the exhaust emissions are measured. Two additional tests-the evaporative emission system purge test and the evaporative emission system pressure test-may be required in some locations.

5.1.0 Evaporative Control System Troubleshooting

Evaporative Emissions System Purge Test. The evaporative emissions system purge test measures the flow of fuel vapors into the engine while performing the IM 240 test. A flow meter transducer is installed into the system purge line between the charcoal canister and the engine intake manifold fitting. A personal computer connects to the flow transducer to analyze data. The computer can then detect if there is adequate purge flow to remove fumes from the canister to draw them to the engine for burning.

Evaporative Emissions Systems Pressure Test. An evaporative emissions system pressure test checks the system for leaks into the atmosphere. It is performed during the IM240 test. Pressure test equipment is connected to the evaporative emission system's vapor vent line. A computer then meters low-pressure nitrogen into the system.

When about 0.5 psi pressure is reached, the computer closes off the system and checks for a pressure drop for 2 minutes. If pressure remains above recommendations, the evaporative emission system passes the pressure test. If the pressure drops too much, repairs are needed to fix the leakage.

Evaporative Emissions Control System Service. A faulty evaporative emissions control system can cause fuel odors, fuel leakage, fuel tank collapse (vacuum buildup), excess pressure in the fuel tank, or a rough engine idle. These problems usually stem from a defective fuel tank pressure-vacuum cap, leaking charcoal canister valves, deteriorated hoses, or incorrect hose routing.

Evaporative Emissions Control System Maintenance and Repair. Maintenance on an evaporative emissions control system typically involves cleaning or replacing the filter in the charcoal canister. Service intervals for the canister filter vary. However, if the vehicle is operated on dusty roads, clean or replace the filter more often.

Also inspect the condition of the fuel tank filler cap. Make sure the cap is installed properly and the seals are in good condition. Special testers are available for checking the opening of the pressure and vacuum valves in the cap. The cap should be tested when excessive pressure or vacuum problems are noticed.

Use a hand vacuum pump to test the charcoal canister vacuum purge solenoids for diaphragm leakage. If a diaphragm will not hold a vacuum, it is ruptured and must be replaced. You can also use the vacuum gauge to check for a vacuum supply to any vacuum canister solenoid.

5.2.0 Emission Control Components Troubleshooting

Vacuum Solenoid Service. Various vacuum solenoids are used to interface emission system electronics with the devices that operate off engine vacuum. They can be used in almost all emission control systems.

When trying to find problems, you should refer to a vacuum hose diagram, which shows the routing of all vacuum hoses. Just as a wiring diagram helps tracing circuit problems, a vacuum hose diagram will give useful information on finding incorrectly routed hoses, leaking or restricted hoses, and bad vacuum components. Figure 115 is a sample vacuum diagram from a service manual. Note how the emissions devices are connected. The service manual will explain the function and testing of each device.

Figure 115 -Vacuum hose routing diagram as it appears in a service manual.

When troubleshooting vacuum solenoids, check for hard, brittle hoses that can leak and prevent normal operation of parts. If the vacuum solenoid is electrically powered, check it for voltage. Connect a volt-meter to the solenoid terminals and start the engine. Make sure you are getting voltage to the unit when needed.

You can also connect a remote source of voltage to a vacuum solenoid to check its operation. When voltage is connected to the solenoid, it will switch vacuum on or off.

PCV System Service. An operative PCV system can increase exhaust emissions. It can also cause engine sludging and wear, rough engine idle, and other problems. A leaking PCV system can cause a vacuum leak and produce a lean air-fuel mixture, causing a rough engine idle. A restricted PCV system can enrich the fuel mixture, affecting engine idle and causing the engine to surge (idle speed goes up and down) and emit black smoke.

PCV System Maintenance. Most auto manufacturers recommend periodic maintenance of the PCV system. Inspect the condition of PCV hoses, grommets, fittings, and breather hoses. Replace any hose that shows any signs of deterioration. Clean or replace the breather filter, if needed. Also, check or replace the PCV valve. Since replacement intervals vary, always refer to the service manual.

PCV System Testing. To quickly test a PCV valve, pull the valve out of the engine and shake it. If the PCV valve does not rattle when shaken, replace the valve. With the engine idling, place your finger over the end of the valve, as shown in Figure 116. With the airflow stopped, you should feel suction on your finger and the engine idle speed should drop about 40-80 rpm.

Figure 116 - With the engine running, place your finger over the PCV valve to check for suction.

If you cannot feel any vacuum, the PCV valve or hose might be plugged with sludge. If engine rpm drops more than 40-80 rpm and the engine begins to idle smoothly, the PCV valve could be stuck open.

A PCV valve tester will measure the exact amount of airflow through the system. To use a tester, make sure the engine intake manifold vacuum is correct. Then, connect the tester to the engine as described in the operating instructions. Start and idle the engine. Observe the airflow rate on the tester. Replace the PCV valve if airflow is not within specified limits.



Do not attempt to suck through a PCV valve with your mouth. Sludge and other deposits inside the valve are harmful to the human body.

Another simple check of the PCV valve can be made by pinching the hose between the valve and the intake manifold with the engine at idle, as illustrated in Figure 117. There should be a clicking sound from the valve when the hose is pinched and unpinched. If no clicking sound is heard, check the PCV valve grommet for cracks or damage. If the grommet is alright, replace the PCV valve.

Figure 117 - Pinch PCV hose with pliers.
Ensure to wrap something around the hose so the hose is not damaged by the pliers

Some manufacturers suggest placing a piece of paper over the PCV breather opening to test the PCV system. After a few minutes of operation, the piece of paper should be pulled down against the breather opening by the crankcase vacuum. If the suction does not develop, there is a leak in the system, such as a ruptured gasket or cracked hose, or the system may be plugged.

A four- or five-gas exhaust analyzer can also be used to check the general condition of a PCV system. Measure and note the analyzer readings with the engine idling. Then, pull the PCV valve out of the engine, but not off the hose. Compare the readings after the PCV valve is removed.

A plugged system will show up on the exhaust analyzer when oxygen and carbon monoxide do not change. Crankcase dilution (excessive blowby or fuel in the oil) will usually show up as an excessive (1% or more) increase in oxygen or a 1% or more decrease in carbon monoxide. This is because the excess crankcase fumes will be pulled into and burn in the engine, affecting your readings.

Thermostatic Air Cleaner System. The thermostatic air cleaner system speeds engine warm-up and keeps the air entering the engine warm. By maintaining a more constant inlet air temperature, a carburetor can be calibrated leaner at startup to reduce emissions. A typical thermostatic air cleaner is shown in Figure 118.

Figure 118 -Thermostatic air cleaner.

Thermostatic air cleaners are not needed with fuel injection systems. An electronic fuel injection system can alter its operation with cold air entering the engine more efficiently than a carburetor system.

A thermal vacuum valve is normally located in the air cleaner to control the vacuum motor and heat control door. A vacuum supply is connected to the thermal vacuum valve from the engine. Another hose runs from the thermal valve to the vacuum motor (diaphragm).

The vacuum motor, also called a vacuum diaphragm, operates the heat control door, or flap, in the air cleaner inlet. The vacuum motor consists of a flexible diaphragm, spring, rod, and diaphragm chamber. When the vacuum is applied to the unit, the diaphragm and rod are pulled upward, moving the heat control door.

The heat control door can be opened or closed to route either cool or heated air into the air cleaner. When the door is closed, hot air from the exhaust manifold shroud enters the engine. When the door is open, cooler outside air enters the engine.

Thermostatic Air Cleaner System Service. An inoperative thermostatic air cleaner system (heated air inlet) can cause several engine performance problems. If the air cleaner flap remains in the open position (cold air position), the engine could miss, stumble, stall, and warm up slowly. If the air cleaner flap stays in the closed position (hot air position), the engine could perform poorly when at full operating temperature.

Thermostatic Air Cleaner System Maintenance. The thermostatic air cleaner system requires very little maintenance. You should inspect the condition of the vacuum hoses and hot air tube from the exhaust manifold shroud. The hot air tube is frequently made of heat-resistant paper and metal foil. It will tear very easily. If torn or damaged, replace the hot air tube.

Testing Thermostatic Air Cleaner System. For a quick test of the thermostatic air cleaner system, watch the action of the heat control door in the air cleaner snorkel.

Start and idle the engine. When the air cleaner temperature sensor is cold, the door should be open. Place an ice cube on the sensor, if needed. Then, when the engine and sensor warm to operating temperature, the door should swing closed, as depicted in Figure 119, View A.

If the air cleaner flap does not function, test the vacuum motor and the temperature sensor. To test the vacuum motor, apply vacuum to the motor diaphragm with a hand vacuum pump. With the prescribed amount of vacuum, the motor should pull the heat control door open. If the door leaks or does not open, it should be replaced. After replacing the motor, recheck the thermostatic air cleaner system operation to make sure the air temperature sensor is working properly, as depicted in Figure 119, View B.

Figure 119 - Checking the operation of a thermostatic air cleaner.

To test the thermal vacuum valve in the air cleaner, place a thermometer next to the unit. With the valve cooled below its closing temperature, apply vacuum to the thermal vacuum valve. It should pass vacuum to the vacuum motor, and the heat control door should open.

Then, warm the thermal vacuum valve to its closing temperature. A heat gun (hair dryer) can be used to heat the unit. When warm, the valve should block the vacuum and the heat control door should close. Replace the thermal vacuum valve if the door fails to open and close properly.

Exhaust Gas Recirculation (EGR). The exhaust gas recirculation system, or EGR system, allows burned exhaust gases to enter the engine intake manifold to help reduce NOx emissions. When exhaust gases are added to the air-fuel mixture, they decrease peak combustion temperatures (maximum temperature produced when the air-fuel mixture burns). For this reason, an exhaust gas recirculation system lowers the amount of NOx in the engine exhaust. EGR systems can be controlled by engine vacuum or by the engine control module.

Vacuum-Controlled EGR. A vacuum-controlled EGR system uses engine vacuum to operate the EGR valve. A basic vacuum EGR system is simple. It consists of a vacuum- operated EGR valve and a vacuum line from the throttle body or carburetor. The EGR value usually bolts to the engine intake manifold or a carburetor plate. Exhaust gases are routed through the cylinder head and intake manifold to the EGR valve. Figure  120, View A shows that, with the throttle closed at idle speed, vacuum to the EGR valve is blocked and the valve remains closed to prevent rough idling. Figure 120, View B shows that when the throttle opens for more engine speed, the vacuum port to the EGR valve is exposed to vacuum. The EGR valve diaphragm is pulled up and exhaust gases enter the engine intake manifold without adversely affecting engine operation.

Figure 120 - Basic EGR valve operation.

The EGR valve consists of a vacuum diaphragm, spring, plunger, exhaust valve, and diaphragm housing. It is designed to control exhaust flow into the intake manifold, as illustrated in Figure 121.

Figure 121 -Back pressure EGR valve.

Vacuum EGR Operation. At idle, the throttle plate in the throttle body or carburetor is closed. This blocks off engine vacuum so it cannot act on the EGR valve. The EGR spring holds the valve shut and exhaust gases do not enter the intake manifold. If the EGR valve were to open at idle, it could upset the air-flow mixture and the engine could stall.

When the throttle plate opens to increase speed, engine vacuum is applied to the EGR hose. Vacuum pulls the EGR diaphragm up. In turn, the diaphragm pulls the valve open.

Engine exhaust can then enter the intake manifold and combustion chambers. At higher engine speeds, there is enough air flowing into the engine that the air-fuel mixture is not upset by the open EGR valve.

Electronic-Vacuum EGR Valves. An electronic-vacuum EGR valve, as illustrated in Figure 122, uses both engine vacuum and electronic control for better exhaust gas metering. An EGR position sensor is located in the top of the valve and sends data back to the ECM. This allows the computer to determine how much the EGR valve is opened.

Figure 122 - Cutaway of an Electronic- Vacuum EGR valve.

With some systems, EGR solenoid valves are used to control more closely the EGR opening. These valves use electric solenoids to block or pass airflow to the EGR valve. They are located in one or more of the vacuum lines going to the EGR valve. The ECM can then energize the solenoids to alter when and how fast the EGR valve opens or closes to improve efficiency, as demonstrated in Figure 123. The diagram shows how the control module can be used to monitor and control a vacuum- operated EGR valve. The electric solenoids can block or allow flow in the vacuum line going to the EGR valve, providing computer control for this system. The engine coolant temperature sensor allows the control to keep the EGR valve closed when the engine is cold and NOx emissions are not a problem.

Figure 123 - Diagram of control module monitoring and controlling vacuum- operated EGR valve.

EGR System Variations and Components. There are several EGR system variations that might be encountered, including:

Electronic EGR System. An electronic EGR system uses vehicle sensors, the ECM, and a solenoid-operated exhaust gas recirculation valve to reduce NOx emissions.

The ECM uses input data from the EGR position sensor, engine coolant temperature sensor, mass airflow sensor, throttle position sensor, crankshaft position sensor, and other sensors. The sensor signals allow the ECM to determine how much duty cycle should be sent to open and close each valve for maximum efficiency and minimum exhaust emissions.

The EGR duty cycle is a measurement of control current on and off time sent from the ECM. The ECM can precisely control the duty cycle from the ECM. The ECM can precisely control duty cycle to meter just the right amount of exhaust gases needed to reduce NOx emissions, as represented in Figure 124.

Figure 124 - Block diagram represents the relationship between a power train control module and the EGR system.

Electronic EGR Valves. An electronic EGR valve, sometimes termed digital EGR valve, uses one or more electric solenoids to open and close its exhaust passages. It works without engine vacuum.

A single-stage EGR valve uses only one solenoid and valve. It is a simple, dependable EGR design, as shown in Figure 125, View A. To open one of the exhaust passages in the EGR valve, the ECM energizes its solenoid. When control current is sent to the solenoid windings, it pulls up on the metal armature connected to the valve. This lifts up the valve to open an exhaust recirculation passage. Exhaust gases flow through orifices to limit engine combustion temperatures and prevent NOx pollution. When the ECM stops current flow to the EGR solenoid, spring tension closes the valve to prevent exhaust flow into the engine.

Figure 125 - Cutaway view showing components of an electronic or digital EGR valve and a multistage EGR valve that uses three separate solenoids and valves.

A multi-stage EGR valve, as illustrated in Figure 125, View B, uses more than one (usually three) solenoid valves to more closely match exhaust gas flow to engine needs.

The solenoids mount on a base plate. When the valves are closed, they contact and seal against the base plate seats.

If only a small amount of exhaust recirculation gas is needed (combustion temperatures only slightly too hot), the ECM will only energize one of the EGR solenoids.

If combustion temperatures become hotter (engine conditions like speed load, or outside air temperature increase), the ECM will energize the other EGR solenoid as needed to increase exhaust gas recirculation flow. The added exhaust gases will decrease combustion temperatures to reduce NOx.

EGR System Service. EGR system malfunctions can cause engine stalling at idle, detonation, and poor fuel economy. If the EGR valve sticks open, it will cause a lean air- fuel mixture. The engine will run rough at idle or stall. If the EGR fails to open or exhaust passage is clogged, higher combustion temperatures can cause abnormal combustion (detonation) and knocking.

EGR System Maintenance. Maintenance intervals for the EGR system vary with vehicle manufacturer. Refer to the service manual for exact mileage intervals. Some vehicles have a reminder light in the dash. The light will come on when the EGR maintenance is needed. Also, check that the vacuum hoses in the EGR system are in good condition. They can become hardened, which can cause leakage, Also check for proper wire routing and for good electrical connections on digital EGR valves.

EGR System Testing (Vacuum Type). To test a vacuum EGR system, allow the engine to warm to operating temperature. Operating the accelerator linkage by hand, increase engine speed to 2000-3000 rpm very quickly. If visible, observe the movement of the EGR valve stem. The stem should move as the engine is accelerated. If it does not move, the EGR system is not functioning.

Sometimes the EGR valve stem is not visible. You will need to test each EGR system component separately. Follow the procedures described in the service manual.

To test the EGR valve, idle the engine. With the engine idling, connect a hand vacuum pump to the EGR valve, as demonstrated in Figure 126. Plug the supply vacuum line to the EGR valve. When vacuum is applied to the EGR valve with the pump, the engine should begin to miss or stall. This lets you know that the EGR valve is opening and that exhaust gases are entering the intake manifold.

Figure 126 - Testing the EGR system with a hand vacuum pump.

If the EGR valve operation does not affect the engine idle, remove the valve. The valve or the exhaust manifold passage could be clogged with carbon. If needed, clean the EGR valve and exhaust passage. When the EGR valve does not open and close properly, replace the valve.

EGR System Testing (Electronic Type). Most problems with electronic or digital EGR valves will trip a trouble code. A scan tool will isolate most problems quickly and easily. EGR valves that provide electrical data to a computer control system require special testing procedures. Refer to the shop manual covering the specific system.



Component damage could result from using an incorrect testing method.

The problem symptoms described also apply to a digital EGR. If not working normally, it can cause rough engine idle, high oxides of nitrogen, and other problems.

To pinpoint test a digital EGR valve, connect a hand-held scope to the wire going to the valve, as shown in Figure 127. Connect the scope to ground and probe through the EGR valve connector. The service manual wiring diagram will tell you which wires to probe. The scope's waveform will measure the voltage applied to the EGR from the ECM and it will also check the condition of the EGR windings. If voltage to the EGR is not indicated, check for a bad electrical connection. There could also be an ECM problem in the control circuit to the EGR valve.


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Figure 127 - An oscilloscope can be used to check digital EGR valves and their ECM control circuits.

Air Injection System. An air injection system forces fresh air into the exhaust ports or catalytic converter to reduce HC and CO emissions. The exhaust gases leaving an engine can contain unburned and partially burned fuel. Oxygen from the air injection system causes this fuel to continue to burn in exhaust manifold or the catalytic converter, as shown in Figure  128.

Figure 128 - Diagram illustrating the air injection system.

The air injection pump is belt-driven and forces air at low pressure into the system. The spinning vanes, or blades, pull air in one side of the pump. The air is trapped and compressed as the vanes rotate. As rotation continues, air is forced out of a second opening in the pump, as shown in Figure 129.

Figure 129 - Rear and side cutaway views of an air pump.

Electric air injection pumps are driven by a small dc motor, instead of being engine driven. This reduces emissions at start-up and with high engine temperatures because a more constant flow of air is produced by the electric motor. Air pump speed does not change with engine speed.

Air Injection System Components. Figure 130 shows the major parts of an air injection system. The diverter keeps air from entering the exhaust system during deceleration. This prevents backfiring, which is an explosive burning of the air-fuel mixture outside the combustion chamber in the exhaust system. The diverter valve also limits maximum system air pressure. It releases excessive pressure through a silencer or muffler. A plastic or rubber hose connects the pump output to a diverter valve.

Figure 130 - The basic parts of air injection system.

An air distributor manifold is used in air injection systems to direct a stream of air toward each engine exhaust valve. Fittings on the air distribution manifold screw into threaded holes in the exhaust manifold or cylinder head. Figure 130 shows a typical air distribution manifold.

An air check valve is usually located in the line between the diverter valve and the air distribution manifold. It keeps exhaust gases from entering the air injection system.

Air Injection System Operation. When the engine is running, the spinning vanes in the air pump force air into the diverter valve. If not decelerating, the air is forced through the diverter valve, check valve, air injection manifold, and into the engine exhaust ports.

The fresh air blows on the engine exhaust valves to keep any fuel burning as it leaves the engine.

During periods of deceleration, the diverter valve blocks airflow into the engine exhaust manifold. This prevents a possible backfire, which could damage the vehicle's exhaust system. When needed, the diverter valve's relief valve releases excess pressure.

Vehicles can also use the air injection system to force air into the catalytic converter. This is done to help the converter burn, or oxidize, the partially burned fuel more completely. A metal line runs from the air pump to the catalytic converter. Air from this line is forced into the converter.

Air Injection System Service. Air injection system problems can cause engine backfiring, other noises, and increased HC and CO emissions. Air injection is used to help burn fuel that enters the exhaust manifolds and exhaust system. Without this system, the fuel could ignite all at once with a loud bang or backfiring. Insufficient air from the air injection system could also prevent the catalytic converter from functioning properly.

Air Injection System Maintenance. Air injection system maintenance typically includes replacing the pump inlet filter (if used), adjusting pump belt tension, and inspecting the condition of the hoses and lines.

If the pump belt or any hoses show signs of deterioration, they should be replaced. Refer to the manual specifications for maintenance intervals.

Testing Air Injection Systems. A four- or five-gas exhaust analyzer provides a quick and easy method of testing an air injection system. Run the engine at idle and record the readings. Then, disable the air injection system and remove the air pump belt or use pliers to pinch the hoses to the air distributor manifold. Compare the exhaust analyzer readings before and after disabling the air injection system.

Without air injection, the exhaust analyzer's oxygen reading should drop approximately 2%-5%, while hydrocarbon and carbon monoxide readings should increase. This shows that the air injection system is forcing air into the exhaust system. If the analyzer readings do not change, the air injection system is not functioning. Test each component until the source of the problem is found.

To test the air pump, remove the output line from the pump. Use a low-pressure gauge to measure the amount of pressure developed by the pump at idle.Typically, an air pump should produce about 2-3 psi of pressure, as shown by Figure  131.

Figure 131 - A pressure gauge can be used to check air pump output.

If a low-pressure gauge is not available, place your finger over the line and check for pressure.

Replace the pump if faulty. When testing the diverter valve or other air injection system valves, refer to the service manual. It will explain testing procedures for the specific components.

Catalytic Converter. A catalytic converter oxidizes (burns) the remaining HC and CO emissions that pass into the exhaust system. Extreme heat (temperatures of approximately 1400°F) ignite these emissions and change them into harmless carbon dioxide and water.

A catalyst is any substance that speeds a chemical reaction without itself being changed. A catalytic converter contains a catalyst agent, usually the elements platinum, palladium, rhodium, or a mixture of these materials. Platinum and palladium treat the HC and CO emissions. Rhodium acts on the NOx emissions. Some newer converters also use cerium to extract and release additional oxygen into the exhaust stream.

The converter's catalyst agent is coated on either a ceramic honeycomb-shaped block or small ceramic beads. The catalyst is encased in a stainless steel housing that is designed to resist heat. The catalyst operating temperature is attained when the catalyst agents are hot enough (above 300°F) to start treating emissions.

Types of Catalytic Converters. A catalytic converter using a ceramic honeycomb catalyst is often termed a monolithic converter, as shown in Figure 132. Monolithic catalytic converters use a honeycomb- shaped block of ceramic material covered with catalytic elements to treat exhaust gases. The catalyst is enclosed in a stainless housing. When small ceramic beads are used, it is called a pellet catalytic converter, as illustrated in Figure 133.

Figure 132 - Monolithic catalytic.

Figure 133 - Pellet type catalytic.

A mini catalytic converter is a very small converter placed close to the engine exhaust manifold. It heats up very quickly to reduce emissions during engine warm-up. A mini catalytic converter is used in conjunction with a larger, main converter, as illustrated in Figure 134.

Figure 134 - This engine is equipped with a mini catalytic converter and a main converter.

A two-way catalytic converter, sometimes called an oxidation converter, can only reduce two types of exhaust emissions (HC and CO). A two-way converter is normally coated with platinum.

A three-way catalytic converter, also termed a reduction-type converter, is capable of reducing all three types of exhaust emissions (HC, C, and NOx). A three-way converter is usually coated with rhodium and platinum.

A dual-bed catalytic converter contains two separate catalyst units enclosed in a single housing. A dual-bed converter normally has both a three-way (reduction) catalyst and a two- way (oxidation) catalyst. A mixing chamber is provided between the two. Air is forced into the mixing chamber to help burn the HC and CO emissions, as illustrated in Figure 135.

Figure 135 - Cutaway of a dual-bed catalytic converter.

Dual-Bed Catalytic Converter Operation. When the engine is cold (below approximately 128°F), the air injection system routes the air into the exhaust manifold. Exhaust heat and the injected air are used to burn exhaust emissions. When the engine warms, the system forces air into the catalytic converter, as illustrated in Figure 136.

Figure 136 - Diagram shows how an air pump forces oxygen into a dual- bed catalytic converter.

First, the exhaust gases pass through the front three-way catalyst that removes HC, CO, and NOx. Then, the exhaust gas flows into the area between the two catalysts. The oxygen in the air flowing into the chamber causes the gases to continue to burn. The exhaust flows into the rear two-way catalyst, which removes even more HC and CO.

Some catalysts are coated with a material that absorbs and temporarily stores NOx emissions. When a saturation level is reached, the on-board computer temporarily enriches the fuel mixture. This causes the converter's internal honeycomb block to glow red hot, breaking up the stored NOx emissions into harmless by-products.

Catalytic Converter Service. Catalytic converter problems are commonly caused by contamination, overheating, and extended service. A clogged catalytic converter, resulting from deposits or overheating, can increase exhaust system back pressure.

High back pressure decreases engine performance because gases cannot flow freely through the converter.

A clogged catalytic converter is a fairly common problem. The increased back pressure will reduce engine power considerably. You may notice a rotten egg odor at the tailpipe. A clogged converter can also overheat, possibly causing a fire.

An exhaust back pressure test will check for a clogged catalytic converter and other system parts. Remove the front oxygen sensor and install a pressure gauge into the threaded hole, as shown in Figure 137.

Figure 137 -Testing the exhaust system's back pressure.

Start the engine and read the pressure gauge at idle and at higher speeds. If the pressure gauge reads too high, the converter, muffler, or an exhaust pipe is restricted. To isolate the exhaust restriction, disconnect parts one at a time. When the back pressure drops, the source of the restriction is found.

After extended service, the catalyst in the converter can become coated with deposits. Those deposits can keep the catalyst from acting on the hydrocarbon, carbon monoxide, and oxides of nitrogen. The inner baffles and shell can also deteriorate. With a pellet-type catalytic converter, this can allow BB-size particles to blow out the tailpipe.

Pellet catalytic converters normally have a plug that allows replacement of the catalyst agent. The old pellets can be removed and new ones installed. If the converter housing is damaged or corroded, replace the converter. Monolithic (honeycomb) catalytic converters must be replaced when the catalyst becomes damaged or contaminated.

Testing Catalytic Converter Efficiency. An exhaust gas analyzer can be used to check the general condition of the catalytic converter. Follow the specified directions provided with the analyzer. Warm and idle the engine. With some systems, it may be required to disable the air injection or pulse air system before performing the test. Measure the oxygen and carbon monoxide at the tailpipe.

If oxygen readings are above approximately 5%, you know there is enough oxygen for the catalyst to burn emissions. However, if the carbon monoxide readings are still above about 0.5% (other systems operating properly), then the catalytic converter is not oxidizing the emissions from the engine and the converter or catalyst requires replacement.

The scan tool can be used to diagnose catalytic converter problems on OBD II vehicles. The catalytic converter's condition is monitored by measuring its oxygen sensors-a pre- converter sensor and a post-converter sensor. Under normal conditions, the pre- converter oxygen sensor switches frequently and the post-converter sensor seldom switches. The pre-converter sensor switches more frequently because it "smells emissions," while the post-converter sensor "sniffs" cleaner gases.

Catalytic Replacement. To install new pellets in a catalytic converter, follow service manual instructions. It is required to use a special vibrating tool to shake the old pellets out of hole in the converter. Then, new pellets are installed and the service plug is replaced in the converter housing. This procedure is not used frequently since it is faster and easier to simply replace the catalytic converter.



Remember that the operating temperature of a catalytic converter can be over 1400°F. This is enough heat to cause serious burns. Do not touch a catalytic converter until it is determined it has cooled.

Catalytic Converter Replacement. On many vehicles, the converter can be unbolted from the exhaust system. Remove the clamps that secure the converter to the exhaust pipes. Then, use a muffler cutter or a chisel to cut or loosen the old converter from the exhaust pipes. Hammer blows to the converter should then free it from the vehicle.

Sometimes the catalytic converter is an integral part of the header pipe. With this design, the converter and pipe may have to be replaced together. When installing the new converter, use new gaskets and reinstall all heat shields, as demonstrated in Figure 138.

Figure 138 - Exploded view of a catalytic converter installation.

After replacing a catalytic converter, turn in the old converter to be recycled. The platinum and other precious metals are valuable.

Computerized Emission Control Systems. A computerized emission control system uses various engine sensors, a three-way catalytic converter, an ECM, electronic fuel injection, and other computer-controlled components to reduce pollution levels from the vehicle.

The ECM analyzes data from the many vehicle systems to monitor closely and control any function that can affect emissions.

Oxygen Sensors. The oxygen sensor monitors the exhaust gases for oxygen content. The amount of oxygen in the exhaust gases is a good indicator of the engine's operational state. The oxygen sensor's voltage output varies with any changes in the exhaust's oxygen content. For example, an increase in oxygen, which would indicate a lean mixture, will make the sensor output voltage decrease. A decrease in oxygen, which occurs during the rich mixture conditions, causes the sensor's output voltage to increase, as diagramed in Figure 139.

Figure 139 - Diagram oxygen sensor monitoring system.

In this way, the oxygen sensor supplies data to the computer. The computer can alter the opening and closing of the injectors to maintain a correct air-fuel ratio for maximum efficiency.

Primary and Secondary Oxygen Sensors. Newer vehicles are equipped with multiple oxygen sensors. The number of sensors used depends on the engine application.

A primary oxygen sensor, also termed front O2 sensor, is used to monitor the oxygen in the exhaust gases as they leave the engine. The signal from the primary sensor indicates whether the engine's air-fuel mixture is lean or rich. All primary sensors are located before, or in front of, the catalytic converters, usually as close to the engine as possible.

A secondary oxygen sensor, or rear O2 sensor, is mounted downstream in the exhaust system. Depending on its location downstream, the rear oxygen sensor can either be used to check the exhaust gases for oxygen content before the catalytic converter or monitor the converter for proper operation. Any O2 sensor mounted after a converter is referred to as a catalytic monitor.

Oxygen Sensor Position. Oxygen sensor position in the vehicle is assigned a number by its location and order in relation to the engine's banks. The sensor closest to the number one cylinder is denoted as Oxygen sensor, Bank 1, Sensor 1. If the engine is equipped with only one oxygen sensor, which is the case with OBD I vehicles, it is referred to as Oxygen sensor, Bank 1, no matter where it is located in the exhaust system. If the engine is a V-type, sensors located in the other bank are considered to be located on Bank 2. Sensors further down the exhaust stream from the engine are consecutively numbered as Sensor 2, Sensor 3, and so on, as shown in Figure 140. In almost all cases, the sensor with the highest number, such as Sensor 3, is the catalyst monitor.

Figure 140 - Oxygen sensor location and identification.

Heated Oxygen Sensors. A heated oxygen sensor (HO2S) uses an electric element to quickly warm the sensor to operating temperature. The heating element also stabilizes the temperature and operation of the sensor. The heating element allows the computer system to use the input signals sooner.

Zirconia Oxygen Sensor. Most heated O2 sensors are also called zirconia oxygen sensors because of their active materials. Zirconia and platinum are commonly used to produce the voltage output that represents oxygen in the exhaust gases. The platinum coating on the sensor surface causes any unburned fuel to ignite, which helps the sensor to maintain a high operating temperature. At any operating temperature of about 600°F, the oxygen sensor's element becomes a semiconductor and generates a small voltage. Figure 141 is an example of a heated zirconia oxygen sensor.

Figure 141 - Cutaway view showing the internal parts of a heated zirconia oxygen sensor.

The zirconia oxygen sensor has an inner cavity that is exposed to the atmosphere. Since the earth's atmosphere consists of approximately 21% oxygen, this percentage serves as a reference for the amount of oxygen in the exhaust gases. The outer surface of the oxygen sensor is exposed to the exhaust gases. The outer surface serves as the positive connection of the sensor circuit. The inner cavity of the sensor serves as the negative connection, or ground.

The difference between the oxygen content in the inner cavity and the oxygen content of the exhaust gases flowing over the sensor's outer surface causes the sensor to generate a voltage. The ECM compares the voltage produced by the sensor to a reference voltage of approximately 450 millivolts (0.45 volts).

For example, if the engine's air-fuel mixture is too rich, there will be almost no oxygen in the exhaust gases. This creates a large difference in oxygen content between the sensor's surfaces and causes the sensor to generate a voltage of about 600 millivolts (0.6 volts). This would inform the ECM to lean the mixture to reduce emissions.

With a lean air-fuel mixture going to the engine, there will be a smaller difference in oxygen content between the sensor's inner and outer surfaces. The sensor will generate a weaker voltage signal of about 300 millivolts (0.3 volts). The ECM will then enrich the fuel mixture and try to maintain a stoichiometric (chemically correct) air-fuel mixture.

Planar Oxygen Sensors. Vehicles are equipped with plannar zirconia oxygen sensors. These sensors work the same way as conventional zirconia sensors, but the zirconia element, electrodes, and heater are combined in a flat, laminated strip, as illustrated in Figure 142. The design of this type of sensor makes it more resistant to contamination and vibration than conventional zirconia sensors. Planar sensors also light-off, or reach operating temperature, in about 10 seconds, allowing the computer control system to enter closed loop twice as fast as systems with conventional heated oxygen sensors. This significantly reduces cold-start hydrocarbon emissions.

Figure 142 - Cutaway of a planar oxygen.

Titania Oxygen Sensors. A few vehicles are equipped with titania oxygen sensors. The main difference between titania sensors and zirconia sensors is the way they produce their output signals. Zirconia sensors generate their own voltage signals.

Titania oxygen sensors, on the other hand, vary their internal resistance to modify a reference voltage. Titania sensors offer several advantages over zirconia sensors. They provide an oxygen content signal almost immediately after cold startup, eliminating the need for a heating element. Titania sensors are smaller than zirconia sensors and are manufactured as sealed units, making them less susceptible to outside contamination.

During operation, a constant reference voltage is sent from the ECM to the titania sensor's positive terminal. As the oxygen content of the exhaust changes, the resistance of the sensor also changes (Figure 143). The amount of resistance formed in the sensor determines the sensor's voltage drop to a predetermined value; the ECM knows that the air-fuel mixture is too rich. If the sensor's voltage drop is below the predetermined value, the ECM knows the mixture is too lean. In either case, the control module can adjust fuel injection pulse width accordingly. Figure 143, View B is a diagram of a basic circuit from the titania oxygen sensor to the ECM.

Figure 143 - A-Cutaway view of a titania oxygen sensor and basic circuit.

Wide-Band Oxygen Sensors. There are vehicles equipped with wide-band oxygen sensors. As the air-fuel ratio changes, the wide-band oxygen sensor generates an internal voltage. The sensor then converts this voltage into a current. When the air-fuel ratio is lean, the current moves in the positive direction. When the air-fuel is rich, the current moves in the negative direction.

Unlike conventional oxygen sensors, which simply toggle their output voltage abruptly to indicate a lean or rich condition, the output of the wide-band sensor changes gradually and is directly proportional to the oxygen content of the exhaust gases. This makes it possible for the ECU to determine the exact air-fuel ratio at points other than stoichiometer, leading to more accurate control of the air-fuel ratio.

Oxygen Sensor Service. After prolonged service, oxygen sensors become coated or fouled with exhaust by-products. As this happens, fuel economy and emissions may be adversely affected. If gas mileage is 10% to 15% lower than normal, suspect a lazy oxygen sensor. A lazy O2 sensor will not alter its output signal fast enough to maintain an efficient air-fuel ratio. The sensor will be slow to change its voltage or resistance with changes in exhaust content. Figure 144 provides a quick reference for emission control system diagnosis.

A bad oxygen sensor will affect engine performance and emissions. If it does not work properly, fuel metering will be too lean or too rich. A bad rear oxygen sensor (catalyst monitor) may not detect an inoperative catalytic converter. A dead O2 sensor has little or no resistance or voltage output change. Even when the exhaust content changes, the sensor's signal remains almost constant.

Testing Oxygen Sensors. Most O2 sensor problems will trip a trouble code with OBD I and OBD II systems. However, there are times when an oxygen sensor will be close to but not out of its operating parameter and will not trip a trouble code. Even if the scan tool shows a problem with the O2 sensor, pinpoint tests will also be needed to verify the source of the trouble.

If the scan tool readout shows that the O2 sensor output voltage is abnormal, you might want to measure the sensor's output voltage with a multimeter. An oscilloscope can also be used to check the signal leaving the O2 sensor. When testing a titania oxygen sensor, it may also be necessary to check the reference voltage supplied by the ECM, as well as the sensor's resistance or voltage drop. Refer to the service manual for details.

By comparing actual voltage (zirconia-type sensor) or resistance levels (titania-type sensor) to scan tool readout values and manufacturer's specifications, it can be determined whether the sensor, wiring, or ECM is at fault.

If using a dual trace scope (scope has two test leads and can display two separate wave forms at once), you can compare the signals from the front and rear oxygen sensors on OBD II vehicles. If the voltage levels from the sensors are too similar, you may have a faulty catalytic converter.

If having trouble isolating an oxygen sensor-related problem, refer to the factory service manual. It will give specific information to help find the source of the problem Testing methods for wide-band oxygen sensors are similar to those for conventional oxygen sensors. Again, refer to the manufacturer's service manual for specific instructions, as testing methods may vary.

Oxygen Sensor Replacement. If the oxygen sensor is defective, first disconnect the negative battery cable. Then, separate the sensor from the wiring harness by unplugging the connector. Never pull on the wires themselves, as damage may result. Spray the sensor threads with a generous coat of penetrating oil. Use a special sensor socket to remove the sensor from the exhaust system, Use care to avoid thread damage. Inspect the sensor for signs of contamination.

Obtain and install the correct replacement oxygen sensor. Start the sensor by hand and then tighten the sensor with a wrench socket. Do not overtighten and damage the sensor during installation. Reconnect the wire connector and check system operation.

Emission Control System Diagnosis
Condition Possible Causes Correction
Excessive hydrocarbon reading 1. Poor cylinder compression

2. Leaking head gasket

3. Ignition misfire

4. Poor ignition timing

5. Defective input sensor

6. Defective output sensor

7. Defective ECU

8. Open EGR valve

9. Sticking or leaking injector

10. Improper fuel pressure

11. Leaking fuel pressure regulator

12. Oxygen sensor contaminated or responding to artificial lean or rich condition

13. Improperly installed fuel filler cap  

Test components. Service or replace as necessary.
 Excessive carbon monoxide reading 1. Plugged air filter

2. Engine carbon loaded

3. Defective sensor

4. Defective ECM

5. Sticking or leaking injector

6. Higher than normal pressure regulator

7. Leaking fuel pressure regulator

8. Oxygen sensor contaminated or responding to artificial lean or rich condition

Test Components. Service or replace as necessary.
Excessive hydrocarbon and carbon monoxide readings 1. Plugged PVC valve or hose

2. Fuel-contaminated oil

3. Heat riser stuck open

4. Air pump disconnected or defective

5. Evaporative emissions canister saturated

6. Evaporative emissions purge valve stuck open

7. Defective throttle position sensor

Test Components. Service or replace as necessary.
 Excessive oxides of nitrogen  1. Vacuum leak

2. Leaking head gasket

3. Engine carbon loading

4. EGR valve not opening

5. Low fuel

6. pressure

7. Low coolant level

8. Defective cooling fan or fan circuit

9. Oxygen sensor grounded or responding to an artificial rich condition

10. Fuel contaminated with excess water

Test Components. Service or replace as necessary.
Excessively low carbon dioxide reading 1. Exhaust system leak

2. Defective input sensor

3. Defective ECU

4. Sticking or leaking injector

5. Higher-than-normal fuel pressure

6. Leaking fuel in

Test Components. Service or replace as necessary.
Low oxygen reading 1. Plugged air filter

2. Engine carbon loaded

3. Defective input sensor

4. Defective ECU

5. Sticking or leaking injector

6. Higher-than-normal fuel pressure

7. Leaking fuel pressure regulator

8. Oxygen sensor contaminated or responding to artificial lean condition

9. Defective evaporative emission system valve.

Test Components. Service or replace as necessary.
 High oxygen reading 1. Vacuum leak

2. Low fuel pressure

3. Defective input sensor

4. Exhaust system leak

Test Components. Service or replace as necessary.

Figure 144 - Emission control system diagnosis quick reference.

Diesel Particulate Filters (DPFs). Diesel particulate filters have been around for a number of years usually in special applications, such as a garbage packer, required to operate inside for long periods of time. On this type of application, they were usually known as particulate traps. A DPF will be found on every highway diesel engine meeting 2007 emissions and beyond. A DPF is an aftertreatment device designed to eliminate soot produced by the engine cylinder combustion process.

There are three general types of DPFs in use:

  1. Catalyzed. This is the most common type of DPF. It is built into the aftertreatment canisters that also contain multistage catalytic converters and what used to be known as the muffler. Catalyzed DPFs will be found on most highway trucks and are capable of both passive and active regeneration cycles.
    1. A passive (diesel) particulate filter, as illustrated in Figure 145, is a standard, ceramic honeycomb structure with alternate plugged channels. A catalyst is typically upstream of the filter but may be integrated into it. In certain cases, a washcoat layer of catalytic material may be placed on the filter in addition to the catalyst.

Figure 145 - Passive (diesel) particulate.

Passive particulate filter systems rely exclusively on chemistry to regenerate. As carbon moves through the exhaust, it is trapped by the particulate filter. That filter begins to build up with carbon and other residual exhaust species, which have also become trapped. This will continue until conditions become favorable for the elements to combust internally off the filter surface. The heat for this regeneration comes from the thermal energy that occurs naturally with exhaust temperatures of typical diesel engines.

  1. Active particulate filters are generally used in instances where passive particulate filters will not work because there are not sufficient natural activation energy levels to achieve regeneration.

Active particulate filters rely on outside energy sources. In an active particulate filter, the particulate matter is trapped as it works its way through the exhaust. The control system then activates the regeneration process by auxiliary means, which raises the temperature enough to activate the chemical reaction. Active regeneration is technically possible in several ways.

  1. Noncatalyzed. This type of DPF is required in diesel-powered vocational vehicles that are operated under low engine loads for long periods. Typical applications are cement trucks, fire trucks, and garbage packers. Non catalyzed DPFs may only be capable of active regeneration cycles that require the vehicle to be stopped.
  2. Low temperature particulate filters. These are usually aftermarket devices fitted to pre-2007 highway diesels. Many have been fitted to school buses. While being capable of significantly reducing PM, they usually do not achieve this to EPA 2007 standards.

DPF Operating Principles. The idea behind a DPF is that engine-emitted soot first collects on the walls of the device. Engine manufacturers design DPFs to function primarily in self-regeneration mode. This means that when soot collection reaches a threshold level, it is burned off in what is known as a regeneration cycle. In most cases, the regeneration cycle will occur when exhaust temperatures are sufficiently high during normal operation. When this regeneration takes place unassisted by additional fuel or air injection to the exhaust gas, it is known as self-regeneration.

Self-regeneration is also known as passive regeneration. The terminology varies by engine OEM.

Operating temperatures of the combined oxidation converter and PDF can exceed 1112°F, so most of these devices have plenty of heat shielding. Depending on the engine and its power rating, both single and duel canister versions are used. Typically, single and dual canisters are specified by horsepower rating as follows:

Active Regeneration. When the operating environment is not conducive to a self- regeneration cycle, regeneration can also occur when assisted by injection of some fuel ignited by a spark plug or induced high exhaust temperatures. This mode of regeneration is known as active regeneration. Fuel for an active regeneration cycle is usually sourced from the fuel subsystem and delivered at the specified charging pressure. Most diesel engine OEMs are using similar DPF but they are managed in different ways.

Frequency of Cycles. Regeneration cycles are designed to occur at set intervals. These set intervals may be as often as once every hour, or as frequently as once every 8 hours of operation, depending on the engine, its power ratings, and how the engine is being operated. Operators should be informed about the expected intervals of the regeneration cycles of the equipment they operate. During both passive and active regeneration, there will be an increase in exhaust gas temperatures.

Ash Residues. Regeneration leaves some ash residues. These ashes primarily originate from the additive package in the engine lube that is burned in the cylinder during normal combustion. Ash residues have to be removed manually. Federal law requires that this cleaning process occurs no more frequently than once a year or every 150,000 miles (whichever comes first).

Temperature Monitoring. Temperature monitoring is by dual and single thermocouples (pyrometers). A thermocouple has a hot end and a sensing end. Two dissimilar wires, one being pure iron and the other constantin wire that is 55 percent copper and 45 percent nickel, are arranged to form a circuit. They are wound together at the hot end, as illustrated in Figure 146. When the hot end is exposed to heat, a small voltage is created. This small voltage is read by a millivoltmeter at the sensing end and displayed in temperature values. Thermocouple devices must be replaced as a complete unit when diagnosed as failed. The DPF is a complex assembly within which the regeneration cycles and temperatures have to be precisely managed.

Figure 146 - A pyrometer.

High Exhaust Pyrometer Readings.

  1. Install a master pyrometer (do not rely solely on the vehicle pyrometer).
  2. Chassis dynamometer test using a full instrumentation readout.

A pyrometer is an operator's instrument as much as it is the mechanic's. A complaint of high pyrometer readings without other symptoms may be an indication that the operator requires some additional training. The operator should consider a pyrometer reading approaching its maximum as a signal to downshift gear ratios.

Some possible causes and solutions are:

DPF Cleaning Station. Most of the engine OEMs state that their DPFs will require routine in-shop cleaning using special equipment. Although some early systems could be cleaned while on chassis, all current systems require out-of-chassis cleaning stations. This high-flow-back pressurizes the system for a period of around 20 minutes. The total time the PDF cleaning procedure usually takes less than 2 hours. Figure 147 shows a DPF cleaning system.

Figure 147 - Diesel particulate filter cleaning system.


An appropriate respirator should be worn when servicing DPF components. Submicron ash particulate is known to cause respiratory problems.

Selective Catalytic Reduction (SCR). Like DPF, Selective Catalytic Reduction is also an exhaust gas aftertreatment process. While the DPF addresses particulate HC, an SCR system attempts to "reduce" oxides of nitrogen back to nitrogen and oxygen.

Although the SCR has been used in Europe for a decade, the EPA did not approve SCR use until 2007. The reason for the reluctance on the part of the EPA in approving SCR systems is that to function, this type of system depends on using a "consumable," specifically urea. The urea that is in aqueous form has been routinely refilled, just like fuel. This means that the EPA approval required some conditions.

Aqueous Urea. SCRs use consumable urea to achieve what rhodium does in gasoline- fueled engines. Urea is composed of crystallized nitrogen compounds sourced from natural gas. The urea, in a solution with water, is known as aqueous urea. It is also referred to as diesel exhaust fluid, or DEF. It is a nontoxic solution of 67.5 % deionized water and 32.5% automotive grade urea. It is stable, odorless, colorless, and meets accepted international standards for purity and composition. The aqueous urea is injected into the exhaust gas stream by a computer-controlled injection system. After injection, the urea reduces to ammonia, which itself reacts with NOx compounds, reducing them back to oxygen and nitrogen.

Selective Catalytic Reduction Management. SCR is managed by the engine ECM. The urea is contained in aqueous/water form in a replenishable vessel. It is injected upstream from the converter/DPF/muffler assembly sometimes assisted by on-chassis compressed air. The urea injection has to be precisely metered by the ECM. Too much urea can result in ammonia discharge through the exhaust system, while too little results in NOx emissions. Figure 148 shows a schematic of SCR components and operation.

Figure 148 - Schematic of a selective catalytic reduction components and operation.

Urea is carried onboard the truck in tanks with capacities ranging from 20 to 50 gallons. An aqueous urea solution freezes at 12°F, so it must be freeze-protected in winter operation. The urea is consumed at a rate that varies between approximately 1.5 percent and 10 percent of the fuel used, the variability depending on how the engine is being operated, that is, how much NOx has to be reduced.

Closed Crankcase Ventilation (CCV). Highway diesel engines meeting 2007 emissions standards are required to have closed crankcase ventilation systems, as shown in Figure 149. CCV is the diesel engine equivalent of automotive positive crankcase ventilation (PCV) systems. The objective of a CCV system is to prevent venting to atmosphere of crankcase gases. Crankcase gases consist of:

Figure 149 - Cutaway of a Closed Crankcase Ventilation System.

CCV Operation. Diesel CCV systems can be directly compared with PCV circuits used on automobiles. The system is plumbed upstream from the turbocharger impeller housing so that some pull is exerted on the crankcase. The CCV piping is routed through a filter assembly and may be located on top of the rocker housing cover. OEM recommendations for CCV service intervals should be observed.

CCO Injectors. Elimination or reduction of nozzle sac volume in hydraulic injection injectors reduces the cylinder boil/dribble of sac fuel at the completion of injection. This nozzle design principle is used in all injectors.

Charge Air Cooling. Effective cooling of intake air lowers combustion temperatures, making it less likely that the nitrogen in the air mixture oxidized to form NOx. Air-to-air charge air coolers cool air more effectively than those that relied on engine coolant. Note that anything that compromises the charge air cooler's ability to cool will result in higher NOx emissions. This is why codes are logged when charged air heat exchangers become plugged.

Variable Geometry Turbochargers VGT). Variable geometry turbochargers that perform effectively over a much wider load and rpm range can make a significant difference to both HC and NOx emissions, especially when ECM controlled, by providing the ability to manage boost on the basis of the fueling and emissions algorithms. VGTs are used rather than constant geometry turbochargers in almost every 2007-compliant highway diesel engine. When a constant geometry turbocharger is used today, it is usually as one of the pair used in series turbocharged engines.

Low Headland Volume Pistons. Low headland volume pistons raise the upper compression ring close to the leading edge of the piston crown. This keeps the headland gas volume close to minimum. Headland gas volume tends to be unclean and can increase HC emissions. The requirement for low headland volume pistons by engine designers has resulted in some radical design changes in diesel engine pistons within a short period of time. Most diesel engine manufacturers today favor trunk-type pistons, such as the Mahle Monotherm®, which features low headland volumes, as shown in Figure 150. Although forged steel trunk pistons have been around for quite some time, primarily in drag racing, they have evolved to being used in diesel engine technology. The advantages of forged steel trunk pistons are reduction of headland volume, thermal expansion, long life, and lightweight.

Figure 150 - Cutaway view of a forged steel trunk Mahle Monotherm® piston.

Opacity Meters. Opacity meters measure visible smoke emissions. They are simple to use and require that the unit's sensor head probe be fitted to the outlet of the exhaust pipe. Two different types are used:

  1. Partial flow
  2. Full flow

In the partial flow type, during the test procedure, a portion of the exhaust gas flow is diverted to a sensing chamber, in which the opacity of the smoke can be read by the opacity measurement directly at the stack outlet.

Light Extinction. Partial and full flow opacity meters are classified as light extinction test instruments. A beam of light from a light-emitting diode (LED) is directed at a photo diode sensor through the exhaust gas. The amount of light blocked from the photo diode sensor (by smoke) is determined by the density of the engine exhaust. The higher the density, the less light can pass through the smoke to be read by the sensor. Smoke density is expressed as percentage reading by the opacity meter. In most cases, the opacity meter is equipped with an extension handle for the sensor head, so the device can be placed at the exhaust stack outlet by the mechanic working at the ground.

Opacity meter readings are displayed in percentages. The actual readings vary according to engine horsepower and the diameter of the exhaust pipe(s). Most should produce accurate results regardless of weather conditions, and many will record the data electronically. Data recorded by the opacity meter can be hard-copy printed or transferred to PC- or Web-based systems for analysis. Any opacity meter can be used to evaluate smoke density in diesel smoke. Those used for official testing and to enforce compliance must be approved and have PC-managed and logged test sequences.

Weather Conditions. Weather conditions can have some impact on smoke density, and changes in air density influence the operation of any internal combustion engine. For this reason, SAE J1667 imposes restrictions on the environmental conditions during the administration of an "official" snap acceleration test.

Environmental Conditions. The following environmental conditions are required when performing a J1667 test:

Snap Test. Each J1667 throttle snap consists of a three-phase cycle that must be precisely executed by the person doing the test:

  1. Accelerator is snapped to high idle and held for 1 to 4 seconds or until prompted by the opacity meter to release.
  2. Engine rpm must be allowed to drop to the specified idle speed.
  3. Engine must run at the specified low idle for a minimum of 5 seconds and a maximum of 45 seconds before initiating the next snap, as prompted by the opacity meter.

SAE J1667 Test Cycle. The SAE J1667 test cycle consists of four phases and, again, these must be precisely executed:

  1. Preliminary Snaps. Three full snaps are required to clear the exhaust system of loose particles and to precondition the vehicle.
  2. Official snaps. Three official snaps should be undertaken, as prompted by test instrument software.
  3. Validation. The difference between the highest and lowest maximum opacity readings of the three official test snaps should be within five opacity percentage points. If the difference is less than 5 percent, the meter software will compute the average. If the difference is greater than 5 percent, additional official snaps must be undertaken up to a maximum of nine.
  4. Drift factor. Validation is required that the drift (rpm at full pedal travel) between three official test cycles does not exceed 2 percent opacity. A variation greater than ±2 percent will result in an invalid test. This is not a tough spec to meet: 2 percent of 2,000 rpm translates to 40 rpm.

Stack Dimensions. Exhaust pipe diameter has an effect on the opacity reading of exhaust gas. Before undertaking a snap test, the diameter of the exhaust stack has to be known. For instance, an engine that tests at 30% opacity when tested with a 3-inch exhaust stack will only measure 20% opacity when tested with a 5-inch tailpipe. This is why you have to input the stack diameter into the opacity meter.

Although there are exceptions because horsepower rating usually determines the stack diameter, it is recommended that the horsepower rating be inputted into the opacity test instrument before the exhaust pipe diameter. Horsepower rating is often specified on the engine ID plate or in the electronic service tool (EST) accessible read-only data.

Any truck diesel engine that fails an opacity snap test has a severe smoking problem, one that could not pass unnoticed under any circumstances. The way we see smoke depends on the state of the emission. All matter has three states: vapor or gaseous, liquid, and solid. It is possible to emit matter in all three states from a diesel engine tailpipe.

Black Smoke. The term black smoke is used to describe anything from grayish tinge, in exhaust smoke, to heavily sooted emission. It is the result of the incomplete combustion of fuel and therefore has many causes.

Blue Smoke. Blue smoke is usually caused by lube oil getting involved in the combustion process.

Some possible causes are turbocharger seal failure, roots blower seal failure, pullover of lube oil from oil bath air cleaner sump, worn valve guides, ring failure, glazed cylinder liners, high oil sump level, excessive big end bearing oil throe-off, low-grade fuel, fuel contaminated with automatic transmission fluid (ATF), or engine lube placed in fuel tanks as an additive.

White Smoke. White smoke is caused by condensing liquid in the exhaust gas steam. Temperature usually plays a role when white smoke is observed, both ambient temperature and the engine-operating temperature. Remember that water is a natural product of the combustion of any hydrocarbon fuel and in mid-winter conditions, it is normal for some of this to condense in the exhaust gas. However, when the white smoke is a problem, the following are some of the possible causes:


- To Table of Contents -


This manual has provided information for the construction mechanic in the overhauling and troubleshooting fuel systems for automotive, construction, and support equipment.

In the manual, you were introduced to gasoline fuel injection systems, which primarily spoke of the throttle body. You then covered small engine carburetors, which are primarily used with support equipment and gasoline power tools.

You covered the various diesel fuel injection systems, covering such components as injectors, fuel pumps, and governors, along with the air induction systems that included superchargers and turbochargers. Additionally, the principles of the diesel engine cold starting devices, such as glow plugs and ether, were covered.

Lastly, you learned the components, purpose, and troubleshooting of the emission system.

- To Table of Contents -

Review Questions

1. Gasoline injection system has several advantages over a carburetor-type fuel system except for _____.

A. Increased engine power
B. Better fuel distribution
C. Higher emissions
D. Improved atomization

2. The fuel injector for an EFI system is simply what?

A. a coil- or solenoid-operated fuel valve
B. a regulated fuel injector
C. a coil- or diode-operated fuel tap
D. a fuel valve only

3. Generation two oxygen sensors are installed where?

A. Only one before the catalytic converter
B. One before or one after the catalytic converter
C. One at the fuel injector and one before the catalytic converter
D. One before the catalytic converter and one after

4.  The oxygen sensor functions immediately when the vehicle is started.

A. True
B. False

5. Which sensor is important for determining computer output?

A. manifold absolute pressure sensor
B. Throttle position sensor
C. Engine coolant temperature sensor
D. Airflow sensor

6. Which sensor sends an electronic signal proportional to the pressure inside the rail to the ECU?

A. Fuel pressure sensor
B. Fuel temperature sensor
C. Fuel injection sensor
D. Intake air temperature sensor

7. The throttle body injection has _____.

A. One or two injectors delivering diesel fuel from one central point in the intake manifold
B. One or two injectors delivering gasoline from multiple points in the intake manifold
C. One or two injectors delivering gasoline from one central point in the intake manifold
D. One injector delivering gasoline to two central points in the intake manifold

8.  On the throttle body injection system, the fuel injector is a cam-operated fuel valve.

A. True
B. False

9. What is the most common type of service performed on gasoline fuel injection systems?

A. Preventive
B. Interim
C. Injector
D. Depot

10. Any pressure less than atmospheric pressure is generally referred as what?

A. combustion
B. flow
C. vaporization
D. vacuum

11. The narrowed portion of the carburetor is known as what?

A. venturi
B. bernoulli
C. throttle
D. vortex

12. Which component of the carburetor holds the fuel for use by the different metering circuits?

A. metering tube
B. bowl vent
C. fuel container
D. fuel passages

13.  The bowl vent may be an external or internal type.

A. True
B. False

14. What is the component that connects the carburetor's bowl to the engine side of the throttle plate?

A. high-speed air bleed
B. idle passage
C. idle air bleed
D. bowl vent

15. Which carburetor system lifts the diaphragm both mechanically and pneumatically?

A. choke
B. primer
C. emission
D. enrichment

16. When using the model number of a Briggs & Stratton carburetor, which number is used to identify the type of carburetor?

A. first
B. second
C. third
D. fourth

17.  On a Vacu-Jet carburetor, multiple fuel pickup tubes are one of the design features.

A. True
B. False

18. The final adjustment on a Vacu-Jet carburetor specifies what rpm?

A. 1675
B. 1700
C. 1750
D. 1775

19. What three components on the Pulsa-Jet carburetor serve the same functions as the gravity feed tank, float, and float chamber on a conventional float-type carburetor?

A. Primer, choke, and fuel tube
B. Fuel tank, fuel pump, and constant level fuel chamber
C. Fuel cup, fuel diaphragm, and constant fuel passage
D. Fuel tank, fuel injector, and constant fuel level float

20. How is the fuel pump actuated on the Pulsa-Jet fuel carburetor?

A. Changes in pressures in the air horn or throat
B. Pulses of air in the fuel nozzle
C. Changes in pressures in the fuel tank
D. Steady pressures in the air horn or throat

21. In a Pulsa-Jet carburetor, the diaphragm separates the air compartment and spring from what compartment?

A. Venturi
B. Oil
C. Lubrication
D. Fuel

22. In a Pulsa-Jet carburetor, what is one the design variations?

A. Adjustable and non-adjustable jets in the fuel pickup
B. Non-integral venturi
C. Non-vented fuel tanks
D. Choke plate shaft dust stop

23. What component does the wet bulb primer eliminate the need of on the Pulsa- Prime carburetor?

A. Manual choke
B. Automatic choke
C. Pump diaphragm
D. Automatic fuel pump

24. Why would you remove the main air jet air bleed on a Pulsa-Jet Prime carburetor?

A. High-altitude compensation
B. Low-altitude compensation
C. Do not really need it
D. Never remove it

25.  The automatic choke on the Vacu-Jet and Pulsa Jet carburetors is opened by atmospheric pressure in front of the venturi.

A. True
B. False

26. During your automatic choke inspection and after going through all the procedures, what may be the cause for the engine to run at idle when you adjust the mixture so lean that the engine should stop and the needle valve is closed?

A. Air leak at the diaphragm
B. Fuel leak in the venturi
C. Fuel leak at the diaphragm
D. Pump spring stuck

27. For the Bimetal choke, the springs have been color coded according to their _____.

.A. Function
B. Gauge
C. Length
D. Strength

28. What is the feature related to the Flo-Jet carburetor relating to fuel feed?

A. Gravity fed
B. Pressure fed
C. Vacuum fed
D. Pump fed

29. Once removing the packing nut and high-speed needle valve, what tool do you use to remove the nozzle?

A. Tapered screwdriver
B. Un-tapered screwdriver
C. Phillips screwdriver
D. Needle-nose pliers

30. What is one of the causes for carburetor leak on a Flo-Jet carburetor that is typically caused by transport?

A. Float-bind
B. Float-freeze
C. Float-jump
D. Float-bounce

31.  Lapping compound to remove corrosion is one of the options restore a nozzle of a two-piece Flo-Jet carburetor.

A. True
B. False

32. Which carburetor has the ability to be used regardless of the position of the fuel tank?

A. Flo-Jet
B. Diaphragm without pump
C. Diaphragm with pump
D. Pulsa-Jet

33. In a diaphragm pump, what type of tip may the inlet needle have to resist exotic fuels and wear?

A. Viton
B. Brass
C. Rubber
D. Carbon steel

34. What is required to start a cold engine with a diaphragm pump?

A. Lean fuel mixture
B. Choke wide open
C. Throttle half open
D. Rich fuel mixture

35. Why is it imperative for a proper seal of the pump gasket between the fuel intake chamber, pulse chamber, and the inlet and outlet valve area on a diaphragm pump?

A. To prevent cross-leaking
B. To prevent air choke
C. To prevent fuel choke
D. To prevent oil leak

36. When reinstalling the inlet needle/fulcrum spring, which is a tedious procedure, you can use what type of grease to help the process?

A. Axle
B. Lithium-based
C. Bearing
D. Teflon-based

37. During the initial adjustment of a diaphragm pump, after closing the high-low speed mixture needles, what is the next procedure?

A. Turn them clockwise to unseat them.
B. Turn the idle screw about 1½-turns.
C. Turn the screws outward about 1½-turns.
D. Turn the screws inward about 1½-turns.

38. If you do not have the specification settings available for the final adjustments of the diaphragm carburetor, you can set the idle between what rpm?

A. 1500 and 2000
B. 2000 and 2500
C. 2500 and 3000
D. 3000 and 3500

39. The main components of a fuel injection pump using a sleeve metering fuel system is _____.

A. The housing, plunger, and bushing
B. The body, barrel, and filter
C. The plunger, sleeve, and casing
D. The barrel, plunger, and sleeve

40. In a Caterpillar sleeve metering fuel injection system, fuel injection begins at what point?

A. As the plunger moves upward and closes the inlet port
B. As the plunger moves downward and closes the inlet port
C. As the barrel moves upward and closes the inlet port
D. As plunger moves upward and closes the outlet port

41. After normal start-up, at what rpm does the governor take over in a Caterpillar sleeve metering fuel injection pump?

A. 300
B. 400
C. 500
D. 600

42. The type governor that will most likely be found on a Caterpillar bulldozer is ______.

A. A limiting speed governor
B. A variable speed governor
C. A constant speed governor
D. A wavering speed governor

43.  An over run occurs when the governor allows the engine to exceed its maximum rated speed.

A. True
B. False

44. Hunting or surging is a continuous engine speed fluctuation and can be controlled by a/an _____.

A. Adjustable spring
B. Bumper screw
C. Adjustable screw or bumper screw
D. Adjustable screw or bumper spring

45. A Stanadyne pump has its own mechanical _____.

A. Plunger
B. Head
C. Sleeve
D. Governor

46. What regulates the volume of fuel in the hydraulic head?

A. Metering valve
B. Cam ring
C. Intake valve
D. Metering port

47. In the Stanadyne pump, before reaching the transfer pump the fuel must pass through which components?

A. A water separator and inlet ports
B. A water separator only
C. A water separator and primary fuel filter
D. A water separator and secondary fuel filter

48. When the engine is running, the pump rotates and fuel is pulled into the end plate by the _____.

A. Bypass valve
B. Transfer pump
C. Pump piston
D. Plunger

49. In the discharging cycle of the distributor injection pump, as the rotor continues to revolve, the inlet passages move out of alignment with the charging ports. What happens next?

A. The rotor closes the discharge port to the heads inlet.
B. Fuel is trapped between the plungers.
C. The rotor discharge port opens one of the heads outlets.
D. The rotor injection port closes one of the heads inlets.

50. What keeps the lines full of fuel so that a full charge of fuel can be injected at the next cycle for a cylinder after the discharging cycle?

A. Rotor
B. Delivery port
C. Distributor valve
D. Delivery valve

51. When fuel pressure decreases because of reduction in the engine speed, fuel from the position area drains through the orifice below what to allow the cam ring to retard?

A. Ball check valve
B. Ball relief valve
C. Retard cam
D. Cam ring valve

52. During the mechanical governor operation of the distributor injection pump, the flyweights transmit force through the thrust valve, causing the lever to _____ .

A. Move upward
B. Pivot
C. Rotate
D. Alternate

53. The governor may be equipped with a/an housed within the governor control cover _____.

A. Electrical rotor
B. Mechanical shut-off device
C. Hydraulic shut-off device
D. Electrical shut-off device

54.  Some energized-to-run systems employ a mechanical override device for emergency use if the coil becomes inoperative due to electrical failure.

A. True
B. False

55. What are some of the causes for low pressure in the transfer pump?

A. Restricted fuel supply or air leaks on the suction side of the pump
B. Worn transfer pump blades or a malfunctioning regulator valve
C. Both A and B

56. When you are testing a Stanadyne fuel injection pump, a reading of how many inches of vacuum indicates a restricted fuel supply?

A. 7
B. 8
C. 9
D. 10

57. Each time a fuel injection pump is overhauled, which of the following parts is/are always replaced?

A. Springs
B. Seal Seats
C. O-rings and seals
D. Timing plate

58. Of all the fuel injection systems, what was most readily adaptable of all the fuel injection systems to electronic control?

A. Electronic element injectors
B. Electronic unit injectors
C. Electrical fuel insertion
D. Electronic component module

59. In the Delphi EUI, what component is a solenoid consisting of a coil and armature with an integral poppet control valve?

A. Control cartridge
B. Control cylinder
C. Plunger
D. Pump element

60. The actuator components of the electronic unit injection system use what kind of action to create the pressures needed for injection?

A. Hydraulic
B. Electronic
C. Mechanical
D. Fluid

61. What happens when the solenoid on the EUI is energized and its armature is pulled upward, closing the poppet valve and descending the plunger?

A. Creates a rapid rise in pressure within the fuel supply chamber
B. Causes a decrease in pressure within the fuel supply
C. The spray tip assembly receives air to mix with the fuel
D. Creates a rise in air pressure

62.  An electronic unit injector's start and duration of injection is controlled by the pulse width from the ECM.

A. True
B. False

63. On the Stanadyne electronically controlled distribution pump, if one or two sensors fail, the engine will run only at limited power. If all three sensors fail, the engine will _____.

A. Idle down and stop
B. Run only at idle
C. Increase idle
D. Run normally

64. Which manufacturer developed the pressure-timed fuel injection system?

A. Cummins
B. Caterpillar
C. Detroit Diesel
D. International Harvester

65. In reference to the PT fuel system, what does the T refer to?

A. Time elapsed between injections
B. Time for fuel to flow into injector body
C. Time between piston strokes
D. Time available for the fuel to flow into the injector cup

66. Which of the following is not a function of the pressure-time governed fuel pump?

A. Control of exhaust smoke during acceleration
B. Idle and maximum speed governing
C. Rail pressure to the injectors
D. Startup of the engine

67. What type of governor is installed in a PT fuel pump?

A. Simple flyweight-operated electronic governor
B. Simple flyweight-operated mechanical governor
C. PT pump is unique not having a governor
D. The governor is mounted on the engine separate form pump

68. The engine's power output can be controlled by the operator, within the established governor limits through the use of what?

A. The throttle
B. The rail
C. Governor link

69. When there is little or no air pressure applied to the AFC diaphragm, the maximum fuel pressure and flow are controlled by what?

A. AFC plunger
B. AFC spring
C. Check valve
D. No-air adjusting screw

70. In the PT fuel pump injector operation, when the cam follower roller is on the inner base circle, _____.

A. The injector return spring lifts the plunger.
B. The plunger lifts the return spring.
C. The check ball moves upward.
D. The plunger moves downward.

71. During which stroke(s) is the metering orifice uncovered and fuel flows into the injector cup?

A. Intake stroke only
B. End of intake stroke and beginning of the compression stroke
C. Compression stoke only
D. End of compression stroke and beginning of intake stroke

72. On a PT-type fuel injection system, when should maximum fuel manifold pressure be obtained?

A. At idle
B. Just off idle
C. After the governor cuts in
D. Just before the governor cuts in

73. To remove carbon from the PT fuel injector tips, you use which of the following methods?

A. A wire brush
B. Reverse flushing
C. A pin vise and the proper size fine wire
D. Sand paper

74.  The aneroid controls the exhaust emissions by creating a lag in the fuel system equal to that of the turbocharger.

A. True
B. False

75. You need to take special care of the PT fuel pump when disassembling it because is made of _____ .

A. Plastic
B. Cast iron
C. Aluminum
D. Bronze

76. When rebuilding a PT-type fuel pump, parts should be discarded at what point?

A. When they only show minor wear
B. Only after they break
C. When they are worn beyond replacement limits
D. At each overhaul

77. To prevent goring of the PT fuel pump and pump parts in reassembly, the mechanic should use which of the following means?

A. Spring steel lock washers
B. Flat steel washers
C. Torque wrench
D. Extreme pressure lubricant

78. When a PT pump has been rebuilt, it should be run at 1,500 rpm for how long to allow the bearings to seat?

A. 2 minutes
B. 5 minutes
C. 10 minutes
D. 15 minutes

79. When servicing a PT fuel injector, you should NOT take which of the following actions?

A. Use cleaning solvent.
B. Use cleaning wire.
C. Use mineral spirits.
D. Soak the orifices to remove carbon.

80. Scavenging takes place during which strokes in a two-cycle engine?

A. End of downstroke; beginning of upstroke
B. End of upstroke; beginning of downstroke
C. Beginning of intake downstroke; dead center exhaust stroke
D. Dead center downstroke; end intake stroke

81. Superchargers pump a greater amount of air into n engine than could be supplied by normal atmospheric pressure. What is the effect on fuel consumption and power?

A. More fuel is burned; power is decreased
B. Less fuel is burned; power is decreased
C. More fuel is burned; power is increased
D. Less fuel is burned; power is increased

82. What are the three primary components of a turbocharger?

A. Turbine wheel, center housing, and compressor wheel
B. Turbine shaft, center support, and injectors
C. Exhaust tube, air pump, and compressor tube
D. Turbine housing, air injection, and compressor shaft

83. What is the component that sends compressed fresh air to the intake manifold?

A. Air inlet
B. Intake tube
C. Exhaust pipe
D. Crossover tube

84.  Lack of engine power, blue smoke, and increased engine power are some of the symptoms of turbocharger problems.

A. True
B. False

85. While inspecting the turbocharger, you discover the inner heat shield is distorted. What action should you take?

A. Replace with a new shield.
B. Replace the turbocharger.
C. Reuse, since it will straighten when bolted back on.
D. Heat up and straighten.

86. What is the proper shutdown procedure of a turbocharger to prevent bearing damage?

A. Idle down then shut off immediately.
B. Idle the engine for a few minutes.
C. Shutdown at high rpm.
D. None of the above.

87. What type of heater is mounted outside the engine and has the advantage of working in a short period of time?

A. Immersion heater
B. Intake air heater
C. Circulating coolant heater
D. Lube oil heater

88. Where on the engine is the lubricating oil heater generally installed?

A. Drain plug
B. Dipstick opening
C. Oil line
D. Both A and B

89. After the engine starts under cold conditions using a glow plug, how does the glow plug turn off?

A. Operator must turn glow plug switch to OFF
B. Putting transmission into drive
C. Control circuit disconnects glow plug
D. None of the above

90. When may ether be used as a diesel engine cold starting aid?

A. Only in extreme emergencies
B. Only in extreme cold weather
C. Any time
D. When the engine will not start after four attempts

91.  Diesel engines can become "dependent" upon ether.

A. True
B. False

92. Which exhaust emission is produced when unburned fuel escapes from the exhaust system of a poorly running engine?

A. Particulates
B. Oxide of nitrogen
C. Carbon monoxide
D. Hydrocarbons

93. What is the function of the exhaust gas recirculation system?

A. Recirculates engine crankcase fumes back into the combustion chamber
B. Injects burned exhaust gases into the engine to lower combustion temperatures and prevent the formation of NOx
C. Chemically changes combustion by-products into harmless substances
D. Forces outside air into the exhaust system to help burn unburned fuel

94. Pressure leakage past the piston rings on the blower stroke is referred to as what?

A. Engine blockage
B. Piston blowby
C. Engine blowby
D. Engine pressure

95. The PCV valve controls the air through the PCV system. It can be located in any of the following locations except the _____.

A. Valve cover
B. Intake manifold
C. Side of the engine block
D. Air breather

96. What is the purpose of the evaporative emissions control system?

A. Makes oil vapors condense and flow into the oil pan
B. Prevents toxic fuel vapors from entering the atmosphere
C. Prevents toxic fuel from entering the cylinders
D. Regulates engine crankcase ventilation

97. What is the function of the liquid-vapor separator in the evaporative emissions system?

A. Used to keep fuel from entering the evaporative emission system
B. Carries fuel vapors to a charcoal canister
C. Stores fuel vapors when the engine is not running
D. Prevents fuel vapors from entering the atmosphere

98. What component of the evaporative emissions control system allows the vapors to enter the intake manifold and is pulled into the combustion chambers for burning?

A. Purge line
B. Air dome
C. Purge connector
D. Purge valve

99. On the enhanced evaporative emissions control system, what component monitors internal fuel tank pressure?

A. Canister vent solenoid
B. Rollover valve
C. Fuel tank pressure sensor
D. Service port

100. On the enhanced evaporative emissions control system, what happens when the system is in diagnostic mode?

A. The control module closes the vent solenoid, blocking air flow into the canister.
B. The control module opens the vent solenoid, blocking air flow into the canister.
C. The control module closes the vent solenoid, allowing air flow into the canister.
D. The control module opens the vent solenoid, allowing air flow into the canister.

101. Maintenance on an evaporative emissions control system typically involves cleaning or replacing which component?

A. Filter in the vacuum pump
B. Filter in the pressure canister
C. Filter in the charcoal canister
D. None of the above

102.  One very basic and simple way to quickly test a PCV valve is to pull it from the engine and see if it rattles when shaken.

A. True
B. False

103.  Another procedure to check the PVC valve is to suck on it to check operation.

A. True
B. False

104. What component is normally located in the thermostatic air cleaner to control the vacuum motor and heat control door?

A. Heat pressure valve
B. Thermal vacuum valve
C. Vacuum motor
D. Air cleaner inlet

105. If the air cleaner flap does not function on the thermostatic air cleaner, what components should be checked first?

A. Vacuum motor and temperature sensor
B. Vacuum sensor and temperature motor
C. Vacuum hose and temperature valve
D. Gas motor and pressure sensor

106. The exhaust gas recirculation system lowers the amount of what emission in the engine exhaust?

A. Carbon monoxide
B. Hydrocarbons
C. Oxides of nitrogen
D. Particulates

107. On the EGR System, what component may be used to prevent exhaust gas recirculation when the engine is cold?

A. Vacuum line
B. Back pressure valve
C. Coolant pressure valve
D. Coolant temperature switch

108. What is the difference between a single-stage EGR and a multi-stage EGR?

A. Solenoids
B. Exhaust gases
C. Stages
D. Valves

109. While conducting a test on a vacuum type EGR System, you plug the supply vacuum line to the EGR valve and apply vacuum with a hand pump. What should the engine do?

A. Rpms will rise
B. Run smoothly
C. Begin to miss or stall
D. Idle down, then rpms rise

110. While running a test on an electronic type EGR System and after going through the procedures, what should check for after checking the condition of the windings and no voltage is indicated?

A. Check for bad electrical connection
B. Check the ECM
C. There is nothing more, replace EGR
D. Both A and B

111. What component of the air injection system keeps air from entering the exhaust system during deceleration?

A. Air distributor manifold
B. Diverter
C. Exhaust pipe
D. Exhaust manifold

112. When testing the air pump on the air injection system, it should typically produce how much pressure?

A. 2-3 psi
B. 2-6 psi
C. 3-4psi
D. 4-5 psi

113. The following are types of catalytic converters except which one?

A. Dual-bed
B. Two-way
C. Mini-stage
D. Monolithic

114. How does the dual-bed catalytic converter dispose of the NOx emissions?

A. Returns to cylinders to burn off
B. Filters through converter, released as hydrogen
C. Stores then releases when full
D. Stores then burns off to harmless by-product

115. What may be one of the symptoms of a clogged catalytic converter?

A. Increased power
B. Rotten egg odor
C. Increased pressure
D. Black exhaust

116. When testing for catalytic converter efficiency, you know there is enough oxygen for the catalyst to burn emissions if the oxygen readings are above what percentage?

A. 4

B. 5
C. 6
D. 7

117.  The oxygen sensor monitors the exhaust gases for oxygen content.

A. True
B. False

118. The primary oxygen sensor holds what designation and is located where?

A. Front O2; near the engine
B. Front O1; near the engine
C. Front O2; downstream of the engine
D. Rear O2; near the engine

119. The difference between the oxygen content in the inner cavity and the oxygen content of the exhaust gases flowing through the sensor's outer surface causes the sensor to generate _____.

A. Amps
B. Voltage
C. Backpressure
D. Negative flow

120. What determination does the ECM make when a titania oxygen sensor's voltage drops below the predetermined value?

A. Mixture too rich
B. Mixture too lean
C. Mixture is correct
D. None of the above

121. A lazy O2 sensor will not its output signal fast enough to maintain an efficient air-fuel ratio.

A. Alter
B. Detect
C. Determine
D. Sense

122. When the oxygen sensor is determined to be defective, what shall be done first before removing it?

A. Disconnect the sensor harness.
B. Disconnect the positive battery cable.
C. Disconnect the negative battery cable.
D. Loosen the sensor.

123. Currently, there are three types of diesel particulate filters as listed below except which one?

A. Precatalyzed
B. Catalyzed
C. Low temperature particulate filters
D. Noncatalyzed

124. What is the idea behind the DPF?

A. Soot collects in the filter system, then burned off.
B. Soot collects in a container, then removed daily.
C. Soot collects on the walls, then rinsed off weekly.
D. Soot collects on the walls, then burned off.

125.  A pyrometer is an instrument only used by the mechanic.

A. True
B. False

126. What is the system on a diesel engine which is comparable to the PCV on gasoline engines?

A. Closed crankcase ventilation
B. Open crankcase ventilation
C. Closed manifold ventilation
D. Closed cylinder ventilation

127. Opacity meter readings are displayed in _____.

A. Percentages
B. Volts
C. Ohms
D. Proportion

128. Before undertaking a snap test, what needs to be considered relating to the exhaust stack?

A. Length
B. Diameter
C. Bends
D. Pressure

129. White smoke is caused by condensing _____.

A. Oil
B. Diesel
C. Fumes
D. Liquid


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