3.1 Gasoline Fuel Systems

The function of the fuel system is to supply a combustible mixture of air and fuel to the engine. Major elements of the gasoline fuel supply system include the following: fuel tank and cap, fuel system emissions controls, fuel lines, fuel pump, fuel filter, carburetor or fuel injection system, air cleaner, and exhaust system. Before learning about these components of a gasoline fuel system, you should understand the composition and properties of gasoline.


Gasoline is a highly volatile, flammable liquid hydrocarbon mixture used as a fuel for internal combustion engines. A comparatively economical fuel, gasoline is the primary fuel for automobiles worldwide. Chemicals called additives such as lead, detergents, and anti-oxidants, are mixed into gasoline to improve its operating characteristics.

Antiknock additives are used to slow down the ignition and burning of gasoline. This action helps to prevent engine ping or knock. Leaded gasoline has lead antiknock additives. The lead allows a higher engine compression ratio to be used without the fuel igniting prematurely. Leaded gasoline is designed to be used in older vehicles that have little or no emission controls.

The fuel used today is unleaded gasoline. Unleaded gasoline, also called no-lead or lead-free, does NOT contain lead antiknock additives. Congress has passed laws requiring that all vehicles meet strict emission levels. As a result, manufacturers began using catalytic converters and unleaded fuel.

Properties of Gasoline

For a gasoline fuel system to function properly, the fuel must have the right qualities to burn evenly no matter what the demands of the engine are. To help you recognize the qualities required of gasoline used for fuel, let’s examine the three properties of gasoline and their effects on the operation of the engine.


The ease with which gasoline vaporizes is called volatility. A high volatility gasoline vaporizes very quickly. A low volatility gasoline vaporizes slowly. A good gasoline should have the right volatility for the climate in which the gasoline is used.

If the gasoline is too volatile, it will vaporize in the fuel system. The result will be a condition called vapor lock. Vapor lock is the formation of vapor in the fuel lines in a quantity sufficient to prevent the flow of gasoline through the system. Vapor lock causes the vehicle to stall from lack of fuel. In the summer and in hot climates, fuels with low volatility lessen the tendency toward vapor lock.

Antiknock Quality

In modern high compression gasoline engines, the air-fuel mixture tends to ignite spontaneously or to explode instead of burning rather slowly and uniformly. The result is a knock, a ping, or a detonation. For this reason, gasoline refiners have various ways to make gasoline that does not detonate easily.

Octane Rating

A gasoline that detonates easily is called low octane gasoline. A gasoline that resists detonation is called high octane gasoline.

The octane rating of a gasoline is a measurement of the ability of the fuel to resist knock or ping. A high octane rating indicates the fuel will NOT knock or ping easily. It should be used in a high compression or turbocharged engine. A low octane gasoline is suitable for a low compression engine.

Octane numbers give the antiknock value of gasoline. A higher octane number (91) will resist ping better than a gasoline with a low octane number (83). Each manufacturer recommends an octane number for its engine.

Air-Fuel Ratio

For proper combustion and engine performance, the right amounts of air and fuel must be mixed together. If too much fuel or too little fuel is used, engine power, fuel economy, and efficiency are reduced.

For a gasoline engine, the perfect air-fuel ratio is 14.7:1 (14.7 parts air to 1 part fuel by weight). Under constant engine conditions, this ratio can help assure that all fuel is burned during combustion. The fuel system must change the air-fuel ratio with the changes in engine-operating conditions.

Lean Air-Fuel Mixture

A lean air-fuel mixture contains a large amount of air. For example, 20:1 would be a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel will be burned; however, too lean of a mixture can cause poor engine performance (lack of power, missing) and even engine damage.

Rich Air-Fuel Mixture

A rich air-fuel mixture contains a little more fuel mixed with the air. For gasoline, 8:1(8 parts air to 1 part fuel) is a very rich mixture. A slightly rich mixture tends to increase power; however, it also increases fuel consumption and exhaust emissions. An overly rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust).

Gasoline Combustion

For gasoline or any other fuel to burn properly, it must be mixed with the right amount of air. The mixture must then be compressed and ignited. The resulting combustion produces heat, expansion of the gases, and pressure.

Normal Combustion

Normal gasoline combustion occurs when the spark plug ignites the fuel and burning progresses smoothly through the fuel mixture. Maximum cylinder pressure should be produced after a few degrees of crank rotation after the piston passes TDC on the power stroke.

Normal combustion takes only about 3/1,000 of a second. This is much slower than an explosion. Dynamite explodes in about 1/50,000 of a second. Under some undesirable conditions, however, gasoline can be made to bum quickly, making part of the combustion like an explosion.

Abnormal Combustion

Abnormal combustion occurs when the flame does NOT spread evenly and smoothly through the combustion chamber. The lean air-fuel mixture, high operating temperatures, low octane, and unleaded fuels used today make abnormal combustion a major problem that creates unfavorable conditions, such as the following:

• Detonation results when part of the unburned fuel mixture explodes violently. This is the most severe engine-damaging type of abnormal combustion. Engine knock is a symptom of detonation because pressure rises so quickly that parts of the engine vibrate. Detonation sounds like a hammer hitting the side of the engine. It can crack cylinder heads, blow head gaskets, burn pistons, and shatter spark plugs.

• Pre-ignition results when an overheated surface in the combustion chamber ignites the fuel mixture. Termed surface ignition, a hot spot (overheated bit of carbon, sharp edge, and hot exhaust valve) causes the mixture to burn prematurely. A ping or mild knock is a light tapping noise that can be heard during pre-ignition. Pre-ignition is similar to detonation, but the action is reversed. Detonation begins after the start of normal combustion, and pre-ignition occurs before the start of normal combustion. Pre-ignition is common to modern vehicles. Some manufacturers say that some pre-ignition is normal when accelerating under a load.

• Dieseling, also called after-running or run on, is a problem when the engine keeps running after the key is turned off. A knocking, coughing, or fluttering noise may be heard as the fuel ignites and the crankshaft spins. When dieseling, the engine ignites the fuel from heat and pressure, somewhat like a diesel engine. With the key off, the engine runs without voltage to the spark plugs. The most common causes of dieseling are high idle speed, carbon deposits in the combustion chambers, low octane fuel, overheated engine, or spark plugs with too high of a heat range.

• Spark knock is another combustion problem caused by the spark plug firing too soon in relation to the position of the piston. The spark timing is advanced too far, causing combustion to slam into the upward moving piston. This causes maximum cylinder pressures to form before TDC, not after TDC as it should. Spark knock and pre-ignition both produce about the same symptoms—pinging under load. To find its cause, first check ignition timing. If ignition timing is correct, check other possible causes.

Gasoline Fuel System Components

A gasoline fuel system draws fuel from the tank and forces it into the fuel-metering device (carburetor, gasoline injectors), using either a mechanical (engine-driven) or electric fuel pump. The basic parts of a fuel supply system include the following:

• Fuel tank (stores gasoline)

• Fuel pump (draws fuel from the tank and forces it to the fuel-metering device)

• Fuel filters (remove contaminants in the fuel)

• Fuel lines (carry fuel between the tank, the pump, and other parts)

Fuel Tank

An automotive fuel tank must safely hold an adequate supply of fuel for prolonged engine operation. The location of the fuel tank should be in an area that is protected from flying debris, shielded from collision damage, and not subjected to bottoming (Figure 3-1). A fuel tank can be located just about anywhere in the vehicle that meets these requirements.

Figure 3-1— Common fuel tank locations.

Figure 3-2 shows the general construction of a fuel tank used on automotive equipment. Fuel tanks are usually made of thin sheet metal or plastic. The main body of a metal tank is made by soldering or welding two formed pieces of sheet metal together. Other parts (filer neck, fuel tank cap, and baffles) are added to the form to complete the fuel tank assembly. A lead-tin alloy is normally plated to the sheet metal to prevent the tank from rusting.

The fuel tank filler neck is an extension on the tank for filling the tank with fuel. The filler cap fits on the end of the filler neck. The neck extends from the tank through the body of the vehicle. A flexible hose is normally used as part of the filler neck to allow for tank vibration without breakage.

In vehicles requiring unleaded fuel, a fuel neck restrictor is used inside the filler neck. This prevents the accidental use of leaded gasoline in an engine designed for unleaded. The restrictor is too small to accept the larger leaded fuel type pump nozzle.

Figure 3-2— Fuel tank construction.


If the restrictor is removed and leaded fuel is used in a vehicle designed for unleaded fuel, the catalytic converter will be damaged. This action is a violation of federal law; therefore, NEVER remove the filler neck restrictor.

Modern fuel tank caps are sealed to prevent escape of fuel and fuel vapors (emissions) from the tank. The cap has pressure and vacuum valves that open only under abnormal conditions of high pressure or vacuum.

Fuel tank baffles are placed inside the tank to prevent the fuel from sloshing or splashing around in the tank. The baffles are metal plates that restrict fuel movement when the vehicle accelerates, decelerates, or turns corners.

Fuel tanks give little or no trouble, and generally require no servicing other than an occasional draining and cleaning.


If a fuel tank is punctured or develops leaks, it should NOT be welded or repaired with or near an open flame until all traces of fuel and fuel vapors have been completely removed from the tank. Before attempting to make any repairs to a fuel tank, consult with the shop supervisor for specific instructions on all safety precautions to be observed.

Fuel Gauges

The fuel gauge indicates the fuel level in the fuel tank. It is a magnetic indicating system that can be found on either an analog or digital instrument panel.

The fuel sending unit is combined with the fuel pump assembly and consists of a variable resistor controlled by the level of an attached float mechanism in the tank. When the fuel is low, resistance in the sender is low; therefore movement of lift of the gauge is low. When the resistance is high, such as with a full tank, the indicator is high, showing the gauge higher up the scale on the instrument panel.

Fuel Filters

The fuel injection system is highly sensitive to foreign particles. Fuel filters prevent water, dirt, and rust particles from entering the system. Contaminated fuel can cause incomplete combustion, smoky exhaust, engine knocking, and difficulties starting the engine. Most heavy equipment has a fuel pressure gauge that indicates when the filters are dirty.

The fuel filter operates by passing fuel through a porous element that removes particles large enough to cause problems in the system (Figure 3-3). Particles are often measured in microns. A micron is one millionth of a meter.

Figure 3-3— Fuel filter operation.

Some filters serve as sediment bowls. These types of filters separate water and larger particles from the fuel. After separation, the water and particles settle to the bottom of the bowl, where they can be removed through a drain plug.

Filter elements can be made of ceramic, treated paper, sintered bronze or metal screen (Figure 3-4). Some filter elements are made of laminated disks that are spaced 0.0003 inches apart. Foreign matter is blocked out as the fuel passes between the disks.

Figure 3-4— Fuel filter elements.

Fuel Pump

A fuel pump is the device that draws the fuel from the tank to the engine’s injection system. All late model vehicles use an electric fuel pump.

The fuel pump can be located either inside the tank or in the fuel system after the tank. There are four types of fuel pumps: the diaphragm, plunger, bellows, and impeller or rotary pump. The in-tank electric pump is usually a rotary pump. The others are usually of the demand style, meaning that when the ignition is turned on, the fuel pump starts and when the pressure in the system is correct, it shuts off. When more fuel is required, the pump starts again.

Most vehicles have an in-tank fuel pump. Some other vehicles also have a secondary pump along the fuel line. Fuel pumps are mounted in the tank to help keep the pump cool.

Nearly all electric fuel pump circuits have some sort of rollover protection. Typically this protection includes the installation of an inertia switch that shuts off the fuel pump if the vehicle rolls over or is in an accident.

Fuel Lines and Hoses

Fuel lines and hoses carry fuel from the tank to the filter and fuel injection assembly. They can be made from either metal tubing or flexible nylon or synthetic rubber hoses. The latter must be able to resist gasoline. The hoses must be nonpermeable so gas and gas vapors cannot evaporate through the hose.

Fuel supply lines from the tank to the injectors usually follow the frame of the vehicle along the underchassis. Generally, rigid lines are used from the tank to the fuel pump or filter. To absorb vibration, these lines can be joined with short lengths of flexible hose.

Many tanks also have vent hoses to allow air in the tank to escape when the tank is being filled. Vent hoses are usually routed alongside the filler neck.

Faulty fuel lines and hoses are a common source of fuel leaks. Fuel hoses can become hard and brittle after being exposed to the engine heat and the elements. Engine oil can soften and swell them. Always inspect hoses closely and replace any in poor condition. Metal fuel lines rarely cause problems; however, they should be replaced if they become smashed, kinked, rusted, or leaky. Remember these rules when working with fuel lines and hoses:

• Place a rag around the fuel line fitting during removal to keep fuel from spraying on you or on a hot engine. Use a flare nut or tubing wrench on fuel line fittings.

• Use only approved double wall steel tubing for fuel lines. NEVER use copper or plastic tubing.

• Make smooth bends when forming a new fuel line. Use a bending spring or bending tool.

• Form double lap flares on the ends of fuel lines. A single lap flare is NOT approved for fuel lines.

• Reinstall fuel line hold-down clamps and brackets. If not properly supported, the fuel line can vibrate and fail.

• Route all fuel lines and hoses away from hot or moving parts. Double-check the clearance after installation.

• Use only approved synthetic rubber hoses in a fuel system. Vacuum hose is NOT to be used as fuel hose.

• Make sure fuel hoses completely cover their fittings or lines before installing clamps. Pressure in the fuel system could force a hose off if not installed properly.

• Double-check all fittings for leaks. Start the engine and inspect the connections closely.



Most fuel injection systems have very high fuel pressure. Follow recommended procedures for bleeding or releasing pressure before disconnecting a fuel line or fitting. This action will prevent fuel spray from possibly causing injury or a fire.

Fuel 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 valve operation.

Fuel Injectors

Fuel injection systems can have one or more fuel supply devices called fuel injectors (Figure 3-5). Fuel injectors are controlled by an ECM. The computer system sends an electrical current to activate the solenoid inside the injector. When the solenoid is activated, the injector nozzle opens and squirts atomized fuel in a cone-shaped pattern. The computer system controls the fuel-air ratio by varying the length of time that the injector nozzle remains open.

Figure 3-5— Fuel injector.

In gasoline engines injectors squirt fuel into the intake manifold. In diesel engines, fuel is delivered directly into the combustion chamber. Spring pressure closes the injector nozzle when the solenoid is deactivated.

Air Cleaner

Air cleaners are used to prevent foreign matter, such as sand, dust, and lint, from entering the intake system (Figure 3-6). Contaminated air in the intake system can cause engine wear, poor combustion, and engine breakdown. In addition to supplying clean air, air cleaners reduce vibration sounds and other noises caused by air entering the intake system.

Figure 3-6 — Air cleaners.

Test your Knowledge

1. Which of the following is NOT a property of gasoline?

A. Volatility
B. Antiknock quality
C. Cetane number
D. Octane rating

2. Which of the following air-fuel ratios is considered to be perfect for a gasoline engine?

A. 20:1
B. 14.7:1
C. 17.4:1
D. 18.8:1

3. The fuel pressure regulator controls the amount of pressure entering the _____.

A. intake
B. valves
C. injectors
D. pump