4.6 General Troubleshooting

When troubleshooting a diesel engine, keep in mind that problems associated with one make and type of engine (two-stroke versus four-stroke) may not occur exactly in the same way as in another. Specifically, particular features of one four-stroke-cycle engine may not appear on another due to the type of fuel system used and optional features on that engine. Follow the basic troubleshooting steps listed below before rolling up your sleeves and trying to pinpoint a problem area.

If possible, use the special tools and diagnostic equipment at your disposal to verify a complaint and pinpoint the general area.

• Determine the cause(s) of the problem and carry out the repair.

• Operate the engine and road test the vehicle to confirm that the problem is corrected.

Exhaust Smoke Color

One of the easiest methods to use when troubleshooting an engine for a performance complaint is to visually monitor the color of the smoke coming from the exhaust stack. There are four basic colors that may exit from the exhaust system at any time during engine operation—white, black, gray, or blue. The color of the smoke tips you off to just what and where the problem might lie.

White Smoke

White smoke is generally most noticeable at engine start-up, particularly during cold conditions. As the combustion and cylinder temperatures increase during the first few minutes of engine operation, the white smoke should start to disappear which indicates the engine is sound. However, if the white smoke takes longer than 3 to 5 minutes to fade away, a problem exists. The problems white smoke may indicate are as follows:

• Low cylinder compression from worn rings

• Scored piston or liner

• Valve seating problems

• Water leaking into the combustion chamber

• Faulty injectors

• Use of a low cetane diesel fuel

Black or Gray Smoke

Black or gray smoke generally is caused by the same conditions—the difference between the colors being one of opacity or denseness of smoke. Black or gray smoke should be checked with the engine at operating temperature of 160°F. Abnormal amounts of exhaust smoke emission is an indication that the engine is not operating correctly, resulting in a lack of power, as well as decreased fuel economy. Excessive black or gray exhaust smoke is caused by the following:

• Improper grade of diesel fuel

• Air starvation

• High exhaust back pressure

• Incorrect fuel injection timing

• Faulty nozzles or injectors

• Incorrect valve adjustment clearances

• Faulty injection pump

• Faulty automatic timing advance unit

Blue Smoke

Blue smoke is attributed to oil entering the combustion chamber and being burned or blown through the cylinder and burned in the exhaust manifold or turbocharger.

Remember: always check the simplest things first, such as too much oil in the crankcase or a plugged crankcase ventilation breather. The more serious problems that can cause blue smoke are as follows:

• Worn valve guides

• Worn piston rings

• Worn cylinder walls

• Scored pistons or cylinder walls

• Broken ring

• Turbocharger seal leakage

• Glazed cylinder liner walls due to use of the wrong type of oil

Quick Injector Misfire Check

Listed below are several quick and acceptable checks that can be performed on a running engine to determine if one or more injectors are at fault on any type of engine.

On four-stroke-cycle engines with a high-pressure in-line pump or distributor system, such as Caterpillar and Roosa Master, you can loosen off one injector fuel line, one at a time, about one-half turn as you hold a rag around it while noting if there is any change in the operating sound of the engine. If the injector is firing properly, there should be a positive change to the sound and rpm of the engine when you loosen the line, since it prevents the delivery of fuel to the cylinder.

On an engine with the PT fuel system, a cylinder misfire can be checked by running the engine to a minimum of 160°F. Remove the rocker covers and install a rocker lever actuator over an injector rocker lever. Hold the injector plunger down while the engine is running at low idle. This will stop the fuel flow to that injector. If the engine speed decreases, the injector is good. If the engine rpm does not decrease, replace the injector.

On the two-stroke-cycle non-electronic Detroit diesel engines, remove the rocker cover; then, using a large screwdriver push and hold down the injector follower while the engine is idling. This action is like shorting out a spark plug on a gasoline engine, since it prevents fuel from being injected into the combustion chamber. If there is no change to the sound and speed of the engine, the injector is not firing. There should be a definite change to indicate that the injector was in fact firing.

Dead Cylinder Test

The dead cylinder Test is another name for the quick injector misfire check. It is performed in the same manner. If you experience a problem while performing this test, you have a “dead cylinder”.


In this lesson you have learned about the diesel fuel system and its components, different methods of injection, superchargers, turbochargers, and cold starting devices, and have been briefly introduced to some troubleshooting techniques. Because there are newer and better innovations every day, you should also refer to the manufacturer’s guide for specific systems. Your knowledge of the diesel fuel system will enable you to evaluate certain engine problems with confidence that the fuel system can be diagnosed.