In learning to recognize the symptoms that may help locate the causes of engine trouble, you will find that experience is the best teacher. Even though written instructions are essential for efficient troubleshooting, the information usually given serves only as a guide. It is very difficult to describe the sensation that you should feel when checking the temperature of a bearing by hand; the specific color of exhaust smoke when pistons and rings are worn excessively; and, for some engines, the sound that you will hear if the crankshaft counterweights come loose. You must actually work with the equipment to associate a particular symptom with a particular trouble. Written information, however, can save you a great deal of time and eliminate much unnecessary work. Written instructions will make detection of troubles much easier in practical situations. A symptom that indicates that trouble exists may be in the form of an unusual noise or instrument indication, smoke, or excessive consumption or contamination of the lube oil, fuel, or water. Figure 3-43 is a general listing of various trouble symptoms that you may encounter.

NOISES

The unusual noises that may indicate that trouble exists or is impending may be classified as pounding, knocking, clicking, and rattling. Each type of noise must be associated with certain engine parts or systems that might be the source of trouble.

Pounding or hammering is a mechanical knock (not to be confused with a fuel knock). It may be caused by a loose, excessively worn, or broken engine part. Gen-erally, troubles of this nature will require major repairs. Detonation (knocking) is caused by the presence of fuel or lubricating oil in the air charge of the cylinders during the compression stroke. Excessive pressures accompany detonation. If detonation is occurring in one or more cylinders, stop the engine immediately to prevent possible damage.

Clicking noises are generally associated with an improperly functioning valve mechanism or timing gear. If the cylinder or valve mechanism is the source of metallic clicking, the trouble may be due to a loose valve stem and guide, insufficient or excessive valve tappet clearances, a loose cam follower or guide, broken valve springs, or a valve that is stuck open. A clicking in the timing gear usually indicates that there are some damaged or broken gear teeth.

Rattling noises are generally due to vibration of loose engine parts. However, an improperly functioning vibration damper, a failed antifriction bearing, or a gear-type pump operating without prime are also possible sources of rattling noises.

When you hear a noise, first make sure it is a trouble symptom. Each diesel engine has a characteristic noise at any specific speed and load. The noise will change with a change in speed or load. As an operator, you must become familiar with the normal sounds of the engine. Investigate all abnormal sounds promptly. Knocks that indicate a trouble may be detected and located by special instruments or by the use of a sounding bar, such as a solid iron screwdriver or bar.

INSTRUMENT INDICATIONS

As an engine operator, you will probably rely more on the instruments to warn you of impending troubles than on all the other trouble symptoms combined. Regardless of the type of instrument being used, the indications are of no value if the instrument is inaccurate. Be sure an instrument is accurate and operating properly before you accept a low or high reading. Test all instruments at specified intervals or whenever you suspect them of being inaccurate.

SMOKE

Smoke can be quite useful as an aid in locating some types of trouble, especially if used in conjunction with other trouble symptoms.

The color of exhaust smoke, a good indication of engine performance, can also be used as a guide in troubleshooting. The exhaust of an efficiently operating engine has little or no color. A dark, smoky exhaust indicates incomplete combustion; the darker the color, the greater the amount of unburned fuel in the exhaust. Incomplete combustion may be due to a number of troubles. Some manufacturers associate a particular type of trouble with the color of the exhaust. The more serious troubles are generally identified with either black or bluish-white exhaust colors.

Figure 3-43.Symptoms of engine troubles.