When learning to recognize the symptoms that may help you 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 will serve you only as a guide. It is very difficult to describe the sensation you should feel when you are 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 you will hear if the crankshaft counter-weights come loose. You must actually work with the equipment before you can associate a particular symptom with a particular problem. Written information, however, can save you a great deal of time and can help you eliminate unnecessary work. Written instructions will make your detection of problems much easier in practical situations.

Symptoms that indicate trouble may be in the form of an unusual noise or instrument indication, smoke, excessive consumption of lube oil, or contamination of the lube oil, fuel, or engine coolant. Figure 11-1 provides a general listing of various trouble symptoms the operator of an engine may encounter. For additional information, you should consult the Naval Ships Technical Manual, chapters 079 and 233.

Noises

Unusual noises that may indicate that a trouble exists (or is impending) are classified as pounding, knocking, clicking, and rattling. You must be able to associate each type of noise with certain engine parts or systems that might be the source of the trouble.

Pounding or hammering is a mechanical knock (and should not be confused with a fuel knock). Pounding or hammering may be caused by a loose, excessively worn, or broken engine part. Generally, 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 cylinder pressures accompany detonation. If detonation is occurring in one or more cylinders, you should stop the engine immediately to prevent possible damage.

Figure 11-1.-Symptoms of engine trouble.

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 caused by vibration of loose engine parts. However, an improperly functioning vibration damper, a failed antifriction bearing, or a gear-type pump that is operating without prime are also possible sources of trouble when rattling noises occur. When you hear a noise, first make sure that it is a symptom of trouble. 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 an engine. You must investigate all abnormal sounds promptly. You can detect and locate knocks that indicate trouble by using special instruments, such as an engineers stethoscope, or a sounding bar, such as a solid metal screwdriver or bar.