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TM 55-1905-223-24-1
2-11. General Symptoms.
a. White Smoke. White smoke is the result of incomplete combustion and is generally associated with
engine startup. At low outside temperature, misfiring cylinders or incomplete combustion causes fuel to be
exhausted which condenses upon contact with the cold outside air causing this condition. As the engine
warms, the misfiring cylinders begin to sustain combustion and the level of white smoke decreases.
Since white smoke is a normal characteristic during startup at low outside
temperature, it is extremely important to determine if the level is significantly
higher than normal before attempting to correct a problem which may not exist.
Compare the level of white smoke from one engine to the other. If there is a
significant difference under the same operating conditions, refer to Table 2-2.
b. General Engine Noise. Engine noises heard at the crankshaft speed (rated engine rpm) are related to
the crankshaft, rods, pistons and piston pins. Those heard at the camshaft speed (one-half engine rpm) are
related to the valve train. There is no definite rule which will positively determine the source of a noise;
however, it can sometimes be isolated by holding the injector plungers down one at a time and listening for a
reduction in the noise level.
The use of a stethoscope is helpful in isolating noise.  A handheld digital
tachometer will help to determine whether the noise is camshaft or crankshaft
(1) Connecting Rod Bearing Noise. Connecting rods with excessive clearance knock at all engine
speeds and under both idle and load conditions. When the bearings begin to become loose, the noise can be
confused with piston slap or loose piston pins. The noise increases in volume with engine speed. Low oil
pressure can also accompany this condition.
(2) Piston Noise. It is difficult to tell the difference between piston pin, connecting rod, and piston
noise. A loose piston pin causes a loud double knock which is usually heard when the engine is idling. When
the injector to this cylinder is held down, a noticeable change will be heard in the sound of the knocking noise.
However, sometimes the knock becomes more noticeable when the engine is operated at a steady speed
(3) Main Bearing Noise. A loose main bearing produces a dull knock heard every other revolution
when the engine is under heavy load. If all main bearings are loose, a loud clatter will be heard. Low oil
pressure can also accompany this condition. Causes that can produce a knock in tight bearings are oil that is
too thin, or no oil at the bearing.
(4) Crankshaft End Clearance Excessive. This condition is accompanied by a sharp intermittent knock.
Changes in engine speed and/or load will cause the noise to change.


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