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 TM 55-1-5-223-24-5
d. Main gearing Noise (refer to Table 2-2. Item No. 37). The noise caused by a loose or worn main
bearing is aloud, dull knock heard when the engine is pulling a load. If all main bearings are loose or worn, a
loud clatter will be heard. The knock is heard regularly every other revolution. The noise is loudest when the
engine is "lugging" or under a heavy load. The knock is duller than a connecting rod noise. Low oil pressure
can also accompany this condition. If the bearing is not loose or worn enough to produce a knock by itself, the
bearing can knock if the oil is too thin, or if there is no oil at the bearing. An intermittent sharp knock can
indicate wom crankshaft thrust bearings resulting in excessive crankshaft end clearance.
Clutch
disengagements can cause a change in the noise.
e. Connecting Rod Bearing Noise (refer to Table 2-2. Item No. 35). Connecting rods with excessive
clearance knock at all engine speeds and under both idle and load conditions. When the bearings begin to
come loose or worn, 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.
f. Piston Noise (refer to Table 2-2. Item No. 36). It is difficult to tell the difference between piston pin,
connecting rod, and piston noise. A loose piston pin causes aloud 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, on some engines, the knock becomes more noticeable when the
engine is operated at a steady speed.
2-16. White Smoke-General Information. White smoke is the result of incomplete combustion and is
generally associated with engine startup at low ambient temperatures. This condition is more predominant on
high horsepower fixed injection timing engines because the fuel and combustion systems are optimized for
maximum performance and for reliability and durability under high load operating conditions.
a. Starting at Low Ambient Temperatures. These engines can have two or three cylinders that misfire or
have incomplete combustion when the engine is started at low ambient temperatures. The fuel that is injected
into the cylinders that are misfiring is exhausted into the atmosphere as unburned hydrocarbons which cool,
condense, and appear as white smoke. As the cylinder temperature and subsequently the coolant temperature
rise, the misfiring cylinders begin to sustain combustion which decreases the hydrocarbon level in the exhaust,
resulting in less white smoke being produced.
b. Level of White Smoke. Since white smoke is a normal characteristic of high horsepower, heavy-duty
diesel engines during startup at low ambient temperatures, it is extremely important to determine if the level of
white smoke is significantly higher than normal for a particular engine model before making any attempt to
correct a complaint. This can be accomplished by comparing the level of white smoke from one unit to another
which has the same chassis configuration and engine model. If a significant difference in the level of white
smoke between the two units is noted under the same operating conditions, refer to Table 2-2, Item No. 34,
" xcessive White Smoke at Idle"for corrective action.
e
Change 2
2-23
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