Engine Noise Diagnostic Procedures - General Information
NOTE: When diagnosing engine noise problems, make sure that noises caused by accessories, such as the air
compressor and power takeoff, are not mistaken for engine noises. Remove the accessory drive belts to eliminate noise
caused by these units. Noise will also travel to other metal parts not related to the problem. The use of a stethoscope
can help locate an engine noise.
Engine noises heard at the crankshaft speed, engine RPM, are noises related to the crankshaft, rods, pistons, and
piston pins. Noises heard at the camshaft speed, one-half of the engine RPM, are related to the valve train. A
hand-held digital tachometer can help to determine if the noise is related to components operating at the crankshaft
or camshaft speed.
Engine noise can sometimes be isolated by holding down the injector plungers one at a time. Refer to Procedure
5-13. If the volume of the noise decreases or the noise disappears, it is related to that particular engine cylinder.
There is not a definite rule or test that will positively determine the source of a noise complaint.
Engine driven components and accessories, such as gear-driven fan clutches, hydraulic pumps, belt-driven
alternators, air-conditioning compressors, and turbochargers can contribute to engine noise. Use the following
information as a guide to diagnosing engine noise.
Main Bearing Noise (Refer to Engine Noise Excessive - Main Bearing Symptom Chart)
The noise caused by a loose main bearing is a loud dull knock heard when the engine is pulling a load. If all main
bearings are loose, a loud clatter will be heard. The knock is heard regularly every other revolution. The noise is
the loudest when the engine is lugging or under 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 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 irregular noise can indicate worn crankshaft thrust bearings.
An intermittent sharp knock indicates excessive crankshaft end clearance. Repeated clutch disengagements can
cause a change in the noise.
Connecting Rod Bearing Noise (Refer to Engine Noise Excessive - Connecting Rod Symp-
tom Chart)
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.
Piston Noise (Refer to Engine Noise Excessive - Piston Symptom Chart)
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, on some engines the knock
becomes more noticeable when the vehicle is operated on the road at a steady speed condition.
SectionT-Troubleshooting Troubleshooting Symptoms Charts
N14 Page T-105
To Next Page
Loading...
