GMC 1976 ZEO 6083 Maintenance Manual page 284

NOTE: When diagnosing engine noise problems, be careful that noises caused by
accessories are not mistaken for engine noises . Removal of accessory drive belts
will
eliminate any noises caused by these units.
In general, engine noises are either synchronized to engine speed or one-half
engine speed . Those that are timed to engine speed are sounds that have to
do with the crankshaft, rods, pistons, and wrist pins . The sounds emitted at
one-half engine speed are valve train noises .
The use of a stethoscope will often aid in locating an engine noise . Caution
must be exercised, however, because noise will travel to other metal parts
not involved in the problem. A timing light will aid in determining if the noise
is synchronized with engine speed or at one-half engine speed .
Engine noise sometimes may be isolated by grounding the spark plug leads
one at a time . If the noise lessens appreciably or disappears, it is confined
to that particular cylinder .
No definite rule or test can be listed that will positively determine the source
of a noise complaint.
Fuel pumps, distributors, flywheels, water pumps, drive belts, or carbon built
up in the combustion chamber may contribute to noisy engine operation. The
following information can therefore, be used only as a general guide to noise
diagnosis . There is no substitute for experience .
NOTE : A loose main bearing is indicated by a powerful, but dull, thud or knock
when the engine is pulling. If all main bearings are loose a noticeable clatter will
be audible.
The thud occurs regularly every other revolution . The noise is loudest when
the engine is "lugging" or under heavy load . The sound is heavier and duller
than a connecting rod noise. Low oil pressure also accompanies this condi-
tion . The knock can be confirmed by shorting spark plugs on cylinders adja-
cent to the bearing . Knock will disappear or be less when plugs are shorted.
This test should be made at a fast idle equivalent to 15 mph . If bearing is not
quite loose enough to produce a knock if oil is too thin or if there is no oil
at the bearing .
Regular noise : worn
GENERAL
Insufficient oil supply .
Low oil pump pressure.
Thin or diluted oil.
A . NOISY MAIN BEARINGS
main
IMPORTANT: Crankshaft End Play -
than a loose main bearing.
cause a change in the rap.
ENGINE NOISY
bearings ; irregular; worn end-thrust bearings .
MECHANICAL
ENGINE
6A- 1 9
Excessive bearing clearance.
Excessive crankshaft end play .
Eccentric or out-of-round crankshaft
journals .
Sprung crankshaft .
Excessive belt tension .
Loose harmonic balancer .
Loose flywheel or torque converter.
Intermittent rap or knock that is sharper
Repeated disengagements of the transmission may