Engine - Knock or Rattle Diagnosis
90chevy15Number: 90-55-6
Section: 6
Date: September 1989
Corp. Bulletin No.: 936119
Subject: ENGINE KNOCK DIAGNOSIS
Model and Year: 1988-89 A CAR 1990 W CAR WITH 2.5L (VIN CODE R) ENGINES AND AUTOMATIC TRANSMISSION
Customer comments may be received regarding engine knock or rattle noises on the following vehicles:
1988-89 A-cars with 2.5L engine (VIN Code R)
1990 W-car with 2.5L engine (VIN Code R)
Usually, this noise, is evident only on vehicles equipped with automatic transmissions. This bulletin provides guidelines wiiich may be used to distinguish between camshaft timing gear knock/rattle noise and force balancer gear knock/rattle noise. These noises sound very similar to one another under certain operating conditions and isolating the source may be difficult.
The chart below illustrates the engine and transmission operating conditions at which force balancer gear or the camshaft timing gear knock/rattle noise is noticeable.
* If the knock/rattle noise occurs cold, it is not the force balancer.
* If the knock/rattle noise occurs within the transmission in park/neutral, it is not the force balancer.
In order determine which system is causing the noise, shift the transmission into gear - if the knock/rattle noise becomes evident, the noise is most likely force balancer related.
IMPORTANT: IF THIS IS THE CASE, REPLACEMENT OF THE FORCE BALANCER AND/OR CRANKSFIAFT WILL NOT CORRECT THE CONDITION.
The knock/rattle condition is caused by the particular way that the teeth of the large crankshaft gear and the small balancer intermediate gear mesh together. This noise will produce no detrimental effects to the engine. After diagnosis and confirming the knock is related to the force balancer gear to crankshaft gear mesh, additional efforts to correct this condition should not be performed.
If the knock/rattle noise is not force balancer related, further diagnosis per the Service Manual is recommended to correct the condition.