REVISED ENGINE KNOCK DIAGNOSIS (NOISES)

Models 1988-89 6000, GRAND AM AND FIERO MODELS Affected: WITH 2.5L (VIN CODES U AND R) AND AUTO TRANSMISSION

Customer comments may be received regarding engine knock or rattle noises on the following vehicles:

1988-89 Grand Am models with 2.5L engine (VIN Code U) 1988-89 6000 models with 2.5L engine (VIN Code R) 1988 Fiero models with 2.5L engine (VIN Code R)

Usually, this noise is evident only on vehicles equipped with automatic transmissions. This bulletin provides guidelines which 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.

ENGINE OIL TEMPERATURE

HOT COLD

TRANSMISSION FORCE BALANCER IN GEAR CAM GEAR CAM GEAR

TRANSMISSION CAM GEAR CAM GEAR PARK/NEUTRAL

* If the knock/rattle noise occurs cold, it is not the force balancer.

* If the knock/rattle noise occurs with the transmission in park/neutral, it is not the force balancer.

In order to 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 CRANKSHAFT 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.

If the knock/rattle noise is not force balancer related, further diagnosis per the Service Manual is recommended to correct the condition.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.