THUMP/KNOCK NOISE AT IDLE INSTALL NEW DESIGN DRIVE BELT

SUBJECT: THUMP OR KNOCK NOISE AT OR JUST OFF IDLE (INSTALL NEW SINGLE BELT ACCESS DRIVE BELT)

VEHICLES AFFECTED: 1992 CHEVROLET W-CAR; U-VAN 1992 PONTIAC H,N,W-CAR; U-VAN 1992 OLDSMOBILE A,C,E,H,N,W-CAR; U-VAN 1992 BUICK A,C,E,H,N,W-CAR W/3300 (VIN CODE N) OR 3800 (VIN CODE L ENGINE)

THIS BULLETIN IS BEING REVISED TO CHANGE BELT PART NUMBERS.

New S.B.A.D. belts were released for production October 1, 1991 and are now available for service on 1992 3300/3800 V6 engines. These new belts, with smoother grooves, greatly reduce "belt pilling". Belt pilling is a condition in which particles of belt material forms small balls or "pills" in the belt grooves. Some degree of pilling is normal, however, excessive pilling can cause a thump or knock noise when the pills pass over various pulleys. The thump or knock noise could be misdiagnosed as an engine or accessory noise.

The following checklist will help diagnose and repair a belt pilling condition:

1. Remove the belt and momentarily run the engine to confirm that the source of the noise has disappeared.

2a. If the noise does not disappear, there are other factors that can cause a thump or knock noise. See Section 6 in the service manual for additional diagnostic information.

2b. If the noise does disappear; inspect the power steering pulley for proper alignment and repair as necessary.

3. Inspect all pulleys for excess paint, debris or pills in the grooves - clean as necessary.

4. Install a new belt from the listed service part numbers in the information section of this publication.

SERVICE PARTS INFORMATION:

MODEL PART NUMBER CODE ----- ----------- ---- W, H, C, E and V 25532180 SAT

A 24502761 SEU

N 24502762 SEV

NOTE: Supercharged 3800 V6 engines were all produced with new smoother groove belts. The part numbers for these belts are:

Supercharged Drive 24501446

Alternator Drive 24501447

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.