ENGINE DETONATION (INSTALL NEW PROM PACKAGE)

Subject: ENGINE DETONATION (INSTALL NEW PROM PACKAGE)

Models Affected: 1990-91 GRAND AM MODELS WITH AUTOMATIC TRANSMISSION AND 2.5L ENGINE (RPO L68)

This bulletin is being revised to add the 1991 model year and a corresponding new PROM package. This bulletin cancels and replaces bulletin 90-6-50 issued 8191. Please discard bulletin 90-6-50.

Condition:

--------- Some owners of 1990-91 Grand Am models may comment on more than normal engine detonation. This level of detonation is more than the burst detonation that is inherent with the 2.5L engine at off-idle tip-in at elevated coolant temperatures. Detonation may occur as steady state detonation, detonation on a slight grade or detonation on a light acceleration. Prior attempts to correct this detonation may include the use of top engine cleaners, decarbing or installation of a new head gasket. However, the detonation will return in a short amount of mileage.

Cause:

----- The octane requirement of the engine increases with mileage. Therefore the spark calibration in the original tables may produce detonation in some vehicles.

Correction:

---------- The correction for this condition is a new PROM package. This PROM will modify the spark tables and raise the TCC (Torque Converter Clutch) engagement point to 44 MPH. The customer should also be advised that this PROM may have a negative impact on the fuel economy.

Model Year 1990 1991 ---------- ---- ---- The new PROM Package is: P/N 16168732 P/N 16176078

Broadcast Code: AZBH BATT

Scanner ID: 8791 6081

Parts are currently available from GMSPO.

Labor Operation: T0500

Labor Time .3 Hrs.

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.