ENGINE DETONATION (NEW PROM PACKAGE RELEASED)

SUBJECT: ENGINE DETONATION (INSTALL NEW PROM PACKAGE)

VEHICLES AFFECTED: 1990 N CAR WITH AUTOMATIC TRANSMISSION AND 2.5L ENGINE, VIN 'U'

Condition:

----------- Some owners of 1990 N cars 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 Convertor Clutch) engagement point to 70 KPH (44 MPH). The customer should also be advised that this PROM may have a negative impact on the fuel economy.

The new PROM Package is: P/N 16168732

Broadcast Code: AZBH

Scanner ID: 8791

Part is currently available from CANSPO.

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.