BASE IDLE ADJUSTMENT REVISED DIAGNOSTICS

MODELS AFFECTED: 1988 REGALS, AND 1988 CENTURYS WITH 2.8L ENGINES (VIN CODE W)

NOTICE: This bulletin supersedes Bulletin 88-6-12 and revises the bulletin number. Bulletin 88-6-12 should be discarded.

The following service information revises the recommended diagnostic procedure (reference sheet 6E3-B [symptoms] of the Service Manual) for rough, unstable or incorrect idle and stalling at idle for 1988 Buicks equipped with a 2.8L V-6 MPFI engine (VIN Code W).

A base idle RPM adjustment is not required and should not be performed on 1988 vehicles.

The following information provides an explanation for the above revision.

All 2.8L MPFI V-6 engine calibrations for 1988 production and 1987 service include a feature which stores in Electronic Control Module (ECM) memory the Idle Air Control Valve (IAC) position at which the engine idles. Each vehicle will have a unique value, depending on the throttle body base air rate setting, deposit buildup, etc. The IAC position information is continuously updated, during vehicle operation, and stored in the ECM "keep alive" memory when the engine is turned off.

NOTICE: If the ECM is without battery power for any reason, then the IAC position infomation for a stable satisfactory engine idle is "lost" and is replaced with a "Default" value.

To correct the IAC position information due to an absence of battery power and improve the engine idle, perform the following procedure:

1. Restore the battery power and place air conditioning controls (if equipped) in the OFF POSITION.

2. Start the engine and shift the transmission to "drive" for automatics or "neutral" for manuals.

3. Allow the engine to run approximately five minutes until the normal operating temperature is attained.

4. After attaining operating temperature, allow the engine to idle for an additional two minutes.

This procedure will allow the ECM memory to be updated with the "correct" IAC position, for the vehicle, and provide a stable idle RPM.

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.