ROUGH IDLE/COLD START STALL NEW CALIBRATION PROM

SUBJECT: ROUGH IDLE/STALL AFTER COLD START

MODELS: 1988-91 S/T TRUCK W/4.3L AND AUTOMATIC TRANSMISSION (4L60)

APPLICATION:

1988-91 4.3L Auto. S/T Truck W/Calif. Emission (NB2)

1988-90 4.3L Auto. S/T Truck W/Federal Emission (NA5)

CONDITION:

---------- Some of the above trucks may exhibit a rough idle, sag or stalling condition after cold start-up at intermediate ambient temperatures of 30-70 degrees F. This condition may be most noticeable while performing a Park to Drive/Reverse shift. The condition improves as the engine reaches normal operating temperature.

Cause:

------ The condition is due to low idle speeds and/or lean air/fuel ratios after cold start-up.

CORRECTION:

------------ To correct this condition install a new service calibration PROM which revises warm-up idle speeds. The new PROM will also raise the normal con- trolled idle speed from 500 RPM to 525 RPM in drive. Reference the chart below for the proper PROM application.

SERVICE PARTS INFORMATION

Service Broadcast Series Engine Trans. Emissions PROM P/N Prom ID Code ------------------------------------------------------------------------ S/T 4.3L Auto. Calif. NB2 16156045 5931 AWLN S/T 4.3L Auto. Fed./Alt. 16147593 3151 ATBA (NA5/NA6)

Note: These PROMs were released to production as follows: The Federal PROM is in all 1991 models. The California PROM will go into production in August, 1990 for 1991 models. Parts are currently available from GMSPO.

WARRANTY INFORMATION

Labor Operation: T0500

Labor Time: 0.8 hr.

NOTE: Labor Operation is coded to base vehicle coverage in the warranty system.

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.