REVISED PROMS/DRIVEABILITY- FALSE 'SERVICE ENGINE SOON'. GMC Truck GMC V Truck

VEHICLES AFFECTED: 1988 C, K TRUCKS; 1987-88 R, V TRUCKS; 1987-88 G VANS WITH 5.0L (VIN CODE H) AND 5.7L (VIN CODE K) ENGINES WITH LIGHT DUTY EMISSIONS

THIS CANCELS AND REPLACES PSB 87-650-113 AND 87-657-109 TO RELEASE NEW PROMS WITH ADDITIONAL DRIVEABILITY IMPROVEMENTS.

Some 1987-88 R, V Trucks and G Vans/1988 C, K Trucks and G Vans with TBI engines may experience one or more of the following driveability conditions and/or false codes. If these conditions are experienced, they can be repaired by replacing the PROM.

Driveability Conditions - 5.0L (VIN Code H) TBI Engine

1. Rolling, surging or rough idles. 2. False Code 54 - Fuel pump circuit (low voltage) 3. False Code 43 - ESC fault 4. Tip-In Sags/Hesitations during engine warm-up

Driveability Conditions - 5.7L (VIN Code K) TBI Engine

1. Rolling, surging, or rough idles.

NOTICE: IN ADDITION TO THE PROM CHANGE, IT WILL BE NECESSARY TO RESET MINIMUM IDLE SPEED, PER THE PROCEDURE TO THE FOLLOWING VALUES:

A. Automatic (Drive) 425 plus or minus 25 RPM B. Manual (Neutral) 500 plus or minus 25 RPM

2. Low speed road load surge 3. Detonation and weak acceleration during warm-up 4. Poor cold starting at -28 degrees Celsius 5. False Code 24 - Vss circuit fault when using 4 wheel drive low lock. 6. Tip-In Sags/Hesitations during engine warm-up 7. Long crank times, hot or cold.

* CONDITION SERVICE ACTION --------- -------------- Driveability On 1988 models, to determine whether or not the revised calibration PROM was built into the vehicle, use a "Scan" tool to check to see if vehicle has a revised calibration PROM (see chart attached). Install revised calibration PROM if vehicle is not so equipped.

Use Labour Time: .6 hr C, K, R, V .7 hr G

L05 (5.7L) TBI ENGINE

NEW NEW BROADCAST PROM BODY EMISSIONS NEW PROM CODE I.D. STYLE SERIES TYPE TRANSMISSION AXLE KIT # (PROM LABEL) (SCAN ----- ------ ---- ------------ ---- ----- ------------ -----

Pickup 15-2500 Fed/Alt 4-Spd Auto. All 16120054 AMUR 0061 -2&4WD (NA5/NA6) (MD8)

Suburban 1500 " " " " All 16120054 " " -2&4WD

Blazer 1500 " " " " All 16120054 " " /Jimmy

Sportvan 15-25-3500" " " " All 16120054 " " /Rally

Pickup 15-2500 California " " All 16120059 AMUS 0071 -2&4WD (NB2)

Suburban 1500 " " " All 16120059 " " -2&4WD

Blazer 1500 " " " All 16120059 " " /Jimmy

Sportvan 15-25-3500 " " " All 16120059 " " /Rally

Pickup 15-2500 Fed/Alt 4-Spd Man. All 16120064 AMUT 0081 -2&4WD (NA5/NA6) (M20)

Suburban 1500 " " " " All 16120064 " " -4WD

Pickup 15-2500 " " 5-Spd Man. All 16120068 AMUU 0091 -2&4WD (MG5)

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.