REVISED PROMS/DRIVEABILITY FALSE SERVICE ENGINE SOON. GMC Truck GMC K Pickup

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

LO3 (5.0L) TBI ENGINE ---------------------- Emissions New Prom Body Style Series Type Transmission Axle Kit # ---------- ------ ---- ------------- ---- ----- Pickup 1500 Fed (NA5) 3-Spd Auto. (M40) 2.73(GU2) 16120046 3.08(GU4) Pickup 1500 " " " " " 3.42(GU6) 16120073 3.73(GU4) Pickup 1500 " " 4-Spd Auto.(MD8) 2.73(GU2) 16120077

Pickup 1500 " " " " " 3.08(GU4) 16120082

Pickup 1500 " " " " " 3.42(GU6) 16120086 3.73(GT4) Pickup 1500 Altitude (NA6) 3-Spd Auto.(M40) All 16120090

Pickup 1500 " " 4-Spd Auto.(MD8) All 16120095

*Pickup 2500 Fed/Alt 3-Spd Auto.(40) All 16120099 (NA5/NA6)

Pickup 4WD 15-2500 " " 4-Spd Auto.(MD8) All 16120105

Pickup 2500 " " " " " All 16120105

Suburban 1500 " " " " " All 16120105

Blazer/Jimmy 1500 " " " " " All 16120105

New Broadcast New Prom Code I.D. Body Style Series (Prom Label) (Scan Tool) ---------- ------ ------------ -----------

Pickup 1500 AMTS 9871

Pickup 1500 AMTT 9881

Pickup 1500 AMTU 9891

Pickup 1500 AMTW 9901

Pickup 1500 AMTX 9911

Pickup 1500 AMTY 9921

Pickup 1500 AMTZ 9931

*Pickup 2500 AMUA 9941

Pickup 4WD 15-2500 AMUB 9951

Pickup 2500 AMUB 9951

Suburban 1500 AMUB 9951

Blazer/Jimmy 1500 AMUB 9951

L03 (5.0L) TBI ENGINE ----------------------

EMISSIONS NEW PROM BODY STYLE SERIES TYPE TRANSMISSION AXLE KIT # ---------- ------- ---- ------------ ---- ------

Pickup 4WD 15-2500 Fed/Alt 3-Spd Auto.(M40) All 16120109

Pickup " " " " 5-Spd Man. (MG5) All 16120114

Pickup 4WD " " " " " " " All 16120118

Pickup 1500 California 4-Spd Auto. (MD8) 2.73(GU2) 16120042 (NB2)

Pickup 1500 " 4-Spd Auto. (MD8) 3.08(GU4) 16120125 3.42(GU6) 3.73(GT4)

Pickup 4WD 15-2500 " " " " " " 16120129

Suburban 1500 " " " " All 16120129

Blazer/Jimmy 1500 " " " " All 16120129

Sportvan/Rally 15-2500 " " " " All 16120139

Sportvan/Rally " " Fed/Alt 4-Spd Auto.(MD8) All 16120143 (NA5/NA6)

Sportvan/Rally " " " " 3-Spd Auto.(M40) All 16120147

Sportvan/Rally " " " " 4-Spd Man. (MY6) All 16120151

NEW BROADCAST NEW PROM CODE I.D. BODY STYLE (PROM LABEL) (SCAN TOOL) ---------- --------------- ------------

Pickup 4WD AMUC 9961

Pickup AMUD 9971

Pickup 4WD AMUF 9981

Pickup AMUH 9991

Pickup AMUJ 0001

Pickup 4WD AMUK 0011

Suburban " 0011

Blazer/Jimmy " 0011

Sportvan/Rally AMUL 0021

Sportvan/Rally AMUM 0031

Sportvan/Rally AMUN 0041

Sportvan/Rally AMUP 0051

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.