HIGH ALTITUDE SURGE LIGHT THROTTLE CRUISE/COAST. GMC Truck Jimmy

SUBJECT: SURGE OR CHUGGLE ON DECEL AND/OR ROUGH IDLE (REPLACE ECM AND PROM)

VEHICLES AFFECTED: 1990-91 S/T TRUCKS WITH 4.3L ENGINE (VIN Z, RPO LB4) AND 5 SPEED (5LM60, RPO MY2) MANUAL TRANSMISSION

THIS BULLETIN SUPERSEDES AND REPLACES PRODUCT SERVICE BULLETIN 90-643-105 (DATED DEC. 31/90). THIS BULLETIN IS BEING REVISED TO ADD 1991 MODEL YEAR VEHICLES AND TO REVISE CONDITIONS AND CAUSES COVERED.

CONDITION:

--------- Owners of some 1990-1991 S/T trucks equipped with 5-speed manual transmissions may comment about one or more of the following conditions:

o An engine deceleration surge or chuggle at light throttle cruise or coast

o Rough idle, cold or hot

CAUSE:

----- These conditions are related to EGR, fuel and spark control calibrations.

CORRECTION:

---------- To correct any of these conditions install a new calibration PROM (Programmable Read Only Memory) in the truck. Use the application chart below to select the proper PROM.

NOTE: 1990 model year trucks require ECM (Engine Control Module) P/N 16144288 in order to use the new prom. 1991 model year trucks should already have this engine controller. Also note that the new calibration is nationwide, replacing both federal and California emission original production calibrations.

SERVICE SCANNER BROADCAST SERIES EMISSIONS TRANS AXLE PROM P/N I.D. CODE ------ --------- ----- ---- -------- ------- ---------

S1 FED/ALT/CAL 5-SPD 2.73 16184103 4201 BCWS NA5/NA6/NB2 MY2 GU2

S1, T1 FED/ALT/CAL 5-SPD 3.08 16184104 4211 BCWT NA5/NA6/NB2 MY2 GU4

S1, T1 FED/ALT/CAL 5-SPD 3.42 16184106 4221 BCWU NA5/NA6/NB2 MY2 GU6

For vehicles repaired under warranty use:

Labour Time: 0.8 Hours

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.