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

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

MODELS: 1990-91 S/T TRUCKS WITH 4.3L ENGINE (RPO LB4) AND 5LM60 M/T TRANSMISSION

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:

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

- Rough idle, cold or hot

CAUSE:

----- These conditions are related to EGR (Exhaust Gas Recirculation), fuel and spark control calibrations, and idle speed.

CORRECTION:

---------- A new service calibration has been developed which contains updates to EGR, Fuel and Spark Control and increases idle speed to 700 rpm from 600 rpm. To correct any of the above conditions install a new prom (Programmable Read Only Memory) using the application chart below.

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 PARTS INFORMATION:

Parts are expected to be available on 10/12/92. In case of limited inventory, parts will be placed on 400 control to waive VIP surcharges. Only verifiable emergency VIP orders will be accepted. GMSPO will make every effort to obtain parts. However, the part will be shipped premium transportation at dealer's expense and all other order types will be placed on backorder until the 400 control is removed.

WARRANTY INFORMATION:

Labor Operation: T0500 Labor Time: 0.8 Hr.

Service PROM Scanner Broadcast Series Emissions Trans. Axle P/N I.D. Code ------ --------- ------ ---- --------- ---- ---- S1 FED/ALT/CA 5-Spd. MY2 2.73 GU2 16184103 4201 BCWS NA5/NA6/NB2

S1, T1 FED/ALT/CA 5-Spd. MY2 3.08 GU4 16184104 4211 BCWT NA5/NA6/NB2

S1, T1 FED/ALT/CA 5-Spd. MY2 3.42 GU6 16184106 4221 BCWU NA5/NA6/NB2

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.