SURGE UNDER LIGHT THROTTLE CRUISE/COAST

SUBJECT: SURGE OR CHUGGLE ON DECEL AND/OR ROUGH IDLE

MODEL AND YEAR: 1990-91S/T TRUCKS WITH 4.3L ENGINE AND 5LM60 MANUAL TRANSMISSION

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 90-425-6E, DATED MAY 1991. THE 1991 MODEL YEAR HAS BEEN ADDED AS WELL AS CAUSE AND CORRECTION REVISED. ALL COPIES OF 90-425-6E SHOULD BE DISCARDED.

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.

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

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

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

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

Part Availability:

Parts are currently available from GMSPO.

Warranty Information:

Labor Operation: T0500 Labor Time: 0.8 Hr.

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.