HIGH ALTITUDE SURGE LIGHT THROTTLE CRUISE/COAST. GMC Truck S15 Pickup

SUBJECT: SURGE UNDER LIGHT THROTTLE CRUISE/COAST AT HIGH ALTITUDE

MODELS: 1990 S/T LIGHT DUTY TRUCKS WITH 4.3L ENGINES AND 5LM60 MANUAL TRANSMISSIONS

CONDITION:

---------- Owners of some 1990 4.3L S/T trucks equipped with 5-speed manual transmissions may comment about an engine surge condition at light throttle cruise or coast while driving at altitudes above 4500ft.

CAUSE:

------ This condition is related to EGR at low barometric pressures.

Service PROM Broadcast Series Emiss. Trans. Axle PROM P/N ID Code ------ ---------- ----------- ---------- -------- ---- --------- S FED/ALT 5-spd (MY2) 2.73 (GU2) 16147384 3071 ATAR (NA5/NA6) S/T FED/ALT 5-spd (MY2) 3.08 (GU4) 16147393 3081 ATAS (NA5/NA6) S/T FED/ALT 5-spd (MY2) 3.42 (GU6) 16147397 3091 ATAT (NA5/NA6) S CALIF (NB2 5-spd (MY2) 2.73 (GU2) 16151257 1201 AULZ S/T CALIF (NB2) 5-spd (MY2) 3.08 (GU4) 16151263 1211 AUMA S/T CALIF (NB2) 5-spd (MY2) 3.42 (GU6) 16151267 1221 AUMB

CORRECTION:

-----------

To correct this condition, install a new ECM (P/N 16144288) and calibration PROM in the truck. The new calibration PROM revises the EGR calibration for high altitude driving conditions. Use the application chart below to select the proper PROM.

NOTE: All of the following PROMs must be used with ECM (PIN 16144288).

Parts are currently available from GMSPO.

WARRANTY INFORMATION

Labor Operation: T0500

Labor Time: .8 hrs.

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.