ROUGH IDLE, SURGE ON DECEL INTERMITTENT CODE 42

MODELS: 1987 M, S/T TRUCKS WITH 2.5L (VIN CODE E) ENGINE

THIS BULLETIN CANCELS AND SUPERSEDES TRUCK SERVICE BULLETIN 87-T-79 DATED AUGUST, 1987 ALL COPIES OF WHICH SHOULD BE DESTROYED. THIS BULLETIN IS BEING REVISED TO INCLUDE UPDATED PROM PART NUMBERS, PROM I.D. NUMBERS AND PROM BROADCAST CODES.

Some 1987 M vans and S/T trucks with a 2.5L TBI engine may experience one or more of the following conditions:

1. Idle is unstable, rolls, hunts both hot and cold.

2. Vehicle surges on deceleration.

3. False Code 42 - electronic spark timing.

4. Detonation (spark knock) on manual transmis sion (45-55 mph).

If normal service procedures in the Fuel, Driveability and Emission Manual do not collect the condition, a 1988 calibration PROM is now available and can be used to correct these conditions on the 1987 models listed.

MODEL PROM APPLICATION ----------------------

PROM PROM PROM Broadcast Model Eng Trans P/N I.D. Code ----- --- ----- ---- ---- --------- MVan 2.5L 4 Spd. Man 16074854 9361 ACJT M Van 2.5L 5 Spd. Man 16074858 9371 ACJU M Van 2.5L 4 Spd. Auto 16086797 6781 ALYF

Blazer/ 2.5L 4 Spd. Man 16073744 9321 ACJN Jimmy (2 and 4 WD)

S Pickup 2.5L 4 Spd. Man 16074884 9441 ACKC (2 WD)

T Pickup 2.5L 4 Spd. Man 16073744 9321 ACJN (4 WD)

T All Models 2.5L 5 Spd. Man 16074844 9341 ACJR (4 WD)

S All Models 2.5L 5 Spd. Man 16074888 9451 ACKD (2 WD)

T All Models 2.5L 4 Spd. Auto 16086793 6771 ALYD (4 WD)

S All Models 2.5L 4 Spd. Auto 16088437 8441 AMJW (2 WD)

NOTICE TO DEALERS: When using the above 1988 PROMS, in the 1987 vehicles described in this bulletin, SCAN tools with 1987 programs and CAMs terminals with software versions prior to 19.000 will read only tbe following information correctly:

1. PROM I.D.

2. TROUBLE CODES (including new code 32 EGR Failure - see attached chart)

3. COOLANT

4. TPS

5. RPM

6. MPH

7. BLOCK LEARN

8. INTEGRATOR

9. IDLE AIR CONTROL

10. MAP

Modifications have been made starting in CAMS software version 19.000 that will allow for correct reading of all information. 1988 versions of scan tool programs will include the ability to read all information correctly.

CAMs terminals with software versions 18.025 and earlier should not be used for diagnosis of the listed PROMS in tbe described vehicles.

For repairs to vehicles under warranty use:

Labor Op: T7590 Labor Time: .8 hrs S/T Truck .6 hrs M-Van

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.