Engine - Unstable Idle/Decel Surge and False Code 42

88chevy07
GMC NUMBER: 87-T-112

GROUP: 6E-Engine Emissions

DATE: April, 1988


CORPORATE
NUMBER: 736518R

SUBJECT: UNSTABLE IDLE, DECEL SURGE AND FALSE 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 transmission (45-55 mph).

If normal service procedures in the Fuel, Driveability and Emission Manual do not correct 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
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 the 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 the described vehicles. For repairs to vehicles under warranty use:
Labor Op: T7590
Labor Time: .8 hrs. S/T Truck .6 hrs M-Van