CORRECTION FOR MULTIPLE FALSE CODES/DRIVEABILITY CONDITIONS

MODELS: 1987-88 S/T/M TRUCK AND 1989 S TRUCK W/2.5L (VIN CODE E) ENGINE

This bulletin cancels and supersedes GMC Truck Service Bulletin 88-T-178 to include the 1989 S-Truck with 2.5L engine. All copies of 88-T-178 should be discarded.

Some 1987-88 S/T/M trucks and 1989 S trucks may experience one or more of the following false codes and/or driveability conditions:

1. False Code 32 2. False Code 42 3. Intermittent High Idle 4. Unstable Idle 5. Decel Surge 6. Detonation(SparkKnock) 7. High Idle On Initial Shift Into Drive Gear

If these conditions are experienced and cannot be corrected through normal service procedures, they may be repaired by installing the appropriate MEM- CAL kit as shown in Table I, "Application Table." The driveability and false code conditions that are corrected by the MEMCAL kits for each specific vehicle application are shown in Table II.

Listed below are some symptoms and circumstances of the driveability conditions and false codes:

1. False Code 32

A. A "Check Engine Soon" light is on; may or may not go off during driving.

B. Code 32 stored in the ECM.

C. No noticeable loss in performance.

2. False Code 42

A. A "Check Engine Soon" light is on; may or may not go off during driving.

B. Code 42 stored in the ECM.

C. No noticeable loss in performance.

3. Intermittent High Idle With Clutch Disengaged

A. Engine at normal operating temperature with manual transmission.

B. Excessive amount of time to return to normal idle speed.

C. Can occur after "lugging" the engine.

4. Unstable Idle

A. Idle is unstable, rolls, hunts with either a hot or cold engine.

5. Decel Surge

A. Vehicle surges on deceleration.

6. Detonation (Spark Knock)

A. Typically occurs under steady throttle at highway speeds.

7. High Idle On Initial Shift Into Drive Gear

A. Higher than normal engine idle speed upon initial drive gear engagement that decreases within a few seconds. This "drive only" RPM condition is similiar to a "carbureted fast idle" or "high cam step" level.

The condition may be brought on by repeated engine "revving" in park/neutral or by a rolling (stopping action while in neutral. This condition will only occur in a non A/C vehicle or with A/C off.

IMPORTANT: IF PROM I.D. in the "Application Table" matches what is currently in the ECM, do not install a new MEM- CAL because it will not correct the condition and the problem may lay elsewhere.

TABLE I APPLICATION TABLE

MODEL VEHICLE TRANS. MEMCAL KIT PROM I.D. BROADCAST CODE YEAR LINE AXLE P/N (SCANNER) (PROM LABEL)

87/88 S 4-SPD.MAN. 16130635 0651 APHF 87/88 S 5-SPD.MAN. 16130830 0741 APHP 87/88 S 4-SPD.AUTO. 16129955 0041 APDJ 89 S 5-SPD.MAN. 16135832 5811 ARFR 4.11 AXLE (HO4) 89 S 5-SPD. MAN. 16135836 5831 ARFT 3.73 AXLE (GT4) 89 S 4-SPD. AUTO 16135843 5851 ARHB (MD8)

87 T 4-SPD.MAN. 16073744 9321 ACJN 87 T 5-SPD. MAN. 16074844 9341 ACJR 87/88 T 4-SPD.AUTO. 16086793 6771 ALYD

87 M 4-SPD. MAN. 16074854 9361 ACJT 87 M 5-SPD. MAN. 16074858 9371 ACJU 87/88 M 4-SPD.AUTO. 16086797 6781 ALYF

GM POSTAL VEHICLE: 87/88 S 3-SPD.AUTO. 16086917 7161 AMAW

Parts are currently available from GMSPO.

NOTICE TO DEALERS:

When the above 1987 vehicles are serviced with the correct MEMCAL kit as specified in this bulletin, scan tools with 1987 programs and CAMS terminals with software versions prior to 19.00 will read only the following information correctly:

1. PROM I.D. 2. TROUBLE CODES 3. COOLANT 4. TPS 5. RPM 6. MPH 7. BLOCK LEARN 8. INTEGRATOR 9. I.A.C. COUNTS 10. MAP

Modifications have been made starting in CAMS software version 19.00 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 MEMCALS in the described vehicles.

For vehicles repaired under warranty, use:

Labor Operation: T0500 Labor Time: .7Hr

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.