INFORMATION ON DIAGNOSING INTERMITTENT ENGINE IDLE RPM. Pontiac Firebird 1991

SUBJECT: INFORMATION ON DIAGNOSING INTERMITTENT ENGINE IDLE RPM INCREASE

MODELS: 1988-89 BUICK CENTURY AND REGAL 1988-89 CHEVROLET BERETTA/CORSICA, CAVALIER AND CELEBRITY 1988-89 OLDSMOBILE CUTLASS CIERA AND SUPREME 1988-89 PONTIAC GRAND PRIX AND 6000 EQUIPPED WITH 2.8L ENGINE (VIN W - RPO LB6)

1989-93 BUICK REGAL 1990 CHEVROLET CELEBRITY 1990-92 CHEVROLET CAMARO 1990-93 CHEVROLET BERETTA/CORSICA 1990-94 CHEVROLET CAVALIER AND LUMINA 1989-93 OLDSMOBILE CUTLASS SUPREME 1989-91 PONTIAC 6000 1989-93 PONTIAC GRAND PRIX 1990-92 PONTIAC FIREBIRD 1991-94 PONTIAC SUNBIRD EQUIPPED WITH 3.1L ENGINE (VIN T - RPO LH0)

1991-94 CHEVROLET LUMINA 1991-94 OLDSMOBILE CUTLASS SUPREME 1991-94 PONTIAC GRAND PRIX EQUIPPED WITH 3.4L ENGINE (VIN X - RPO LQ1)

1993-94 CHEVROLET CAMARO 1993-94 PONTIAC FIREBIRD EQUIPPED WITH 3.4L ENGINE (VIN S - RPO L32)

1994 BUICK REGAL AND SKYLARK 1994 CHEVROLET BERETTA/CORSICA 1993-94 OLDSMOBILE CUTLASS SUPREME 1994 OLDSMOBILE ACHIEVA 1994 PONTIAC GRAND AM AND GRAND PRIX EQUIPPED WITH 3.1L ENGINE (VIN M - RPO L82)

THE FOLLOWING INFORMATION MAY ASSIST IN THE DIAGNOSIS AND CORRECTION OF AN INTERMITTENT INCREASE IN ENGINE RPM WITH CLOSED THROTTLE (ACCELERATOR PEDAL NOT DEPRESSED). TYPICALLY THERE IS NO MALFUNCTION INDICATOR LIGHT ASSOCIATED WITH THE CONDITION.

THE ENGINE CONTROL SYSTEM SENSORS ARE SUPPLIED A 5 VOLT REFERENCE SIGNAL TO ENSURE PROPERLY SCALED READINGS TO THE ENGINE CONTROL MODULE (ECM). IF A LARGE CURRENT DRAW FLOWS THROUGH THE CIRCUIT SUPPLYING THE 5 VOLT REFERENCE, THE INPUT SIGNALS, AS READ BY THE ECM, WILL BE SKEWED LOW. THE ECM PROGRAM WILL STORE AN INCORRECTLY LOW THROTTLE POSITION SENSOR SIGNAL AS ZERO. IF THE REFERENCE VOLTAGE RETURNS TO NORMAL, THE ECM WILL INTERPRET THE RESULTANT HIGH THROTTLE POSITION SENSOR SIGNAL AS AN OPEN THROTTLE AND IDLE AIR CONTROL STEPS (THROTTLE FOLLOWER) WILL BE ADDED ACCORDINGLY, CAUSING AN INCREASE IN THE ENGINE RPM AT IDLE.

THE A/C PRESSURE TRANSDUCER HAS BEEN FOUND TO BE ESPECIALLY VULNERABLE TO SHORTS DURING THE ENGINE CRANK PROCESS. ELECTRICAL WIRES IN THE ENGINE HARNESS THAT ARE UNPROTECTED AND RUB ON STRUCTURAL MEMBERS ALSO WILL CAUSE THE PROBLEM.

INSPECT ALL WIRING AND SENSORS ASSOCIATED WITH THE 5 VOLT REFERENCE SIGNAL FOR SHORTS AND CORRECT AS NECESSARY.

IF THE VEHICLE ALSO HAS A CONDITION ASSOCIATED WITH THE AIR CONDITIONING SYSTEM, DIAGNOSE AND CORRECT AS NECESSARY.

WITH A TECHLINE DIAGNOSTIC TOOL, READ THE THROTTLE POSITION SENSOR, MANIFOLD ABSOLUTE PRESSURE SENSOR, AND A/C PRESSURE TRANSDUCER VOLTAGE WITH THE KEY ON-ENGINE OFF.

IF ALL THREE VOLTAGE READINGS ARE SKEWED BELOW THEIR NORMAL RANGES, INSPECT THE A/C PRESSURE TRANSDUCER.

IF ALL THREE VOLTAGE READINGS ARE WITHIN NORMAL RANGES, DISCONNECT THE BATTERY FOR ONE (1) MINUTE, RECONNECT AND START THE ENGINE. IF THE INCREASE IN ENGINE RPM AT IDLE CONDITION IS GONE, INSPECT THE A/C PRESSURE TRANSDUCER.

NOTE: ON THE LB6 (2.8L VIN W), LH0 (3.1L VIN T) OR LQ1 (3.4L DOHC VIN X) ENGINES, IF THE ECM IS WITHOUT BATTERY POWER FOR ANY REASON, THEN THE IDLE AIR CONTROL VALVE (IACV) POSITION INFORMATION FOR A STABLE SATISFACTORY ENGINE IDLE IS "LOST" AND REPLACED WITH A DEFAULT VALUE. TO CORRECT THE IACV POSITION INFORMATION, PERFORM AN IDLE RELEARN PROCEDURE USING A TECHLINE DIAGNOSTIC TOOL. YOU CAN ALSO MANUALLY RELEARN THE IDLE BY FOLLOWING THE PROCEDURE BELOW.

1. REMOVE BATTERY POWER FOR ONE (1) MINUTE. RESTORE BATTERY POWER AND PLACE AIR CONDITIONING CONTROLS (IF EQUIPPED) IN THE "OFF" POSITION.

2. FIRMLY APPLY THE PARKING BRAKE AND BLOCK THE DRIVE WHEELS. START THE ENGINE, SHIFT THE TRANSMISSION TO DRIVE FOR AUTOMATICS AND NEUTRAL FOR MANUALS.

3. ALLOW THE ENGINE TO RUN UNTIL THE ENGINE COOLING FAN HAS CYCLED ONCE, OR A MAXIMUM OF 10 MINUTES.

4. TURN THE IGNITION KEY TO THE "OFF" POSITION FOR AT LEAST 5 SECONDS.

5. REPEAT STEP 2.

6. ALLOW THE ENGINE TO RUN FOR AT LEAST FIVE (5) MINUTES.

THE CORRECT IACV VALUES FOR PROPER IDLE CONTROL WILL NOW BE STORED.

FIGURES: 0

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.