Beretta V6-173 2.8L (1989)
Idle Speed: Customer Interest
Engine - Diagnosing Intermittent Idle Speed Increases
Group Ref.: Engine Fuel & Emission
Bulletin No.: 476510
Date: May, 1994
INFORMATION
SUBJECT:
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
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
WITH 3.1L ENGINE (VIN T - RPO LHO)
1991-94 CHEVROLET LUMINA
1991-94 OLDSMOBILE CUTLASS SUPREME
1991-94 PONTIAC GRAND PRIX
WITH 3.4L ENGINE (VIN X - RPO L01)
1993-94 CHEVROLET CAMARO
1993-94 PONTIAC FIREBIRD
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
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.
