Grand Prix V6-3100 3.1L MFI VIN M (1994)
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.
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), LHO (3.1L VIN T) or L01 (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.
Technical Service Bulletin # 476510
Date: 940501
Engine - Diagnosing Intermittent Idle Speed Increases
Group Ref.: Engine Fuel & Emission
Bulletin No.: 476510
Date: May, 1994
INFORMATION
