Diagnostic Aids
A slow, unstable or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out-of-control range IAC scan tester counts will be greater than 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem.
- Vacuum Leak (High Idle) If idle is too high, turn ignition off. Fully extend (low) IAC with tester. Start engine. If idle speed is greater than 800 RPM, locate and correct vacuum leak (including PCV system). Also check for binding of throttle blade or linkage.
- System Too Lean (High Air/Fuel Ratio) Idle speed may be too high or too low. Engine speed may vary up and down and disconnecting IAC does not help. Code 44 may be set. Scan O2 voltage will be less than 300 mV (.3 volt). Check for low regulated fuel pressure, water in fuel or a restricted injector.
- System Too Rich (Low Air/Fuel Ratio) Idle speed will be too low. Scan tester IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke exhaust. Scan tester O2 voltage will be fixed above 800 mV (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensor will be indicated by a slow responding O2 sensor voltage when monitored with scan tester.
- Throttle Body Remove IAC and inspect bore for foreign material.
- IAC Valve Electrical Connections Carefully check IAC valve connections for proper contact.
- PCV Valve An incorrect or faulty PCV valve may result in an incorrect idle speed.
- If an intermittent condition, poor driveability or idle symptoms are resolved by disconnecting IAC, carefully recheck connections, check valve terminal resistance or replace IAC.