Diagnostic Aids
A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 80 if if too low, and zero counts if too high. Other causes for improper idle are as follows:
- Idle Too High - Stop engine. With ignition on, ground diagnostic terminal. Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than minimum rate, look for possible vacuum leaks. If no leaks are detected, adjust minimum idle speed.
- Air/Fuel Ratio Too Lean - Idle speed may be either high or low. Engine speed may fluctuate and disconnecting IAC may not help. "Scan" tester and/or voltmeter will read oxygen sensor output less than .3 volt. Check for low fuel pressure or water in fuel. A contaminated O2 sensor (usually silicone) will cause a lean air/fuel mixture with oxygen sensor output fixed greater than .8 volt. This may also set Code 45.
- Air/Fuel Ratio Too Rich - Idle speed will be too low and "Scan" tester counts will usually be greater than 80. System may be rich, with Black smoke from tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than .8 volt. Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging bore.
- Throttle Body - Inspect area beneath throttle body for "coking". If coking exists, throttle body must be cleaned before setting minimum air rate or replacing IAC.