Diagnostic Aids
A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be above 80 if if too low, and zero counts, if too high.
- If idle is 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 above minimum rate, look for possible vacuum leaks. If no leaks are detected, adjust minimum idle speed.
- If air/fuel ratio is too lean the idle speed may be either too high or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or voltmeter will read an oxygen sensor output less than 300 mv (.3 volts). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (usually silicone) will cause a lean air/fuel mixture with an oxygen sensor output fixed above 800 mv (.8 volts). This may also set Code 45.
- If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be above 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed above 800 mv (.8 volts). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.
- Inspect area beneath throttle body for "coking". If coking exists, the throttle body must be cleaned before setting minimum air rate or replacing the IAC.