Diagnostic Aids
A slow or unstable idle may be caused by a system problem that cannot be overcome by IAC. If idle is too high, stop engine and plug IAC valve air inlet with Test Tool (BT-8528A). If idle speed is greater than 900 RPM, locate and correct vacuum leak. System Too Lean (High Air/Fuel Ratio)
Idle speed may be too high or to low. Engine speed may vary and disconnecting the IAC does not help. A Code 44 may be set. The "Scan" tester or voltmeter will read an Oxygen (O2) sensor output less than .3 volt. Check for low fuel pressure or water in fuel. A lean air/fuel ratio with O2 sensor output fixed greater than .8 volt indicates a contaminated O2 sensor, usually by silicone. This may also set a Code 45. System Too Rich (Low Air/Fuel Ratio)
Idle speed is to low. "Scan" tester counts are usually greater than 80, and voltmeter will read an O2 sensor output signal fixed greater than .8 volt. Check for high fuel pressure or fuel injector leaking. Throttle Body
Remove IAC and inspect bore for foreign material and evidence of valve dragging in bore.