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 above 900 RPM, locate and correct vacuum leak.
If air/fuel ratio is too lean, "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 above .8 volt indicates a contaminated O2 sensor, usually silicone. This may also set a Code 45.
If air/fuel ratio is too rich, "Scan" tester or voltmeter will read an O2 sensor output fixed above .8 volt. Check for high fuel pressure or fuel injector leaking. Remove IAC and inspect bore for foreign material and evidence of valve dragging in bore.