System Too Rich
WARNING: This page is about a different car, the 1992 Pontiac Grand Prix, 1992 Oldsmobile Cutlass Supreme, and 1992 Chevrolet Lumina. However, it is still accessible from the selected car via links, so may be relevant.
If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. System may be obviously rich, with black smoke from exhaust pipe. "Scan" tester and/or voltmeter will read an O2 sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure and injectors leaking or sticking. Remove IAC, and inspect bore for foreign material and evidence of IAC valve dragging bore.