I finally fixed the idle problem on my 1993 F150 5.8L EFI. It was idling rough and stalling when letting off the accelerator. The truck has 190,000 miles on it so I did an extensive tune-up. I replaced air/fuel filters, fuel pressure regulator (free from Ford for the recall), plugs, wires, cap, rotor, pcv, etc. The problem didn't go away. I then broke out the Haynes manual and started testing. The KOEO showed no codes. The truck would start but wouldn't idle without some help. So I placed a brick on the accelerator and started the truck. It idled just enough to stay running and I was able to run the KOER test, which gave codes 411, 412. After testing the IAC, TPS, MAP, EGR, and EVP sensors I became very frustrated since they were all within their respective ranges. I then began checking for vacuum leaks. Nothing. I then went back to the IAC. I took it off, cleaned it and put it back on. It still had the same idling problem. Finally I took it off and applied voltage to it to see if it was physically working regardless of what the voltmeter said. Guess what? It didn't budge! So, I bought a new one and now she purrs like a kitten. The moral of the story: Seeing is believing!

 

Cool man I got a 1988 Mustang thats got a idle problem. 302 that fluctuates but not all the time, bounces off 550-1200rpm, I cleaned my Iac and it still did it, but I also replaced it with another unit, never changed, but if I disconnect the Iac It idles out fine, weird, but Im leaning toward O2 sensors, dont believe this is the problem... See I got a stock computer with ported heads, and a few mods, but the only thing changing the air flow is the heads, but like I said it doesnt always do it. Makes me crazy sometimes