Got an 86 F150 EFI 302, have had it since it was new. For the past few weeks it has had a high idle that gets stuck; it appears to do this when the temp. outside has dropped considerably. I cleaned old idle air control valve, and replaced it with another and still no remedy. Got a new throttle positioning sensor; no cure. Checked throttle body and IAC valve is allowing too much air to bypass. When I disconnect the IAC it begins to stall out. After reconnecting it I will then disconnect throttle positioning sensor and engine falls to normal idle condition. I suspect it is something telling the IAC to open up all the way. Any thoughts would be greatly appreciated.

 

The tps wires should be pointing at the Iac, If you have a VOM you can check the
voltage if it is off it will give a screwwy reading(search for TPS voltage) Have you
replaced all the vacuum hoses yet? My hard lines were cracking and I had to replace them. Also check the torque on your intake and inspect for blown out gasket, etc.
Your truck should idle at around 550 or so with the IAC wires unhooked adjust the
throttle hard stop to get it there.

 

Found some craked hard lines and reconnected with rubber hosing. As of right now, no high idling. We'll see how long it last. Thxs Blue Beast