I have a 1991 F150 302 4x4 automatic. It has been running very bad....sometimes. At times it wil not idle and will die when you put it in gear or will "lope", other times it runs perfect. I seems to do this more when the truck is cold. I don't have a check engine light. There have not been any mods done to the truck yet and everything appears stock. I have tuned it up and changed the fuel filter. I thought the fuel regulator may be bad but haven't replaced it yet. I will be putting a IAC on it today. The truck only has 106,000 miles on it and looks pretty good. I hate to get rid of it because of something stupid, but it is driving me nuts.

Thanks for any suggestions, Mike

 

Then stop randomly replacing parts. Buy a Haynes manual (PN 36058) and read it. Then do some diagnosis. Check fuel pressure. Test the ignition system (p.5-5). Listen for vacuum leaks with a 3' piece of garden hose. Check timing. Did you install new plugs & wires in the tune-up? Did you route the wires like this?



The only time the IAC should be replaced without diagnosis is if you have the exact symptoms described here: