Help! I have a 1991 F-150 XLT. Sometimes when I start the truck, I'll be driving along and when I let off the gas it will idle up to about 3000 rpm for about 2 or 3 seconds and then go back down. It will do this the entire time I am driving the truck. Now, if I just shut it off and restart it, it won't do it anymore. I've been told it could be the IAC (idle air control) valve, but I've replaced it twice and that's not it. I've also been told it could be a vac hose/line somewhere but I just replaced the engine and have been all over that entire thing and nothing is loose. It has done this since I have bought the truck(used). I think it must have something to do with the computer because it has done it with two seperate motors. Any input whatsoever would be of great help.
Smitty

 

Hey do you have a hypertech or superchips installed? Wierd how it does it even on both motors. My Supercoupe did the same thing but the IAC fixed it. Might wanna try a new puter. Check on ebay. they may have one

Scott

 

I also don't think it's that air invector valve. Had one go bad on my TBird and when that happened it would go up to bout 3000 RPMs but it never came down unless you hit it with a hammer or disconnected the battery.

I'ld start checking the repair manual for tests. Good luck

 

Thanks for the replies, I have looked in the Haynes manual for anything on the subject, but they don't get that specific. I am going to Ford today to pick up something anyway, so I am going to try and talk to a Master Tech out there. Thanks again.

Smitty.

 

Oh yeah, no I don't have a chip.