Late winter, early spring I was having idle problems with my truck where the idle would jump up to 3 grand durring shifts while out driving. The rough idle would last anywhere from a minute, to the whole trip. I checked everything out under the hood and found that one of the screws for my IAC was loose, causing the IAC to not seal properlly, I tigtend the screw and the problem went away, untill this week..I started getting the same problems yet again, this time i look under teh hood and the IAC is bolted down completly. What can cause this to happen??? My TPS is new, my IAC, is new...I cleaned my TB a few months ago along with the new IAC...Whats the deal??

 

Hey MSM,
I am a newby to ford trucks, but have some general mechanical experience, and will try to give you some help with your problem. First I would look for a vacuum leak, check all hoses fittings etc for looseness and cracks. Secondly, try tapping on the IAC while doing its high idle thing. If the idle comes right back to normal after tapping, pull the IAC and clean it real good. I used to work on BMWs back in the late 80s and high/eratic idle was a common problem. BMW reccomended replacing both the IAC and its controller, they said if the slide in the IAC got hung up it would send an incorrect resistance value back to the controller and ruin it.

I hope I have given you an accurate place to start in diagnosing/fixing your problem, I also hope I have not given you any bad info.

 

Thats a good idea 'ToeCutter66' I'll try it If I have the chance to pull over and tap on it, Like i said this comes about while I'm driving inbetween shifts..

 

I also have a slow returning idle..If i rev it up it takes forever for the RPM's to drop back to ide. Do all these do it? btw mine is a 5spd if that matters...

 

Clean the IAC, lube the linkage, check the return spring and make sure its not broken, check the throttle cable & the cruise controll cable for binding (i'm not sure if these can be lubed or not. I've never had a problem with them, i just wipe the grim off best I can.)

All this I consider routine 2xyear mainenance. Once before winter and once after.
It takes all of 10 minutes, if that.

Cheers,
Popa Tim

 

Mine does the same thing. Very annoying to drive at low speed because the idle rises and falls every 10 seconds or so as I try to maintain speed. The truck chugs along and gives whiplash to anyone inside. Not really that bad but annoying just the same. Probably wouldn't be as bad if it didn't have a manual transmission. I don't have a check engine light but thought I might try to get some codes anyway. How do you get the codes from a 90 F-150?

 

Its the same procedures for the entire range of years this forum supports excluding the '96 F150 w/OBDII.

http://fordfuelinjection.com/?p=13
http://www.midimonkey.com/~frederic/f350/eec-codes.html
http://www.troublecodes.net/Ford/
http://www.thorssell.net/hbook/eectest.html

Have fun!
Popa Tim