Been having high idle issues on my '89 f150 for a while now.Out of ten times driving the truck it would be idling high 8 times. when Ive had spare time ive been trying to lock down the exact cause of the issue. So Ive been thoroughly checking for vac leaks, checking gaskets, replacing gaskets etc.

Finally on the weekend after reading some info online I checked the IAC valve when the truck should be at idle and the IAC valve was open, If i covered the holes in the TB for the IAC idle dropped back to where it should be.

So I put on the truck an old but genuine ford IAC and got rid of the "new" aftermarket IAC. straight away when I started it up it was idling a lot better and I drove it for a couple of days with no high idle issue. However last night I went for a drive and the high idle was back. I pulled over turned the truck off and then started her again, Idle was back to normal.

I think and I am pleased that I am finally getting somewhere with this issue but does anyone have any idea about why it wouold still occasionally idle high? what should I be checking ?

 

Check the throttle position sensor.

 

What, if any, active or stored fault codes did you have?

 

It can take up to week for the ECU to learn the idle after a reset.

In my experience, the aftermarket IAC valves are all junk.

 

If you haven't, I would clean the inside of the IAC with a q tip and throttle body cleaner or brake cleaner. Use a new IAC gasket. I don't like the idea of soaking it. It may have stuck that one time because it could be dirty etc.

 

Not getting any codes at all My4Fordtrucks. Im learning that quite a lot of aftermarket parts are junk Jas88, from now on I go genuine where ever possible.

That was what I was thinking Sandymane, It has done it to me a couple of other times since I first posted but each time if I shut it off and restart it comes good. Ill give the cleaning a go this weekend and report back.

Thanks all for the help so far.