I took the engine, trans, wiring, fuel tank, etc. out of a 91 E150 and stuffed it all into my 78 F100 a few years ago. Which is why I put this thread here instead of the proper forum for that style of truck. Anyway, everything was good and well for some time. Then it would start to idle kind of high at random. Usually only after it was warmed up. Then it would idle high quite often. Now it will idle at around 2400 rpm's almost all the time. Once in a while, when it hasn't been driven for a while it will idle fin for a minute or two before the idle jumps up. I've checked for vacuum leaks and couldn't find any. I've replaced the IAC and EGR valves, and MAP sensor. But nothing changed.

Anyways, how many things are there that can make these things idle so damn fast? I haven't driven this truck much over the past couple years because of this. I haven't checked the vacuum pressure as of yet. What should the vacuum pressure be at? I'll have to go dig out my manual because I can't remember that info off the top of my head right now. But due to the way it acts, I guessing it's something electrical and not a vacuum leak. But I've been made a fool of myself before so I'm not gonna rule out the possibility as of yet.

Now that I'm done with college and back to work, I can start putting some more money into this project and get it on the road more often. It does more time parked by my garage than on the road the way it is right now. So ANY help fixing this idle problem would be much appreciated.

Thanks, Bill

 

If that engine has one maybe it could be the throttle position sensor ? A bad one would make it run really rich which would make it idle high.

 

It's hidden so well under the throttle body that I never even thought of it. Doh! You would think that would've been one of the first things I'd have checked. Thanks for the reminder, I change it out and see how it acts afterwords and let ya know thew result.

Thanks, Bill

 

There is an electrically controlled valve that lets air bypass the butterfly. That is how the idle speed is adjusted. The TPS and temperature sensor are in the same circuit and electrically affect the idle speed. To troubleshoot the sensors check them with a meter. You need a meter with a needle to check the TPS and then it's still a little voodoo. You can check the bypass valve by making a gasket that blocks airflow through the valve. Good luck.

 

I assume your talking about the IAC, (Idle Air Control) valve? If so I had already changed that out last summer without any changes. But thanks for the input.

I haven't been able to change the TPS as of yet with the 4th of July weekend and all. I'm hoping to get that done on Monday. EBSCOhost shows a couple ways to test it and many other sending units and engine controls while still on the vehicle. I might try doing that first.

Thanks again and have an awsome 4th. of July!
Bill

 

Sounds like a vacuum leak to me.

 

I put the vacuum gauge on it today an it has plenty vacuum indicating no vacuum leak. I tested the TPS as the Haynes manual describes and it has 5 volts going through it when the throttle is in any position. It should only have .5 - 1 volt at an idle and about 5 volts when wide open. So i have a new TPS on its way to the local Car Quest. I'll pick it up on my way to work this evening and hopefully I'll have time to get it in after work in the morning.

Bill

 

Well I got up a couple hours early and went and got the TPS and put it on. It's back down to idling at about 950 rpms again. Sounds great! Thanks WorkVan58 for the little reminder. I don't know why it didn't cross my mind when I was doing all that fiddling around with it before. It was so simple, yet so far from my mind. Doh!

And thanks everyone else for your input.

Bill

 

Wow, when mine went bad it ran fine, but would flood after 15 minutes and quit.