I have a 90 F-150 that had an engine change with the previous owner. He put a mustang engine in it and it will idle great most of the time, then there is the time when it idles up. Sometimes as high as 2500 - 3000.
Anyone have a guess at what could be causing this. Nothing is sticking around the throttle cable. I think the previous owner had replaced positioner.

Thanks,

Wade

 

If it's idling really high, like 3,000 RPM, it's probably a sticking idle air control valve.

 

Where is this located!

 

IAC on a truck engine:


IAC on a Mustang engine:


A quick test: unplug the IAC while the engine is running. If the idle RPM drops to normal or the engine stalls your issue is electronic. If the idle remains high the IAC could be stuck or you have a vacuum leak.

 

Why does everybody want to guess at these problems, ultimately the computer controls idle speed so pull the codes to see what it thinks is wrong and go from there. If the computer has no codes set then the problem is mechanical... could be a sticking throttle cable or a gummed up IAC valve as mentioned above, but it's just as likely to be an out of range TPS sensor which should set a code.

 

Thanks rla2005, will try that.
Conanski, the computer hook up under the hood doesn't have the same connection as the later model vehicles. Maybe there is a tester for this?

 

I have similar problems with my 95 250 and I cant figure out whats wrong with it. Possible vacuum leak but what could it be electronically? Im putting in a new IAC soon but i dont think its that

 

Yes the equus 3145 is the code reader to get.

 

Paraphrase from Paul's earlier response: "ultimately the computer controls idle speed so pull the codes to see what it thinks is wrong and go from there."

 

And to expand on that.. the computer controls idle speed via the IAC valve and based on feedback from the TPS sensor, so I'd suspect a bad TPS sensor before a stuck IAC valve but I wouldn't touch anything before pulling the codes.

 

Unplugged it and the engine died. Any guess to what the opportunity may be?

Thanks,

 

How thick is your wallet?

Why guess.....ultimately the computer controls idle speed so pull the codes to see what it thinks is wrong and go from there.

 

That was the plan, get an analyzer to pull the codes, just thought I'd throw that out to see if any others had seen such in the past.

Thanks,