Ridiculously high idle on startup - Forums at Modded Mustangs
 
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post #1 of 7 Old February 2nd, 2018, 10:11 PM Thread Starter
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Ridiculously high idle on startup

This just started this morning. Its a stock 93 5.0 with AOD. It was maybe 32*F outside, but thats never been an issue before. Perhaps the cold is finally getting to the old girl, i dont know.

Anywho, i turn the key and she fires right up but the rpms hit 2000 immediately and slowly got higher. The rpms usually hit 1700 or so and gradually come down as she warms up. So i shut her off and try again because this is NOT normal. Second start, same thing. Hits 2000 rpm immediately and slowly got higher. I feather the pedal and the rpms go up and the rpms keep climbing from there. I shift into D and the rpm immediately falls to 1000-1100, which is where it usually sits. I shift back into P and the rpms jump back up to 2000 immediately and slowly got higher. I shut it off and pop the hood. The throttle linkage and cable are not binding. The air tube is firmly secured at the throttle body and the MAF. I try starting a third time and the rpms hit 2000 immediately. Again, i shift into D and the rpms drop to 1000-1100. I shut her down and left her sit as i wasnt in a very trusting mood so my uncle let me borrow his truck instead of me driving the Mustang to work.

The only thing i can think of is the IAC, which is maybe 2 years old, Motorcraft. But since the rpms do fall when i shift into D, that would suggest the computer has control of the IAC, correct? Im just not sure why it would jump to 2000 and keep going when its in P. I have not tested the TPS nor the IAC yet as ts so blasted cold. Nor have i checked for codes. Right now im just looking for opinions.

Another observation is that last night after work, it took three tries to start. It was just as cold then. She cranked and tried to start, but wouldnt. I had to turn the key a little further than usual, too. On the third try it did the same so i feathered the pedal and she started (the last time this happened it was the PIP failing). The rpms did not hit 2000 last night like they did this morning. They were normal. Drove home no problem. Not sure if any of that is a clue, but there it is.

Thanks.

I do have a 1993 Ford Powertrain Control/Emissions Diagnostic Service Manual (the HUGE manual dealers used) i am currently looking thru in hopes of figuring this out, but so far nothing pops out at me. This particular issue doesnt seem to be covered. I also have a Breakout Box to go along with it...somewhere. Cant remember where i put it. Not even sure if it still works.
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post #2 of 7 Old February 2nd, 2018, 11:33 PM
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Didnt someone else have this same problem a few months ago too? Let me see what I find.

My first thought is either tps or iac.

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post #3 of 7 Old February 3rd, 2018, 12:01 PM Thread Starter
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Could the throttle body itself be part of the problem, or the entire problem? I just took another look and found the linkage coated in oil (none on the valve cover), so apparently the shaft seals are shot. Makes sense that if oil can get out, air can get in. But would it be enough cause all of this? I didnt bother trying to start the engine, i just closed the hood and started looking for my spare low mileage throttle body. I also pulled the oil cap and smelled it just because. Smells like oil. No gas.

As for that diagnostic manual, the exact issue at hand is not covered. While it does cover a high/hanging idle, it does not cover the gradual rpm increase on its own while in P.

Replacing the throttle body is on the list only because of the oil. Ill start there.
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post #4 of 7 Old February 3rd, 2018, 12:44 PM
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post #5 of 7 Old February 3rd, 2018, 01:39 PM Thread Starter
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Thanks for the link. It is similar, but mine goes a step further. The rpm hits 2000, but keeps rising gradually on its own. I havent done much other than looking for parts in storage in the event that i will need them. Found the TPS and IAC as well as the throttle body, if i do.

I went ahead and started it again just to see what happens. It idled just fine. The rpm hit 1700 or so like usual and came back down. Its just as cold outside now as it was yesterday so im at a loss here. Whatever it is, its intermittant. Wonderful.
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post #6 of 7 Old February 8th, 2018, 10:56 AM
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Is it possible the cold temps could be making the pintle valve stick on the iac valve?? Then again,if its still occurring after the engine heats up its likely not the cause since engine heat would probably help to unstick it.It wouldn't hurt to test the tps though.Foxbodys tend to have a mind of their own at times.Check the purge valve to make sure its not sticking.Its a known cause for a high idle. You could just disconnect the intake vacuum hose from it,plug the hose and drive it to see what happens.Ignore the temporary gas fumes of course.

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QA1 Coilovers
Weld ProStar XP'S

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post #7 of 7 Old February 17th, 2018, 11:17 AM Thread Starter
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Quote:
Originally Posted by wbrockstar View Post
Is it possible the cold temps could be making the pintle valve stick on the iac valve??
That may have been it as it has not done it since. There were two days after that that were colder and it didnt stick at all so who knows. But so far, so good.
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