5.4 idles too high

[arctictraveller]

My 2001 5.4 has a new bad habit, when cold, the idle speed is increased for an excessive period of time. It goes up noticeably for three or four minutes, and then when I stop long enough for it to cool a bit, it increases again for a couple minutes. In addition, once in a while it runs roughly for a second or two, but other than that, it runs perfectly once warmed up. Any idea what controls the cold idle speed, or what could cause this? Thanks...........

[FVShootingStar]

Originally Posted by arctictraveller

My 2001 5.4 has a new bad habit, when cold, the idle speed is increased for an excessive period of time. It goes up noticeably for three or four minutes, and then when I stop long enough for it to cool a bit, it increases again for a couple minutes. In addition, once in a while it runs roughly for a second or two, but other than that, it runs perfectly once warmed up. Any idea what controls the cold idle speed, or what could cause this? Thanks...........

The throttle position sensor, TPS is what controls it, you might have a damaged ribbon in the TPS.

[carringb]

Actually, Idle Air Control valve controls idle speed. It can get fouled up with PCV oil mist after a while. That said, a bad TPS can cause the IAC to operate improperly.

Any data stream display (scan gauge or whatever) can monitor TPS, so it's easy to rule out. You can also monitor the channel Key On Engine Off, to ensure it's its getting linear signal.

Lastly, an air leak behind the throttle body (cracking manifold, massive vacuum leak etc) can cause erratic idle speeds as well. You track those down by squirting something flammable at your engine (carb cleaner usually). It it surges when you hit a component with the spray, you found a leak. Definitely do this test cold. You know, the whole flammable liquids + hot engine parts thing....

[arctictraveller]

Originally Posted by carringb

Actually, Idle Air Control valve controls idle speed. It can get fouled up with PCV oil mist after a while. That said, a bad TPS can cause the IAC to operate improperly.

Any data stream display (scan gauge or whatever) can monitor TPS, so it's easy to rule out. [QUOTE

Easy if you know what to look for, but I"m not that smart. I do have a Scangauge and I' m just guessing here, but I assume I would look for the same signal (voltage level?) anytime the throttle is closed, no matter the idle speed?



[QUOTE You can also monitor the channel Key On Engine Off, to ensure it's its getting linear signal. [QUOTE


So, what does that look like on the Scangauge, what am I looking for and what do the values display represent? (I assume voltage)


[QUOTE Lastly, an air leak behind the throttle body (cracking manifold, massive vacuum leak etc) can cause erratic idle speeds as well. ....

I'm really hoping it's not a vacuum leak, I was wanting to not work on the engine for at least another 100K miles.

[JWA]

Originally Posted by arctictraveller

I'm really hoping it's not a vacuum leak, I was wanting to not work on the engine for at least another 100K miles.

I hear ya there---I've had enough DIY mechanical repairs just this year to last me a lifetime.

The only upside to most vacuum leaks on the Ford Modular Motors is they're relatively easy to find and fix, most times anyway.