PovertyPony

Okay, I can't figure out what could be causing the problem, but my car started pulling this number a few days ago. I haven't changed anything in months, so I dunno what's got it doing this.

The only thing that changed is I got gas at a different than normal gas station the same day, but since then I've run 2 and some change tanks of high grade fuel from my normal gas station, problem still persists.

Issue:

My car intermittently will idle high (1500) when it's rolling. I notice it most frequently when coasting to a stoplight or sign. Also, when it IS doing this, if I rev it up, it hangs at the high rpm then EVER SO SLOWLY creeps back down to ~1,500-2,000 RPMS. As soon as I stop my car, I mean the very second it stops, it idles back down to ~1,000, which is still a bit high for my normal idle.

So far it's pot luck as to when it does this, and when it stops. My current solution is cutting it off and back on, stops the issue for a few miles usually.

Any insight would be appreciated, thanks in advance!

Details on my car:
1998 GT
2004 GT motor swap
Stock 1998 Computer

----Edit----
Currently suspecting IAC.

----For clarification----

I know Mustang GT's tend to idle high when rolling for things such as stall prevention and the brakes. This isn't what I'm experiencing, it's intermittent, it doesn't do it all the time, and it's not sitting at 1,000-1,100 RPMS, it hangs around 1,500-2,000 and also hangs at very high RPMS if I rev it or ride a gear longer than normal.

Zrxpilot

Not sure if this car has one but it sounds like an aic sensor. Air idle control. My viper did this. Was a cheap and easy fix.

cliffyk

The IAC valve (it's a solenoid controlled valve, not a sensor) is telling you that its days are numbered--some have reported a temporary fix by cleaning them with TB cleaner, however Ford says in the shop manual that it "cannot be cleaned or adjusted".

So save yourself the cost of the TB cleaner and just replace it...

PovertyPony

Okay, thanks a million guys!

Will this still be a cheap and easy fix?

----Edit----

I actually have a throttle body laying around. It's a NON performance improved throttle body. (Leftovers from the PI Swap)

Will that have the IAC, and if so, will it be swappable? I'll probably get a new one anyways, just curious in case this one takes a dump before I get to an auto parts store.

everett

I think you iac is up by the maf sensor

PovertyPony

That doesn't make sense to me, I know it's by the throttle body.

Some searching has yielded it's actually on the plenum, checked my spare one and it doesn't have it. This may be why my current one is failing, it was swapped and is old/dirty.

Google are my friend.

Smoothie247

If the IAC Valve trick doesn't do it, just clean out the throttle body. Had that issue many a time w/an Eclipse POS I had back in the day.
Also, once you fix it, you may wanna run Lucas Fuel Treatment in there once a month or so. Works wonders, and no I don't work for them.

PovertyPony

Hah, go figure, my throwout bearing is bitching at me periodically as well. I'm gonna change that over summer when I have the money and time. Also, I ran Lucas fuel treatment through it yesterday, I hadn't treated it in a while and figured it probably needed it since I got gas at that sketchy Citgo.

I went to Advance and they said they do not carry, nor can order it. Oreilly's told me it would cost SEVENTY dollars to get a new one, that scared me away. I'll look into the IAC valve trick for sure.

Will cleaning the throttle body with cleaner perhaps fix it? Or Sea-foaming the intake?

I don't suspect my fuel filter to be dirty, the high idle doesn't seem like the result of that, especially since it idles down the very second I stop rolling. I won't rule it out though.

PovertyPony

Gonna let my roommate drive it to work and check it out, probably clean the throttle body and check for vacuum leaks. From what I've heard, even though Ford says to not clean the throttle body (Actually says that, the throttle body itself), it's okay to do it. I don't mind spraying it out more often in the event that it eats away the magic coating.

Worst case scenario we ruin the valve that's messed up already? And I guess Oreilly's price isn't bad, Motorcraft's price online was $130. I can damn near find a used BBK throttle body for that, and that's what I would do, just get aftermarket TB and plenum. I think they come with the IAC... Used one probably would anyways.

----

Also, I think we're gonna do the IAC mod just for ****s and giggles anyways, my roommate is coincidentally getting gasket material to do his tonight, may as well make him do mine while he's at it.

Z28KLR

OP, just in case you didn't already find it the IAC is bolted onto the forward facing side of the plenum, not near the MAF or elsewhere as others have suggested.