Thread: testing IAC?

Has anyone here tested an IAC on these cars? Looking for specific voltage input at idle, off idle, etc

My Grand Prix has decided to randomly rev up to like 2800-3k on startup sometimes. It normally revs up to 1300 or so to get the fuel atomized better when it's cold, then lowers down to idle - but it shoots right up to around 3 grand and won't come down at all when it does this.

In gear with the drag from the trans (holding the car in place with brakes, of course) is around 1400-1500 revs. I had it happen to me when I was out for a drive, around 50km/h downhill where the car is normally idling it was at 1400 RPM. Slip it in to neutral and it shoots up to the aforementioned 3k.

Throttle position has absolutely no effect on it whatsoever. It does all of this with the throttle closed.

I cleaned the bejesus out of the IAC valve when I got the car. No DTCs and no running problems to speak of other than this phantom throttle.

It went away on its own when I was out driving, but the only way to get it to stop prior to that was to kill the ignition and start it again, hoping it has gone away. I pulled the connectors for both IAC and TPS to see if they were dirty but they were okay..blew them out and hooked em back up and it started normally.


I suspect either the TPS or IAC valve, but I didn't get codes for either of them. Either the TPS is telling the computer I've got my foot down, or the IAC thinks it's being told I have my foot down. At least that's my n00b theory.

did you clean the tb when you cleaned the iac?.maybe theres carbon build up thats messing with the tps?
theres 2 ways of testing the sensors, you can ohm them out or do a voltage sweep
on the voltage there should be about 5v going into the sensor and coming out dpends on how open the thottle is, at closed should be realy low(about .5v) then work its way up to about 4.7v at wot..

but i dont think those are your problem, if they were then you would have drivably problems and the car might not start at all or if it does then then it would have a hunting idle
maybe the iac has a bad same bad wires in the solenoid..ohm out the solenoid and if you get a reading then its good
if your sure that the iac is good then i would start looking for leaks, broken lines, stuff like that

Yeah, I should have mentioned that I cleaned the bejesus out of the TB. Did it again today too. Both sides of the throttle blade (through the IAC ports) too

Actually had to give it a little bit of throttle to start it a few times just now, otherwise it would crank, not quite fire, die. Just barely had to crack the throttle and it would start.


Got P0102 but it was after I had cleaned the throttle body/intake again today. Had the intake tube *after* the MAF disconnected so the TB was exposed, so there was no air flowing over the MAF. I assume that's what it was from..I cleared it and it did not come back. It only appeared AFTER disconnecting the intake tube, to be clear.


Couldn't find any vac leaks.

yeah the code was from the maf, so that shouldnt come back
maybe you just have a bad iac?..i think there like 10 or 15 bucks so it wouldnt hurt to replace it but you could test it with the car running by
seeing if the car runs rough or stalls when using a/c, power steering while idling

On the Fords I'm used to I have found that junkyard OEM idle air valves are usually better than the replacements at parts stores. Is that the case with these as well?


Any idea what other motors use the same valves as the 3100 SFI?

nota clue,
if the iac test out ok then you could start looking at other sensor's, like all the other inputs for the iac. like the baro, maf, map, tps, and all the other ones...start unpluging them one at a time and it the rpm changes back to normal then replace the one you unpluged

thanks for your help!

nota problem,
keep the thread updated on what you find out