IAC problem?

archerman0531

Registered User
Ok, I've been trying to figure out why my car(90 xr7) isn't ideling correctly. Apon start up it revs to 1700-2000rpm and slowly does to about 1500 and then eventually 1200. I replaced the temp sensor which has helped quite a bit, It takes it much less time to drop the Rpms from start up an if you flip the throttle. But still doesn't idle down correctly nor fast at all. I unplugged my IAC today an drive around town and everything and drove GREAT! Idled down correctly, no surging idle, no sticking idle. Is this a bad IAC or do I have something else wrong?
 
Ok, I've been trying to figure out why my car(90 xr7) isn't ideling correctly. Apon start up it revs to 1700-2000rpm and slowly does to about 1500 and then eventually 1200. I replaced the temp sensor which has helped quite a bit, It takes it much less time to drop the Rpms from start up an if you flip the throttle. But still doesn't idle down correctly nor fast at all. I unplugged my IAC today an drive around town and everything and drove GREAT! Idled down correctly, no surging idle, no sticking idle. Is this a bad IAC or do I have something else wrong?

im assuming you have a bad iac because the motor should stall when you unplug it. i have been having the same idle issues you describe but my iac is good so i think i have a small vacuum leak somewhere.
 
The motor should be set up so that it can just barely hold a steady idle if you start the car with IAC unplugged. If you unplug the IAC and start it up and you get anything higher than, like, 600rpm, then chances are good that you have a vacuum leak.
 
Thing is I've fogged it twice and both times nothing showed up. I have steady idle vacuum at 20in/lbs. and when down shifting while drivin to slow down I have 23-24 in/lbs. boost is up to where it's supposed to be. So I'm stuck as to what else would cause this
 
Thing is I've fogged it twice and both times nothing showed up. I have steady idle vacuum at 20in/lbs. and when down shifting while drivin to slow down I have 23-24 in/lbs. boost is up to where it's supposed to be. So I'm stuck as to what else would cause this

What throttle body is on it?
 
Stock. Everything is stock except it has a k and N air filter, but it's nothing special like a cone filter or anything, just a plain airfilter.
 
Stock. Everything is stock except it has a k and N air filter, but it's nothing special like a cone filter or anything, just a plain airfilter.

Hmm, odd. There are a couple other leak sources you could check. The PCV hose under the rubber intake, for one. And also at the charcoal canister under the bumper. That may not show up all the time, depending on when the solenoid is open.
 
I have checked the PCV hose on the intake tube and no leaks there. Haven't checked the charcoal canister. I plugged the iac back in today, drove it for about 50 miles, turned it off fr about 15 minutes, came back out and idled at 800rpm, put my foot on the throttle to go and it reved up to 1500, took my foot off and would only drop down to 1200.
 
I've noticed when I unplug the IAC, when it's ideling the car makes an almost whooping noise sound like its coming from the passenger side, Cause it never seems to do it when I'm out of the car and hood popped, lol. This sound is also heard when the IAC is plugged in, but when i turn the car off, right at the very end you can hear a little whooping sound. Any idea of what the noise is?
 
Could be the IAC. I have heard of them making a whooping noise when failing. If you pull the IAC off and look inside, there is a little accordion-like boot in there. You may see cracks in it. That would be a sure sign that it is bad.
 
Ok, Thanks. I guess thats what i will doing Saturday. And the thermostat gasket....again( Oreallys gave me the wrong one, the first time. So for future reference when going to Oreillys to order that gasket have them look under the "water inlet gasket", that's where they found it under).

If the IAC shows no signs of failure I guess I will have it smoked again. Even with the IAC UNPLUGGED, after driving it up to operating temp the idle like to rest around 1000 and surge up to 1300 or 1400 and back down. I'm not sure why. Unless my MAF is messed up and this is the whole problem in the first place. That's the only thing I can think of that would give the ECM a signal to raise idle, but I don't understand why it drops it of that is the case.
 
Ok, Thanks. I guess thats what i will doing Saturday. And the thermostat gasket....again( Oreallys gave me the wrong one, the first time. So for future reference when going to Oreillys to order that gasket have them look under the "water inlet gasket", that's where they found it under).

If the IAC shows no signs of failure I guess I will have it smoked again. Even with the IAC UNPLUGGED, after driving it up to operating temp the idle like to rest around 1000 and surge up to 1300 or 1400 and back down. I'm not sure why. Unless my MAF is messed up and this is the whole problem in the first place. That's the only thing I can think of that would give the ECM a signal to raise idle, but I don't understand why it drops it of that is the case.

I believe that, if the boot is torn, air will leak through the IAC even when it is not being pulsed. What you could maybe do is to pull the IAC off and put some duct tape over the ports on the TB. I think that should prevent any air from passing through the IAC circuit (unless I am remembering the design wrong). If the surge stops, then it was probably caused by the IAC. If not, then look elsewhere.
 
I just don't understand why it's pulsing in the first place. To my knoweldge the things that effect idle are: MAF, ECT, and IAC. This is a steady rhythmic pulse, a steady rhythmic drop and increase in vacuum as well. They drop to the same in/lbs and raises to the same. I would think if it's a torn or ripped boot in the IAC I would have an unsteady vacuum when this surging happens.

Maybe I'm thinking wrong.
 
I just don't understand why it's pulsing in the first place. To my knoweldge the things that effect idle are: MAF, ECT, and IAC. This is a steady rhythmic pulse, a steady rhythmic drop and increase in vacuum as well. They drop to the same in/lbs and raises to the same. I would think if it's a torn or ripped boot in the IAC I would have an unsteady vacuum when this surging happens.

Maybe I'm thinking wrong.

One of the main things the computer does to maintain the target idle, is advance and retard ignition timing. If it's making a goose honking noise just after shutting the motor off, it most likely needs a new IAC valve.

I suggest getting it used from someone on here or if you want a new one, get it from the Ford dealer along with a new gasket, because the new ones being sold at some of the auto parts stores don't work correctly.

David
 
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