Fuel Idle Air Control issues

Stugots50

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Feb 6, 2022
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I just got my first Fox Body, and I am looking for help with my IAC motor. The car starts fine but has a higher than normal idle of 1000 RPMs. Everything I do to lower the idle does not help, whether I try to adjust the idle screw or set the min. idle. When the car is at idle, I unplug the IAC motor, and the idle does not change. Is this a normal condition?
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Unplug the IAC. Then use the idle set screw to back the idle down. You should be able to back it down to the point where the car will stall. You want to stop just above that.

Reset the ECU (disconnect battery for 15 min), plug the IAC back in, and let the car relearn idle.


With the IAC disconnected, if you can't lower idle down with the set screw, you very likely have a vacuum leak somewhere that is feeding additional air to the engine. You'll want to do a smoke test to find it
 
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Stugots50

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Feb 6, 2022
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I did a smoke test and found a leak on the left side of the engine bay at the multiple vac harness connector. I repaired that leak and checked the tps voltage again (.99) and performed an idle learn. It doesn't sputter, stall or is hard to start. The idle is just high (1000 rpms) and the idle screw makes no difference and when I unplug the IAC there is no change. I have previously removed the IAC and cleaned the bore with no change.
 

manicmechanic007

Mustang Master
Sep 26, 2017
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The ect sensor may be out of whack and/or the thermostat might be the cause
When the coolant hits around 195 degrees f... The ECT sensor should read about .6 volts and the idle speed should be low or correct at about 700
You can check the heater vent temperature (should be within 2 degrees of the T stat )
If you are lucky you just need a new thermostat
Good luck
 

KRUISR

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Apr 16, 2015
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Unplug the spout connector AND IAC when you adjust the idle with the set screw. Set idle down around 600-650. Then reset the computer and do a base idle reset to relearn idle.

You also didn't mention if the car was stock or modified and whether there was a tune or chip on it
 

KRUISR

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Apr 16, 2015
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Yes, the same plug you disconnect when setting the timing.

If you leave it plugged in the computer will keep adjusting the timing to try and compensate for idle - and not always in the best way. I found I was chasing idle until I set it with both disconnected. I haven't touched the TB set screw in 2 years now.
 

junkyardwarrior

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Jan 10, 2011
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yup gotta disconnect IAC AND SPOUT in order to do it correctly but the majority of internet "base idle reset" won't tell you that.

ideally you'd watch IPSIBR if you had a way to log it (quarterhorse, tweecer, binary editor, tunerpro, etc) but you don't have that so don't worry about it.

600-650 rpm at a HOT idle is about correct with the iac and spout disconnected. Once you get to that point, shut the key off, disconnect battery (which clears the KAM) and then reconnect spout and iac. Reconnect battery and go for a drive to let everything relearn. I had to learn that the hard way on a 4 cylinder...fought with it for YEARS until someone smarter'n me guided me the right way.

0.99v on the tps is chasing your tail, anything from about 0.50 and 1.19 is fine and 0.99 makes no difference in the long run. The ecu watches the lowest voltage that it sees from the tps (aka TP) called RATCH--which it then determines as closed throttle (CT). Once the throttle voltage increases .04v over RATCH, it then considers it "part throttle" (AKA, APT)