Idle Air Control Over Speed Error Code - Replaced Everything

Very first thing. When the intake/throttle body was installed did you perform the manufacturer's recommended procedure regarding establishing the idle set screw?

What happens when at hot idle, the IAC electrical connector is disconnected? Do the RPM's drop or the motor die?

Also note that when the PCM is reset, this forces the PCM to re-learn new IAC idle trim values. It is NORMAL for the idle to be poor for a few drive cycles after a PCM reset. My recommendation is to avoid resetting the PCM unless you have actually done a REPAIR or are trying to reset a hard DTC code.

Here's some more information on how to trouble shoot IAC/idle problems.

Troubleshoot IAC idle problems 1996-2004 | Mustang Forums at StangNet

scottyp said:

The intake manifold was installed by the previous owner and no noticeable difference in rpm when I unplug iac

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^^^^^with no drop in RPM, this confirms either:

• The IAC is totally non-functional electrically or mechanically.
• The IAC's duty percentage is 0% and there's no adjustment left.

What's the most common reason for the IAC's duty percent being zero? Vacuum leak or throttle body idle stop screw set tooooooooooooo high.

Since your car has an aftermarket intake and throttle body, I would first confirm that the manufacturer's recommended procedure regarding establishing the idle set screw was followed (or repeat the procedure to be sure it was done correctly). Then look for a vacuum leak.

Bottom line. The RPM's must drop when the IAC electrical connector is disconnected for the PCM to be able to control the idle. The IAC works be adding bypass air to adjust the idle. Once the duty percent reaches ZERO there's no more adjustment left.

scottyp said:

It has cold air intake but the stock throttle body which does not have an idle set screw for this model.

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So you don't think that the lack of an idle set screw on the throttle butterfly might be part of the problem? All 96+ model year Mustangs have an idle stop set screw on the stock throttle body. It performs two functions. It establishes the minimum air level and ensures that the throttle returns to the SAME point each time. This is necessary for correct operation of the TPS sensor. If not set correctly (or missing) the throttle linkage may bind or close at different TPS point. This was covered in the trouble shooting resource link.

scottyp said:

Whenever I unplug the hose connected to the iac and put my finger where the hose would have been, I push on the throttle and it tries to suck my finger in. So I think that means it works mechanically?

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The answer to this question depends which type of IAC that you have (black vent or not). For the black vent model the operation that you describe is normal. For the non black vent model your test might prove that's stuck open.

scottyp said:

I've been through 3 iac's just to make sure I didn't receive a defective one.

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You might just be one of the un-luckiest person in the world to get 3 bad new parts. It seems far more likely there's a different problem. For example:

• Incorrect IAC for the application. IE, black vent installed in a non-black vent application.
• An installation error. For example something making a chronic vacuum leak.
• The problem has nothing to do with the IAC itself.

scottyp said:

Whenever I unplug the hose connected to the iac and put my finger where the hose would have been, I push on the throttle and it tries to suck my finger in. So I think that means it works mechanically?

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This test doesn't really prove anything. Bottom line. The idle will never work correctly until the RPM's drop when the IAC is disconnected electrically.

Reminder: When the PCM is reset, this forces the PCM to re-learn new IAC idle trim values. It is NORMAL for the idle to be poor for a few drive cycles after a PCM reset. Resetting the PCM in the "hope" this will fix the issue might actually be making it more difficult to gauge any changes. My recommendation is to avoid resetting the PCM unless you have actually done a REPAIR or are trying to reset a hard DTC code after a repair.