Iac Blues...


New Member
Jan 15, 2014
First post but nowhere near new. I've been lurking for quite sometime. Im in a bad spot guys and I need your help!! Here's the deal....

I've had a surging idle and gurgling sound coming fr my intake. My knowledge tells me IAC (you can pinch off vaccum and it stops gurgling). I replaced this thing with 3 different IAC with terrible side effects. They are as follows.

High idle followed by engine stall/die.
Horrible horn sound.

I looked intO the intake port of the IAC and all the new valves have a spring loaded plunger type thing, my old one does not. I also noticed Ford lists two iac for my car...are they different???

I really need to get this thing going as its my daily driver. I have re-installed my old valve and it just has a slight gurgle and fluctuating idle (200-400 rpm fluctuations).

The gurgle started intermittently and is now consistent. Car is stock except for 150 wet shot and programmer. I have returned the tune to stock for now. I've cleaned the IaC with the same results.

My computer reads nearly stoich with old IAC and with new IAC it reads +32 ST FTRM (both banks).

  • Sponsors (?)

Should we assume that the model year of your Mustang is 2002?

Have you done a through search for vacuum leaks? Especially in the PCV system?

The 96-2004 Mustang uses two different IAC valves depending on my MY. One has a black vent and the other does not. The vented IAC is used to provide starting air. The vent is designed to close once the motor starts.

It is important that the IAC match the PCM programming.

The posts does not state which IAC your car has (vented or not). Consider a possible situation. The old IAC was defective and allowed some air in via the vent. The tune was done taking this into account. The new IAC does not allow the un-metered air in. This causes the motor to go lean (PCM adds fuel). If true, the problem could only be fixed with an updated tune.

Expect driveability issues if the IAC is changed to a different type than what the PCM is programmed for.

Have you removed the large square box in the IAC bypass line? If so, likely that is the source of the noise.

Here's some more information on how to trouble shoot IAC/idle issues.
It is a 2003. No vent. The tune has been completely removed from the car. I've been motioning fuel trim with a OBD scanner. The restriction box is still in Place.

What else could cause these symptoms? I have wet down the engine bay with starting fluid to rule out vaccum leaks.

I did notice some fresh oil in the intake-PVC tube. Could a bad PVC cause this?

Not a large amount but noticeable. I've pulled the plugs and they show no signs of oil/wetness.

No CEL either. I'm hesitant to buy a 200 dollar Motorcraft part when it may or may not be my problem.
I guess this is one of those deals where the aftermarket may be the issue. I have a drop at idle and have been trying to figure it out for months. Got my tuner to work on it and thought it had cleared up but it did it again the other day. I may got your route and get a Motorcraft part to see if that settles it.