P1507 Idle Air Control Under Speed Error

Hello,

2001 Cobra stock, 4.6L, bbk headers, cold air intake, sct tuned.

Having a low idle issue when the AC is on. I am getting P1507 Idle Air Control Under Speed Error code.
Car some times turns off when coming to stop with the ac on. Idles really low. Car sometimes is hard to start, need to press the gas to keep it from turning off.

I noticed that there is a constant vacuum leak coming from the IAC Pressure Vent/Cap. It never did this before. I have tried 3 ford iac and 2 after market iac. They all do the same thing. Loud constant vacuum leak coming from the iac pressure cap. Anybody have any ideas? I removed the after market tb and put the stock tb, but it still did the same thing, Returned the tune to stock, but issue still persist.

Hey GB! I'm wondering about the IAC circuit, and if the PCM is commanding the IAC to open (or even has the ability), as all your symptoms point to an IAC that's stuck closed. Well, not STUCK closed, 'cause you've tried other valves. At least not being opened.

The vacuum leak you found may be because the IAC is fully closed with the engine running, and the intake is sucking hard on that IAC breather cap. Most Ford models with this kind of valve don't have that vacuum leak noise, because the IAC is partially open at idle.

There's a couple quick tests I can suggest. One would be to remove the IAC from the intake, but leave it plugged in. Keep your eye on the IAC pintle while someone else starts the engine. There'll be a big ol' vacuum leak, sure, but you should see some kind of pintle movement. If you see no reaction, that would indicate there's a control problem.

The other thing you can do is use a test light on the IAC with the engine running. With the test light connected to B-, the red wire should shine brightly. With the test light connected to B+, the white/light blue wire should show some kind of partial brightness or pulsing. If you see no pulsing, that would suggest a lack of control from the PCM.

Hello Tyler,

Thank you for replying! I will run these test on Saturday and will keep you updated. Again thanks!!

Hello Tyler,

I did the test light test. The light did shine when I attached the test light to the Red wire and did also did shine on the white/light blue wire. I when I throttled the car from the TB, the white/light blue wire was pulsating.

What I also did was unplug and then removed the iac and looked at the location of the pintil, I then reattach it, drove the car to the store and back with the ac on. With the car still on, ac also on, I unplugged the IAC, (car idle stayed the same), then I turned the car off. Removed the iac and noticed the iac pintil in the same location as when the car was cold. Seemed like it did not move.

Any ideas?