I have a 1999 mustang gt conv. 4.6l 5 speed. my car recently developed a stalling problem. when it is cold it wont idle. once it warms up it is ok though. the iac is clean and working. the throttle body has been cleaned also. any ideas.

 

If the IAC is indeed operating properly then I would look for a vacuum leak.

 

the iac wouldnt have anything to do with this most likley. Probably a vaccum leak or MAF problem.

 

Does it only acted up on idle, and only when its cold?

 

it only seems to be idle and slightly off and only when its cold. when its cold if you have to give it a little gas to start (which i know you shouldn't have too) and if you just let off it dies. if you slowly let it down it will usually run. then if you hit the throttle quickly it will die. slowly it is ok. When its warm it seems ok.

I will smoke the vacuum system to look for a leak. also it set a code for iac underspeed. i work at a garage but dont have alot of time to work on my own car with school. as i said eariler though the iac appears to be working ok. just that something appears to want it to be working ok.

 

You might want to clean the MAF and see what happens. Also do you have a cold air Intake? I ask because don't most CAI's remove the MAF screen? I've haven't heard of this causing stalling issues on GT's, but I know sometimes there is a problem with removing the screen's on the Mach 1 CAI's.........I know it's a way different engine, but it might be possible.

Earlier I was going to say check your throttle position sensor....maybe it is going bad and not getting enough voltage at idle, but that doesn't explain the problem getting better when the car warms up.

 

If it were a vacuum leak i would think it would do it even when its warm. It set a code for the IAC? what code was it? I would swap out the darn thing. find someone willing to lend you theirs if you dont want to buy one.

 

Youd be surprised. I remember a 89 GT with this exact same problem.

The fix turned out to be a rotted vaccum hose that was leaking. That was under the intake manifold.

 

Got you, different temps on that hose causing it.

 

i checked for a vacuum leak this weekend and found nothing. it did decide to do something weird this weekend though. it was idleing at 2 grand. i unpluged the iac and it came down. plug it in and it would go back up. it was also coming down slow with the iac plugged in. thanks for the ideas but any other ones. i always come up with the best problems. lol.

 

Run a carbon cleaner through the system?

 

Since your car is a 99, I would suspect you have a spark problem. I struggled with this for a few years until replacing two bad coil packs. Now, it just hesitates slightly for a few seconds and then runs fine. I have colder plugs, which also hurts cold start too, but I would bet that if I were to replace all the other coils the problem would go away completely.

Changing plugs might help if they are old.

 

I know you said you did this, but your problem was IDENTICAL to my problem with my 01 GT. Not start when cold, but did when warm, and would have to slowly let off gas for it not to stall. Anyways, I took off the IAC and cleaned thoroughly with Carb Cleaner and havent had a problem since.

 

Do you know how to check the coil packs. I could probably look it up when i get home but just curious if you could save me some time.
i got the car in sept of 05 and replaced all of the plugs and the fuel filter, kept up on oil changes, ect. so maintance shouldnt be a problem

 

I wanted to say thanks to all of you who have tried to help me. i think that i finally got it fixed though. i'm suprised that more of you havent had similair problems after seeing the design. Under the battery there is a channel to hold the wiring harness. it also holds water. i fixed a couple of corroded wires on friday and havent had a problem since.