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TLDR: Replaced IAC and cleaned throttle body and MAF to stop erratic idle, and it worked for a little while, but two days later my throttle is getting stuck closed, car is shifting into 3rd at around 30mph and doesn't downshift until I come to a complete stop, causing the car to stall out. Help is appreciated, more details are below.

Okay, so, I've spent almost all my free time over the past week trying to chase down an idling problem with 2002 GT. Over the last few months, I've been having intermittent problems with intake grumbling and RPMs bouncing around below 1k. I cleaned the throttle body, MAF, IAC, got a new air filter, and turned my idle up with the throttle adjustment screw, and it fixed the problem... for one day.

Next day, hesitation and idle got worse, so I said "screw it" and bought a new IAC. First time I started the car up, it immediately shot up to 2krpm in park, and stayed there for a solid 2 seconds, before dropping down to regular cold start rpm. Weird, but I figured the computer was adjusting to the new part. I reset the computer beforehand, btw.

I took it for a drive after that, and it completely fixed my problem. But once again, only for the night.

Next day, whenever you'd rev the car up, it would hang there for a second or two, before coming back down to idle. I read that turning the throttle adjustment up could cause this, so this morning, I went outside and tried to turn it back down to factory spec. I marked where it was with a sharpie before I ever messed with it.

Now, here's the weird part. When I went out there to do that, I noticed a lot of resistance on the throttle linkage. It wasn't doing that yesterday. When I loosened the throttle body adjuster, the linkage didn't move from its spot. Even putting a little force on it couldn't make it move down from what I set my idle to before. Starting the car didn't do anything. I put the screw back to where it was and figured I'd fix it later, since I had to get to school.

I take my car out to drive to school, and notice that it's shifting into second and third WAY sooner than it should. I'm talking 3rd gear at 30mph. It also didn't want to shift down until you came to a complete stop, which would cause the car to stall at every light. After the second stall I put on my flashers and limped it home.

I'm at a loss here. Any suggestions for what to do are appreciated. Thanks.

EDIT: Tried the starter fluid trick to check for vacuum leaks. Found nothing. Going up to O'Reilly's later on today to exchange the IAC. It's a Borg Warner, if anyone is wondering.
 

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Any engine codes?

Motorcraft seems to be the brand to get for th IAC.
 

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Any engine codes?

Motorcraft seems to be the brand to get for th IAC.
 

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Discussion Starter #4
Any engine codes?

Motorcraft seems to be the brand to get for th IAC.
Before I get a Motorcraft, I decided to exchange the IAC just in case the one I got was busted. Still was acting up. I ended up getting a new TPS, and it ran fine on the drive home from O'Reilly's, but the real test will be how it acts on a cold start.

To answer your question: nothing that I know of. My OBDII reader got stolen, but I can check it tomorrow at work. No lights are on though.

Also, the throttle plate seems to be warped and won't close all the way. I can't adjust it back down to factory idle. I'm hoping the IAC can compensate for that. The throttle plate catching on the housing is what was causing my pedal to fight me.

I'll update tomorrow on the car.
 

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Have you checked the throttle return springs? Yes there are TWO of them. One straight spring and a coiled one.

Expect problems if you are trying to the use the idle stop screw in an attempt to "adjust" the idle. It may appear to work but will fail when the weather changes.

The Ford idle strategy and requires that the throttle return to the SAME position each time. Otherwise the TPS will not send the correct "closed throttle" signal to the PCM. IMO you should expect idle problems if the throttle linkage is binding.

OBTW, the IAC works by ADDING bypass air to a closed throttle. So if there's too much air entering the throttle body in the first place, then the IAC can't compensate for this by removing air.

Here's some information on how to trouble shoot IAC/Idle issues:
http://www.allfordmustangs.com/foru...-dieing-idle-idling-rough-14.html#post2260649

If you turn the key on but do NOT crank, does the check engine light come on (lamp test)? If so, the light works.

Does the check engine light come on while driving? If not, then you do not have any DTC codes. Don't need an ODB2 reader to find this out.
 

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Discussion Starter #6
The return springs are fine, but there's a good amount of slack in the throttle linkage. The car ran fine for a few days, but started stalling out again when I come to a stop. Check engine light DOES work. Hasn't turned on other than when it tests itself. I'm still going to check for codes soon. Later today probably.

The throttle body is definitely warped a little bit. If I turn the idle stop screw back to where it was from the factory, the throttle body won't close that far, and ends up getting stuck. I guess that's another new part I'm going to have to throw at my car. Oh well, I can use it as an excuse to get a good aftermarket one.

I'm going to do an oil change and run Seafoam through my car to clean up everything. Also going to take the IAC and TPS back off and clean them on the inside just in case some loose carbon could have gotten in there. I'm practically completely lost at this point.
 

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For an automatic transmission if the motor stalls when coming to a stop but otherwise idles normally think lock up torque converter NOT releasing as it should.

Expect idle problems if the throttle linkage is binding as this will mess with the TPS sensor's ability to send a good "closed throttle" signal to the PCM.
 

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I planned on changing the transmission filter when I get access to the lift, so I'll check out the torgue converter while I'm at it. Don't know when that'll be.

I think the problem is the throttle body. Like I said, it won't close enough to go back to factory spec. I'm probably going to order a cheap one and see if it fixes it. If that does turn out to be the problem, I'll buy a good one soon. Probably going to get a BBK one and an intake while I'm at it.

Any suggestions for what else I should check? I just ran Seafoam through it.

I'll attach a list of everything I've done to the car, if it'll help.
 

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