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Old 11-22-2012, 18:51   #1 (permalink)
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IAC problems

Hello all,
I new here and I've got a problem with my son's 97 Thunderbird 4.6L Automatic w/OD. My son got the car real cheap for having a new engine and trans, knowing that it had a drivability problem. My son got it knowing that it stumbled and died at idle after it warmed up. Now my son lake in the brain dept. for when he told me about it he didn't tell the whole story, but he told me about the stumble. The first thing I thought off the top of my head was the IAC and sure enough after checking the codes I got a P1504 - IAC fault. After starting a pin point test I found that the IAC tested good, but it was froze open, so I changed the IAC. Well, my son help me, and when my back was turned he put it on and plugged it in. We took it for a drive and it ran like crap. So I started checking it the next day, and I found that the plug wasn't seated, so sure it wasn't working. After fixing that problem, boy it sure was hard, I started it and it ran pretty good, checked for codes and had none. I then took it for a drive and when it finally got to operating temp it started running like crap again, stumbling, missing and dying at idle. I got it home, cleared the codes, ran the KOEO test and the only code was P1000. I ran the KOER test and it threw P0505. I tested every ting in the pin point test and everything checked fine. I did notice that taking of the PCV hose at the TB, it picked up the idle and smoothed out quit considerably. I thought, that maybe the EGR was stuck open, but it checked OK. Really started thinking about the timing chain jumping a tooth, or the trans operation causing the problem. Well today, I fine out the original owner had this same problem and bought a new engine and transmission from Ford, figuring it would fix the problem, but it didn't so he got pissed and sold the car to my son. Well that shot down the ideas I had, real quick, so now I'm back to square one. Now, I have a question, and it may sound strange but I'm not that much of a FoMoCo mechanic, I'm a GM mechanic, so no wise cracks, please? My question, out side of what might I be missing, is this. I heard that I have to drive the car, or something like that, to make the computer relearn that it has a new IAC??? Is this correct, how long do I drive it, or is there another way that I'm supposed to reset the computer, or is this a bunch of bull?
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Old 11-23-2012, 10:06   #2 (permalink)
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Re: IAC problems

First and foremost, I will not bust on you for GM, they do a fine job all on their own (case in point, dropping the Pontiac, but keeping the Buick). They've all built great autos and they've all built junk.

I'd check the idle vacuum, forget looking for leaks. It takes longer to find a leak than it does to check vacuum and change your hoses.

If vacuum is ok, I've got a link that walks you through idle maintenance and setup, it's for a 5.0, but it should work for the 4.6

Ford Mustang IAC IAB - Solving your idle problems

have fun with it.
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Old 11-23-2012, 10:16   #3 (permalink)
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Re: IAC problems

Yes, I know what you mean about they build good, they build bad. Technology is the same, engineering is just different, that is what I meant. I'm just used to GM not Ford. Your case in point about the Pontiac and Buick is correct, I own a Pontiac. But you know, my Pontiac has a Buick engine in it! Thanks for the heads up, is there a certain spec on the idle vacuum?

Nick
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Old 11-23-2012, 10:27   #4 (permalink)
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Re: IAC problems

Yeah, the 3.8L has always been a Buick engine, but when they dropped the Riv, I lost all interest.
I can't find the spec for the 4.6, but as a rule of thumb, below 14" I start changing hoses and bushings, above 16, I start backing down the base idle.

I am a backyard mechanic, so I'm sure that if those numbers ain't right, someone will be piping up shortly.
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Old 11-23-2012, 10:38   #5 (permalink)
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Re: IAC problems

Yeah, after an accident, a long time ago, I had to retire as a mechanic and just do it under a shade tree. Went into engineering. Anyway I keep dabbling in it just to keep up with it.
Talking about vacuum, you know, when I take off the PCV hose it idles up and smooth's out, that made me think blockage, but TPS might be out of range, you did give me some valuable info though.

Nick
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Old 11-23-2012, 11:07   #6 (permalink)
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Re: IAC problems

Glad I could help. Got a 73 pick up with a 91 5.0 that was all over the board, that set of links helped me out quite a bit.
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