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IAC Problem! -- Don't know!

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I realize that there are many posts on this subject, but so far none of them has helped much. Maybe someone can help me out! I have a video below.

I bought a 1994 GT a few years ago from this lady. She sold it because her husband ran off with their horse trainer to the NC mountains, never to be seen again. She said that the engine was built by Holcombe Motorsports in Lumberton, NC. At that time it had about 6000 miles on the new engine and transmission. I drive it around on the weekends. It is like driving an old farm tractor. I am the only person that can drive it. It takes a lot of nursing to get from place to place, without stalling over and over. It has some sort of big cam? Don’t know what! Trick Flow intake, smog pump block off, It has never wanted to idle and sometimes dies, when changing gears, coming to a stop, and running the A/C.

After reading a bunch of posts yesterday, I removed the IAC. Someone put a solid gasket, with no holes, between the IAC and it’s mount. No air could go through, period. After looking at the old dirty IAC, I decided to go and buy an new one. I installed the new one and “awesome” it now idles with or without the A/C and does not stall between gears or coming to a stop.

But, it has serious hanging RPM now and the check engine light is on off, on off, every few minutes. When I shift gears and let off of the gas, instead of the rpms dropping, they shoot up a few hundred rpms and stay there until I let off of the clutch. Once this morning, the rpms stayed at 2000 at a stop light the whole time. A few minutes later in 4th gear, the engine started to surge from 2000 to 1500 rpms, back and forth about 10 times.

Any ideas? This thing is driving me nuts!




Here is what I learned today.

1) The computer has no chip in it.

2) There is am idle valve control spacer with Allen wrench adjustments right after the IAC, like this one on the link below.



3) While the IAC was totally blocked off, for the past several years, the engine would rev fast and come back down very fast, but stall, not idle, or idle with the AC running, always.

4) Yesterday, after installing the new IAC -- Duralast / Wells part number TV206, it idles great, but the engine does not slow down after pressing in the clutch, while shifting for about 3 seconds. It actually speeds up a little, instead of dropping rpms.

Big Question: I replaced the new IAC exactly like the old one. Could it be upside down? The way that mine is installed, the electrical connector comes in on the top. This is the way I installed it, like it was!

Here is a photo: In this photo of another car it is upside down from mine, and the wiring connects from the bottom!



Is mine upside down?

Also, should I delete the idle valve control spacer, with the Allen wrench idle adjustments?

Now, the check engine light will not go off. It stays on full time!


Edited by Steve@CJPP
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Sounds like it needs tuned to me, auto or manual, if its an auto a higher stall converter will be in order as well, our cars don't like the lack of vacuum and the EEC's for sure don't like cam upgrades, unless they are degreed properly.. I'd look for a tuner local to get that thing straightened out.. And hanging idle is a common problem with these cars..

Sent from my iPhone

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Welcome to the site. There is always a way to make your car run right without deleting things (that's the easy way out).

Usually surging idle can be tracked down too a vacuum leak. That'd be the first thing I'd try to search for. You can do the carb cleaner test where you spray around any gaskets and listen for the idle to change, or pull off a vacuum line leading to the engine and smoke test it.

After that, id do the base idle reset for our cars. Its very simple to do and I've seen it help a lot of cars that have the same problem as you.

And last if that doesn't help, i'd take force Fed's advice and look into a dyno tune. Our ECU's don't particularly like aftermarket cams.

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I'm just going to interject my trouble shooting technique. I always start with the things that I know aren't right. I know a car of that caliber needs to have a chip, and it doesn't have one. We know that's not right, so start there. It has the typical signs of a stock ECU having trouble managing a big cam engine. The big cam causes much lower than stock vacuum pressure, and the stock computer just throws it's hands up in there and tries to do the best it can because it doesn't understand. There is no good reason to start chasing vacuum leaks and checking your distributer cap and all that stuff when you know the car needs to be tuned. The guy who put that engine combo together looks like he knew what he was doing. He probably had a lot of experience with old Fox body Mustangs which had much more foregiving computers and therefore elected to not to do a chip for it based on past experiences with a different ECU.


Edited by revhead347
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I cleaned the throttle body, mass air flow sensor and wiring, checked the voltage on the throttle positioning sensor ( .98 volts ) removed the brand new IAC Valve that I just installed, blocked off the IAC ( like it was ) installed the idle air adjust spacer again (like it was)

Things different: Turned the Ford idle valve adjustment plate screw counter clockwise 2 turns, cranked the bottom Allen screw on the idle valve adjustment spacer all of the way in, and set the top Allen screw halfway in or about 50%.

Now, the engine idles at about 1000 RPMs. After it runs and / or sits running without hitting the gas pedal, it will go down to about 800 RPMs. Now, it does not cut off at stop signs with A/C off, and does not cut off at stop signs when the AC is set on Max, and no hanging RPMs at all. They just hang enough to keep the engine from cutting off between gears and when going into neutral after letting off of the gas pedal

I have not idea how this car works? ---- Or, what I did! ---- It runs better than ever, but now the check engine light is on full time! It does not sound as cool as it did with an idle of 600 and that giant cam lope, but at least I can drive it now without nursing it to death.

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Nice glad to see its getting better! Unfortunately the codes its throwing you may not be able to get rid of without a dyno tune bit pull codes and see if its just something silly.

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I bought a code reader today. --- Here's what I have:

12 - system cannot raise engine idle above normal idle

121 - TP sensor voltage higher or lower than expected or signal voltage inconsistant with engine intake air flow

157 - Mass Airflow senser signal voltage is too low

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Hmmm idk what any of those might be. I don't wanna just say something and have you throw parts at it without knowing for sure. I'd try to go the dyno tune route now. They'll be able to pin down the small qwerks and get her running right

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Well Guys, --- Thanks!

After reading million posts from different Mustang sites and watching a bunch of videos on youtube, my engine is running great. It is better than it has ever been and I can’t imagine it being better. All check engine codes are clear and the check engine light is off.

I cleaned the Mass Airflow Sensor and wiring, throttle body, adjusted the TPS and Idle Speed Screw, returned the new IAC that I purchased to the store and blocked off the IAC, like it was before, ( for the past several years ) Goofed around with the Idle Air Adjustment Plate ( with the two Allen screws) and Idle Valve Adjust Screw.

It now Idles around 850 and maybe 750 with the A/C running. It never cuts off at stop signs or sudden stops, no hanging RPM’s, and no surging RPM’s at all.

It does not sound as cool as it did when it was idling really slow, with the Harley Davidson type lope, but no more nursing the car and goofing around with the gas pedal when coming to a stop sign!

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