Iac Problem?

Telewanger

New Member
Apr 4, 2009
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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!

Thanks!

Click the little link under the video to go to youtube, if the embedded video does not work.


View: https://www.youtube.com/watch?v=dpKJEf5d5x8

 


 
 
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That's a beautifully well done Mustang. It just needs a tune, that's all. The computer doesn't like mods. The reason the rpms shoot up when you shift gears is due to a driveability feature called "rolling idle." The computer keeps the rpms up when you push in the clutch to make shifting easier. It is supposed to keep the rpm constant, but since the computer has not been readjusted for the bigger engine the rpms increase instead of staying steady. All else being the same, that car will run great with a good tune. Check to see if someone has installed a chip on it. The computer is located under the kick panel in front of the passenger side door. A chip will be attached to it externally if there is one. Most people use electrical tape to hold it on. If it doesn't have a chip, then it definately needs one. If there is one, it hasn't been done right.

Kurt
 
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That's an excellent explanation, my modded 5.0L had the "rolling idle" and I thought something was wrong until the tuner told me that is how the 94-95 5.0L's are. I never understood why it did that but now I do. My tuner installed a chip and after a few dyno pulls was able to give me 18 hp more and it idled much smoother.

That's a beautifully well done Mustang. It just needs a tune, that's all. The computer doesn't like mods. The reason the rpms shoot up when you shift gears is due to a driveability feature called "rolling idle." The computer keeps the rpms up when you push in the clutch to make shifting easier. It is supposed to keep the rpm constant, but since the computer has not been readjusted for the bigger engine the rpms increase instead of staying steady. All else being the same, that car will run great with a good tune. Check to see if someone has installed a chip on it. The computer is located under the kick panel in front of the passenger side door. A chip will be attached to it externally if there is one. Most people use electrical tape to hold it on. If it doesn't have a chip, then it definately needs one. If there is one, it hasn't been done right.

Kurt
 
That thing sounds and looks sweet!!! Congrats on the find. What exhaust is that? It sounds a bit like my '90 which had a full Mac exhaust and an F303 cam. You could slap an indicator and degree wheel on it and get the specs of the cam. It would be a bit of work, but would be good information to know.
 
Five things to do before you jump to possible erroneous conclusions.

1. Unless you know the last time the car had a basic tune-up do one. Plugs, wires, filters, cap and rotor.
2. Clean the MAF
3. Check the battery to engine and engine to chassis grounds. They need to be absolutely solid otherwise you end up chasing bogus issues.
4. Check for trouble codes. Clear the old one to make certain what you see are current.(Buy a reader for $20, it's a lot of a lot easier than counting beeps or flashes)
5. If the O2 sensors have seen 60k+ miles just replace them.

Once you have gone through these steps you will be more successful in addressing issues vs. guessing and buy parts when not needed.
 
Thanks!

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.

……….
http://www.supermotors.net/getfile/912900/fullsize/iac-(32a).jpg

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!


http://www.stang.cc/tmcagallery/d/23598-1/iac.jpg

 

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!

Thanks!
 
I think toyman has given you some pretty good advice. I would do that first. He seems to be a pretty smart guy ( even if he is from Canada ;) )

Your car has that many mods and no tune? That would probably be your next step.

I have never seen that idle spacer. Very curious little piece. I am sure someone here can give you info on it.

I don't think it matters which way the IAC is facing. It's just a servo that moves a diaphragm in and out right?

Did you clean the TB and all its passages of all the carbon buildup when you had the IAC off?
 
Thanks!

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
 
Well Guys, --- Thanks!
After reading a 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!