Is my IAC bad?

sj9ers

Member
May 19, 2007
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Is my IAC bad? (update: read my last post)

So lately I've been having some surging and idle issues with my 95 Mustang GT after doing a tune up a couple weeks ago. I've already cleaned my MAF, IAC, checked the TPS, reset the base idle, but it's still surging. Today I unplugged the IAC with the car running and the idle didn't change at all no RPM drop nothing. Does this mean my IAC is stuck open and isn't working properly?
 
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Too many variables. If your base idle is set above the commanded idle, the IAC is probably at a small DC and not much would change if you disconnected it.

IAC's use PWM.
 
Improvement after cleaning the IAC usually just indicates a bad IAC. I find that the cleaning only leads to very temporary relief of the problem.

Kurt
 
Suggesting it to be cleaned was only intended to be a form of diagnostic. If its cleaned and shows improvement, I'd feel a lot better going to buy a new one as opposed to hoping that's the problem and shelling out 50 bucks...
 
Suggesting it to be cleaned was only intended to be a form of diagnostic. If its cleaned and shows improvement, I'd feel a lot better going to buy a new one as opposed to hoping that's the problem and shelling out 50 bucks...

Totally agree with you. But if cleaning it fixes the problem, just go ahead and replace it.

Kurt
 
So I found out my IAC was still good. I unplugged it during a cold start and the car stalled out and died. However I'm still having idle issues and the car is eating a bunch of gas. Right now I'm at half a tank and I only got 66 miles. I've already done a base idle reset and the car is idling at about 700RPM now, but it still surges every few minutes and the idle isn't smooth and the RPMs and voltage will drop every now and then after the car warms up. In low RPMs the car also seem sto studder once in a while. I pulled codes with a scanner and did a cylinder balance test and everything checks out. I was wondering what else should I look at? TPS?
Here's a list of things I've done
New MSD Cap and rotor
9mm Ford racing wires
Motor craft copper plugs gapped at .054
Cleaned MAF, IAC
Fixed vacuum leaks
Pulled codes and did cylinder balance test and everything was fine
Reset idle and computer
Had alternator checked by 2 places once off and on the car and it checked out fine both times
New battery
 
Did you check the fuel system? Your problem sounds like mine. I found that my fuel pressure is maxed out all the time, regardless of vacuum, which means the FPR could be bad or the return line is clogged. I'll find out tonight if it's what is causing my surging and rough/lopey idle.
 
So I replaced my distributor because I read some other threads that had this problem and it fixed it but no luck. I still don't know what could be wrong. The car is also getting bad gas mileage as well. Also when using the power steering when the car is idling the RPM's drop significantly and one time it even stalled out when I turned the wheel and held it there. Also tonight while using the power steering and having the RPMs drop the check engine light came on for a split second then went away. I tried to get it to come back on but it never did. I'm really at a loss what else to check right now. Anybody have any other ideas?
 
If you havent done the procedure outlined at this site ... Ford Mustang Base Idle Reset
I would do that before going much further.

I was experiencing very similar problems with my car. Back then it was still AODE. Half the time when I went from park to reverse it would stall. Under parking lot turning it would stall. Again, my first thought was IAC. But I happened to do the exact smae reset procedure as outlined in that site... and havent had a problem since. Just some food for thought.
 
If you havent done the procedure outlined at this site ... Ford Mustang Base Idle Reset
I would do that before going much further.

I was experiencing very similar problems with my car. Back then it was still AODE. Half the time when I went from park to reverse it would stall. Under parking lot turning it would stall. Again, my first thought was IAC. But I happened to do the exact smae reset procedure as outlined in that site... and havent had a problem since. Just some food for thought.


Thats the exact same procedure I tried when I first reset my base idle and it didn't help. I'm thinking about just taking the car to shop so they can hook it up to a diagnostic machine so they can tell me whats wrong.
 
Today I pulled codes again because the check engine light came on for a split second last night then went away. And I got code 157 for MAF sensor fault for low voltage and code 158 for MAF sensor fault for high voltage, and code 116 for engine coolant temperature higher or lower than expected. To do some further tests I unplugged the MAF when the car was idling and nothing happened it ran the same so I revved it and it still ran the same. I thought if you unplug the MAF the idle should be affected and it shouldn't rev correctly? My plan now is to replace the MAF and ECT. I'm hoping this has been the cause of all my problems and if someone else could chime in on the problem before I drop more money on new parts that would be great.
 
Hi, I'm a new member here, but this looks like the proper thread for this question. I have a 95 GT 5-speed that I've owned for almost 6 years now, and I've never had any major problems with it. The only things I have replaced on it are the fuel filter, SP wires, distributor cap, rotor, and ignition coil about a year and a half ago in a futile attempt to fix what turned out to be the PIP sensor. I used to have this occasional idling problem only in the winter where sometimes when I would put it in neutral, the rpm's would hover in the 2-2.5k region and would only come back down to almost normal when I would come to a stop. I say almost normal because I've forged a strong bond with my car and I know when it's not acting quite right, normal is pretty much dead on 1k and I can tell it's acting up if its up in the 1.1-1.2k area. I haven't had this problem for a while, and it started up a few weeks ago and got progressively worse(meaning it was happening virtually every time I took the car out), I should mention that the weather is far from cold, so this was even more unusual. After reading through many threads, I narrowed down the problem and I removed and sprayed the IAC unit and the intake ports. For good measure I also cleaned and recharged my K&N air filter and checked the MAF for build-up and removed my battery cables, cleaned the terminals and left it disconnected while doing everything else to (hopefully) reset the computer so I could attempt to have the car "relearn" its idle. I put everything back together, took it out for a test drive and it was still doing the same thing. I guess I should mention that within the past few weeks, the CE light has come on a couple of times, but has always gone off. I don't have a tester, so I haven't pulled any codes off of it but I was under the impression that those codes don't get stored on 95's and earlier and the best way to pull them is if the problem is happening at the time, but I could be wrong. Anyway, as I said I'd been reading all these threads and what I did was when I came back from the drive, I left the engine running, popped the hood and disconnected the IAC plug. I have read conflicting opinions on what should happen when you do this, but my car died almost immediately. I plugged it back in, started the car, and it has been absolutely fine after a couple of test drives. So my question is: what does that mean? Is that indicative of a failing IAC? I mean, I obviously had the unit unplugged while I was cleaning it, but that didn't seem to do anything, so why would it affect it just because I unplugged it while it was running? Also, what is the easiest way to remove this unit? Because it was a beyotch getting to that bottom bolt with the fuel lines in the way and I had to get it out at like 1/4 turn a time. Thanks in advance and sorry for the long post, just wanted to be as thorough as possible.
 
If you havent done the procedure outlined at this site ... Ford Mustang Base Idle Reset
I would do that before going much further.

I was experiencing very similar problems with my car. Back then it was still AODE. Half the time when I went from park to reverse it would stall. Under parking lot turning it would stall. Again, my first thought was IAC. But I happened to do the exact smae reset procedure as outlined in that site... and havent had a problem since. Just some food for thought.[/QUOTE
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I did every step in the idle rest. My car idled higher with the IAC unpluged. Like 1500. That's as low as I could get it. Once I plugged it in. 600 to 700.
 
Dude....I just had a similar experience. The vacuum line from the purge valve, had come off the bottom of the intake....slid it back on, good to go...not sure if you checked that, but couldn't hurt..
 
You don't need a code reader to dump the codes, a paper clip will do. No matter what year the Mustang is, there is always some useful information in the code dump.


You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 200,000 hits, which indicates it does help fix idle problems quickly and inexpensively.