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Old 05-28-2015, 07:23 PM   #1
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IAC issues

Okay so I've been trying to figure this out for too long now lol. My car idles between 800-900 rpm, when I turn the a/c on, it drops down to 400rpm (almost stalling) and than back up to 600 rpm, it keeps fluctuating between that every few seconds and the a/c compressor is clicking on and off. Compressor is good, freon is good. IAC is 3 weeks old, now when I unplug the IAC at idle NOTHING HAPPENS. I took my old IAC and tried plugging that in and NOTHING HAPPENS. Its almost like my car is just not sensing that there even is a damn IAC. The only thing thats keeping me to believe the new IAC is NOT bad is that my old one wasn't completely shot, so in theory when i plug thd old one in something should be happening. Nothing at all with either IAC. I'm stumped lol..anything guys? Its like my idle is being controlled by the throttle stop screw and nothing else. I even tried unplugging my IAC, loosening the throttle stop screw until the car barely idles, and when I plug the IAC back in NOTHING. I have to tighten the screw back up, which is just opening up my TB more and more to keep the idle where it should be.
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Old 05-28-2015, 07:27 PM   #2
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I had BAD iac problems on my 94 3000GT. When I replaced the IAC, I had to unhook the battery for an hour so the car could relearn the iac stuff as well as other stuff. Maybe that would help?

Also, did you clean out the area where it goes in as well as the TB to make sure it isn't gunked up?
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Old 05-28-2015, 07:27 PM   #3
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Did you get an autozone aka corner parts store replacement? They are pretty well known to suck. Lots of people find they are no better than the one they are replacing. Gotta get a motorcraft one.


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Old 05-28-2015, 07:48 PM   #4
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Quote:
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I had BAD iac problems on my 94 3000GT. When I replaced the IAC, I had to unhook the battery for an hour so the car could relearn the iac stuff as well as other stuff. Maybe that would help?

Also, did you clean out the area where it goes in as well as the TB to make sure it isn't gunked up?
Yea I did the whole KAM reset and all multiple times, solved nothing. The TB was pretty clean when I looked inside, def not gunked up.enough to a point it would be blocking out the IAC .

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Old 05-28-2015, 07:53 PM   #5
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Very odd, I would at least take the one you bought back and swap it just to eliminate the possibility of the new one also being bad. With technology, stuff can break at random times. I have seen people drive off a car lot in a new car just to be broke down right down the street. lol.

Have you also checked for vacuum leaks?
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Old 05-28-2015, 07:54 PM   #6
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Did you get an autozone aka corner parts store replacement? They are pretty well known to suck. Lots of people find they are no better than the one they are replacing. Gotta get a motorcraft one.


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Ugh I did. The motorcraft was like $150 or something compared to the off brand for like $80..I mean it's possible that could be it, but I realllllly don't wanna shell out more cash for another IAC to find out its going to do the same thing. I want to make damn sure 1000% that the IAC in there now is no good. But like I said my old original motorcraft stock IAC was still working (to my knowledge) at the time of replacement. What happened was I replaced my manifold, had an idle issue, replaced the IAC, and than found out the idle issue was because of a vacuum leak. So my original IAC shouldn't be shot either, probably pretty damn dirty after 150k miles but it was still working when I replaced it. Now if I take the stock IAC and plug it in, nothing happens..what the f lol

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Old 05-28-2015, 08:00 PM   #7
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Yea that is odd. Another thing to try, is put the new IAC on another mustang like yours if your buddy or someone has one. If it still idles fine, then you know it's good. Better then spending money.

Yea IAC issues are a pain to isolate. Just like vibration issues when driving. Don't you just love cars? Lol.

Now I did have your same symptom of it dropping when the A/C gets turned on. Someone mentioned it was the AC oriface or something? I am not familiar with A/C systems but there are shops that can do entire A/C inspections. I don't remember if the A/C on your year has it's own dedicated pulley and stuff. I have heard on other cars of the pulley assembly getting messed up. Like I said, super hard to isolate these things. It's always "one thing at a time".

Also, are you dyno tuned, chipped, or anything?
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Old 05-28-2015, 08:02 PM   #8
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You can also test the voltage on the iac. YouTube it. They walk you through testing it. It's resistance voltage. If you have a multimeter you are good to go


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Old 05-28-2015, 09:19 PM   #9
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Yea that is odd. Another thing to try, is put the new IAC on another mustang like yours if your buddy or someone has one. If it still idles fine, then you know it's good. Better then spending money.

Yea IAC issues are a pain to isolate. Just like vibration issues when driving. Don't you just love cars? Lol.

Now I did have your same symptom of it dropping when the A/C gets turned on. Someone mentioned it was the AC oriface or something? I am not familiar with A/C systems but there are shops that can do entire A/C inspections. I don't remember if the A/C on your year has it's own dedicated pulley and stuff. I have heard on other cars of the pulley assembly getting messed up. Like I said, super hard to isolate these things. It's always "one thing at a time".

Also, are you dyno tuned, chipped, or anything?
Yea I thought about that, than remembered I don't personally know anyone who drives a 99-04 GT lol. I really don't think it has ANYTHING to do with the A/C at all, I've been using my A/C for weeks, even the day my manifold blew, worked PERFECTLY, than I replace my Manifold, IAC, and it starts doing this..I really, really think it may have something to do with that TB idle stop screw. When I first replaced JUST the manifold, I had a problem with car idling at like 2000 RPM, my first initial reaction for whatever reason is maybe I hit the TB in some way and moved that throttle screw a bit, so I completely loosened the screw all the way. Well obviously that didn't work, and than I replaced my IAC, the idle got a bit better but not even close to being okay, than i found the vacuum leak . When I fixed the vacuum leak the car would start but die right away, and than it hit me duhhhh I loosened the TB idle screw all the way. So I tightened up the screw and set my idle perfectly right around 800. Now I'm having this problem with my IAC not bumping up my idle to compensate for extra load of the A/C . idk if I somehow ****ed everything up by messing with that throttle screw, my guess is yes lol
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Old 05-28-2015, 09:41 PM   #10
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Yea my mechanic friend told me a while back to never mess with it unless you really knew what you were doing. Like the Windows Registry. Lol.

Maybe get a used TB or now is a good time to upgrade your TB if you havent D:
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Old 05-29-2015, 08:27 AM   #11
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Yea my mechanic friend told me a while back to never mess with it unless you really knew what you were doing. Like the Windows Registry. Lol.

Maybe get a used TB or now is a good time to upgrade your TB if you havent D:
Yea I was thinking the same thing, but there has to be a way to figure this damn thing out, I hate to just keep throwing money at it..since January I have replaced my Clutch, Manifold, Alternator/Belt, IAC, EGR valve, 4 Coil Packs, And many other little things like plugs, tires, brakes, etc. Its just been nothing but a crazy money pit lately lol. In all honesty I love this car to death, but nothing has pissed me off more in my life than this car the past few months lmao
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Old 05-29-2015, 09:03 AM   #12
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Now I do see you said you replaced the manifold and that is when you first started your journey down the idle issue? I don't remember off the top of my head, but were all new gaskets used, etc during the install? Have you tried inspecting the manifold to make sure it is ok as well?

Man, you are extremely on par with what I went through with my 3000GT. LOL! Safe to say, that thing is out of my life now.

I assume on this model, the TB And IAC connects to the manifold? That was how my old 3000GT was.
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Old 05-29-2015, 04:15 PM   #13
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Now I do see you said you replaced the manifold and that is when you first started your journey down the idle issue? I don't remember off the top of my head, but were all new gaskets used, etc during the install? Have you tried inspecting the manifold to make sure it is ok as well?

Man, you are extremely on par with what I went through with my 3000GT. LOL! Safe to say, that thing is out of my life now.

I assume on this model, the TB And IAC connects to the manifold? That was how my old 3000GT was.
Yea all new gaskets, no leaks at all, no vacuum leaks, nothing, my buddy smoke tested it, we found 1 small vacuum leak that was easily fixed but did not fix the issue. On this model yes the IAC connects to the TB and the TB connects to the top of the manifold. Lol this is such a pain in the ***..if I didn't have so much invested in it lately trust me I'd be dumping this REAL quick, I'm stuck with it for a bit now lol

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Old 05-29-2015, 04:31 PM   #14
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My next guess would be a sensor maybe isn't reading right? Have you cleaned your MAF Sensor, checked O2 sensors, etc?

Also, cleaning other things out can help try to isolate this. Fuel system cleaner to get rid of any bad fuel buildup, seafoam to get the gunk out of the engine, etc.

Have you checked your fuel injectors? I have seen leaky injectors cause weird issues like this also.
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Old 06-05-2015, 01:23 PM   #15
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Andddd it was the POS $80 AutoZone IAC. I finally said F it and bought motorcraft and problem solved...DO NOT BUY NON OEM IAC!! lol thanks guys!

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Old 06-05-2015, 01:39 PM   #16
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Oh dang. Well if you cleaned everything else, at least those are less things to worry about. I never shop at Autozone. Sometimes Orielly for small tools and little odds and ends when I am in the middle of projects. But never actual car parts.
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