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Discussion Starter #1
Its been having this weird surging idle issue off and on for the past year, which is also accompanied by a strange vacuum/whooshing noise from the driver's side firewall area when the idle fluctuates. Also when i am coming to a stop and upon letting the clutch out and putting it in neutral, the rpms drop below 400rpm, then kicks up to 1000rpm before going back down to normal idle.

Car is pretty much stock besides exhaust, and sct tuner.

I am thinking maybe iacv?

Video of cold start-up.

2002 Mustang GT Idle issue - YouTube
 

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Mine has the same exact issue. Only been doing it for a couple months though. I went and purchased a IACV and will be installing it tomorrow morning. I'll let you know if it resolves it or not, but from the trouble shooting I've done, this is the problem with my car. But I'm not sure what your vacuum noise is. Have you checked for vacuum leaks?
 

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Discussion Starter #3
Mine has the same exact issue. Only been doing it for a couple months though. I went and purchased a IACV and will be installing it tomorrow morning. I'll let you know if it resolves it or not, but from the trouble shooting I've done, this is the problem with my car. But I'm not sure what your vacuum noise is. Have you checked for vacuum leaks?
I was under the hood the night before and saw nothing out of the ordinary, and please let me know if it fixes your problem.

The vacuum/whooshing noise only happens when it surges to high idle, then goes away when it drops back to normal idle.

It has never thrown a code for any of this either.
 

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Nor has mine ever thrown a code. I think I know what you are talking about, maybe. Is it coming say from behind the passenger dash it sounds like, a hissing or whining or some sort of air swirl, if that makes since.
 

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Nor has mine ever thrown a code. I think I know what you are talking about, maybe. Is it coming say from behind the passenger dash it sounds like, a hissing or whining or some sort of air swirl, if that makes since.
yep, i havent been able to pinpoint it, but yes it makes that noise whenever it surges to around 1k rpm.
 

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I'll have to see how mine sounds tomorrow. I know mine makes a sound of a sort, but I'm quite sure it isn't related to the problem. But I'll definitely let you know. I should be putting the IACV on sometime tomorrow or Wednesday, I'm hoping. But I will keep you updated.
 

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does it sound like a buzzing noise? If so, the spring inside the IAC valve has taken a dump.
 

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does it sound like a buzzing noise? If so, the spring inside the IAC valve has taken a dump.
I've heard a similar noise in mine before but couldn't pin point it. Sounded like is was coming from the dash near the headlight switch.
 

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I've heard a similar noise in mine before but couldn't pin point it. Sounded like is was coming from the dash near the headlight switch.
yup. i thought mine was the throw out bearing in my clutch because mine got so bad it would vibrate the entire floor, pedals, and steering wheel lol. it was the IAC going BLOOM!
 

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Oh, mine sounds like it is behind the passenger side of the dash, not driver side. But like I said, I'll definitely let you know if it resolves the idle fluctuation issue.
 

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I just replaced it, I let it run for a bit, not fully warm up though, and doesn't seem to be doing it. I'll drive around a bit today to definitely let you know a for sure answer though.
 

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Car idled much better today, but it didn't completely resolve the issue. Before it would go from 1500 or so down to 350-400, and now it fluctuates barely at all. Just from 900-1200 or so. I stopped by the store got some a bottle of MAF cleaner, and going to see if that resolves the issue. The IAC did take away most of the problem though.
 

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Car idled much better today, but it didn't completely resolve the issue. Before it would go from 1500 or so down to 350-400, and now it fluctuates barely at all. Just from 900-1200 or so. I stopped by the store got some a bottle of MAF cleaner, and going to see if that resolves the issue. The IAC did take away most of the problem though.
If you are above 5mph, the car should be idling around 1200 - 1500, once it drops below 5mph, it should go to whatever your idle is set at. Are you sure this is not what you have left now?
 

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If you are above 5mph, the car should be idling around 1200 - 1500, once it drops below 5mph, it should go to whatever your idle is set at. Are you sure this is not what you have left now?
Well that makes sense. Now generally what is idle set at <5mph for a 2000 5spd GT? This may be a dumb question, but I am new to mustangs, and have been researching and reading up on them a very good amount, in general.
 

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After some research, I found a couple websites stating around 750-800RPM's can anyone verify this for me? We all know the internet, can have a great deal of false information on it. Thanks in advance!

And to the OP I'd just clean your IAC really well and your MAF sensor, before you go out and buy stuff, because that alone may just resolve your issue. And I found this site, http://www.modularfords.com/f4/2000-mustang-gt-idle-issues-151775/, figured I'd share it with you to help trouble shoot. Good luck to you!
 

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Discussion Starter #19
After some research, I found a couple websites stating around 750-800RPM's can anyone verify this for me? We all know the internet, can have a great deal of false information on it. Thanks in advance!

And to the OP I'd just clean your IAC really well and your MAF sensor, before you go out and buy stuff, because that alone may just resolve your issue. And I found this site, http://www.modularfords.com/f4/2000-mustang-gt-idle-issues-151775/, figured I'd share it with you to help trouble shoot. Good luck to you!
Thanks man, i have some carb cleaner sitting in the garage, i'll try to get to it this weekend, it is original with almost 100k miles on it, so i wouldn't be surprised if it is going bad.


I also repped you.
 

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Check the TB. It could help. I remember my friends GT doing the same and it was his throttle body. It wasn't a stock one though, but it could be the problem.
 
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