2k RPM Idle at startup

Mods in signature. Just recently bought this car with mods and was told it was tuned. When cold cranking, the idle immediately surges to 2k RPM or so for about 8-10 seconds, then goes down to 1k-1500 where it stays. The car runs well and after warming up, the idle stays around 1k, but it surges up between shifts regularly. Any ideas?
 
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AeroCoupe

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Mustang5L5

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Unplug the IAC and start it up. What does it do?

With the IAC unplugged disconnect the SPOUT and try to lower the idle down to around 700-800 rpm. If it maintains a steady idle, disconnect the ECU for about 15 mins.

Hook it back up, plug the IAC and spout back in and start it up. Hopefully it’s much better.

I would also pull codes. Running rich, or vac leaks will also cause idle surges. A smoke machine is a good way to test for the latter. Can be built cheaply via Home Depot/Amazon.
 
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Unplug the IAC and start it up. What does it do?

With the IAC unplugged disconnect the SPOUT and try to lower the idle down to around 700-800 rpm. If it maintains a steady idle, disconnect the ECU for about 15 mins.

Hook it back up, plug the IAC and spout back in and start it up. Hopefully it’s much better.

I would also pull codes. Running rich, or vac leaks will also cause idle surges. A smoke machine is a good way to test for the latter. Can be built cheaply via Home Depot/Amazon.
I unplugged the IAC and she idles perfectly. A little rough because of the cam, but that's all good. I was told the IAC was brand new and it looks to be. I guess it needs to be replaced. I'll try it with a full cold start tomorrow morning and see what it does.
 

90sickfox

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5L5 was saying you do a base idle reset procedure. Thats what he described in the post. If you didn't do all the steps it'll go back to running like it was when the IAC is plugged back in. The fact that it smoothed out and held a reasonably steady idle means it probably just needs the idle reset done. The IAC is easy to check by blowing through one side. It's a valve and should be closed when unplugged. If air comes out the other side it's bad.
 
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mustystang

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I'm confused, you said the car was tuned. Obviously it's not going to be stock tune. It sounds like you want to change the tune. Do you know if it's a custom tune or a pre-set tune like bama? Just get the equpiment to change the tune. I don't think anything's wrong until you set it back to stock and then are having issues.
 
I'm confused, you said the car was tuned. Obviously it's not going to be stock tune. It sounds like you want to change the tune. Do you know if it's a custom tune or a pre-set tune like bama? Just get the equpiment to change the tune. I don't think anything's wrong until you set it back to stock and then are having issues.
The car was tuned by Mo's Speed shop in Georgia. It will be getting a fresh tune within a couple weeks. Maybe they can fix the idle issue.
 

Dan02gt

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The car was tuned by Mo's Speed shop in Georgia. It will be getting a fresh tune within a couple weeks. Maybe they can fix the idle issue.
Yes I'm sure they can. Until you know what the idle is set at in the tune it's going to be hard to fix the issue. In the mean time I would do the base idle reset and try to get it to idle around 650 RPMs with the IAC and SPOUT disconnected.
 
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jrichker

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Yes I'm sure they can. Until you know what the idle is set at in the tune it's going to be hard to fix the issue. In the mean time I would do the base idle reset and try to get it to idle around 650 RPMs with the IAC and SPOUT disconnected.
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 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
 
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