Erratic Idle when stopped and AC is on... 1999 Mustang GT - Ford Mustang Forum
 
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post #1 of 11 (permalink) Old 05-27-2010 Thread Starter
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Erratic Idle when stopped and AC is on... 1999 Mustang GT

I have a 1999 GT and I am having really bad idle surges when the AC is on and I come to a complete stop. The idle is fine when I have the AC on and the car is in in traffic...the idle surges from 300rmp to about 1400rmp and does this for the duration of time that I am stopped....sometimes it almost shuts off...

I've cleaned the MAF, clean the air filter (K&N), replaced the IAC valve, the EGR valve, PCV valve and I have a new ECM (about 3 months old). Am I not checking something??? Could it be low on freon or something.

It only happens when I come to a stop!!! Any advice...Thanks.


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post #2 of 11 (permalink) Old 05-27-2010
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Is your CEL on? I had the exact same symptoms when my upstream o2 sensors were not responding. As soon as I replaced them it cleared the idle right up.


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post #3 of 11 (permalink) Old 05-27-2010 Thread Starter
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No CEL...all 4 o2 sensors were recently changed...she runs strong other then when stopped and the damn AC running...

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post #4 of 11 (permalink) Old 05-27-2010
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Could have to do with your IAC valve...

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post #5 of 11 (permalink) Old 05-27-2010 Thread Starter
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Quote:
Originally Posted by Black_35th View Post
Could have to do with your IAC valve...
Thought that might be the problem but I just replaced it this morning...

2011 5.0, Sterling Grey, Brembo Pkg and 3.73 Gears, GT/CS Valance, Roush Grille
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post #6 of 11 (permalink) Old 05-27-2010
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Yeah IAC would have been my guess..

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post #7 of 11 (permalink) Old 05-27-2010
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Vacuum leak or ignition problem.

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post #8 of 11 (permalink) Old 05-27-2010 Thread Starter
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Quote:
Originally Posted by racin366 View Post
Vacuum leak or ignition problem.
Would a EVAP test show any signs of a VAC leak? I had a EVAP test done and everything was fine...

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post #9 of 11 (permalink) Old 06-02-2010 Thread Starter
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The TPS was no good...replaced that and now she is good to go...no longer idling all over the place when I come to a stop with the AC on!!!!

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good deal nice fix

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Quote:
Originally Posted by 99redmustang View Post
The TPS was no good...replaced that and now she is good to go...no longer idling all over the place when I come to a stop with the AC on!!!!
I'm going to assume TPS is Throttle Position sensor or am I missing an acronym??


How hard was this to replace? My 2000 GT just started showing these exact symptons this weekend when it was 96 degrees outside.

Mine is also blowing hot air out the vents, so I don't know if this is exactly the same problem, but I'm willing to give it a shot.
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