Erratic Idle when stopped and AC is on...

[99redmustang]

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...Thank

 

[SaleenGT2001]

is this a blower car?

 

[99redmustang]

No blower...

 

[StangSims39]

what about the idle screw on the TB? might have gotten lose and its not in the right spot? i dunno just an idea.

 

[wmburns]

Is the AC low on Freon? Is the AC compressor cycling on/off?

What may be happening is the compressor is cycling too fast for the IAC to re-learn the AC on idle trim.