I have a 05 RS and the idle drop and fluctuate between 1000 and 400 RPM when I start the car, sometime the car will stall. It only does this with the AC on.
Any idea why and how I can fix this?

 

Dont start the car with the ac on. Puts a lot of strain on the motor. Try turning it on after the car is running

 

there is an option you can edit in the ecu to up the idle when the ac is turned on.

or can try turning it on once the car is on and at running temp.

any mods done to the car?

 

Does it do it when you put your car in neutral coming to a stop as well? could be a bad ISC.

 

Does the car have cams? I know my idle drops when I turn the ac on and almost stalls. It did this after a cam install.

 

Nope, no cam. Just the regular bolt on (air intake, exhaust, intercooler,...)

 

It might b ur iac sensor..I had the same problem replaced it then worked fine ..then I put in some cams and im.back to the same spot lol ..

 

My idle also sounds worse with AC on. That's why I almost never use it.

 

Yep, it does that as well. Can you explain about the ISC?

 

The idle air control motor does just that in your case when the a/c is cut on the compressor creates and extra bit of load for the engine thus pulling the idle down, iac tells the ecu to bump up the idle to compensate.

 

+1
Have the same problem with idle when I put my car in neutral...

 

ICS, IACV or IAC, they are all the same exact part. No clue why some places call it different. It's located on the driver side of the throttle body, a small black harness mounted on with three screws.
People have had luck simply taking them off, cleaning then, slapping them back on. I have had the luck of replacing mine like 3 times now. Local autozone should carry them. They are expensive, I want to say close to $250.00 for the part. If you purchase a new one, buy a Tactrix cable and get evoscan. Turn it on SAS mode, and adjust idle by turning the biss screw to around 750-800 rpms. Once it's set, turn of sas mode, and it should idle at whatever evoscan is set to idle at. Hope this helps. On midnight shift and my lack of sleep is hitting me.

 

is your car tuned?