I've got a 92 740T Wagon that runs great, but when the ac is on it develops a very rough idle. The car is idling normally at i'm guessing 750-800 rpm's maybe a little higher when the ac is off. When the ac is switched on the idle drops down to around 500 rpm and then engine just idles very rough, it does not nescessarily surge and hunt, it is consistant, just lower than normal and rough.Carrunsout fine either way and gets good fuel mileage, just sometimes whensitting at a red light you think it's going to quit any minute. Any ideas? Fuel filter, battery, basic things are new. I haven't done anything to the throttle body or plugs since i've had the car. Thanks in advance.

 

Sounds like the compressor is going out. Do you ever hear it squeal? When the compressor is notturning well it is puting the motor under a load and making it want to cut out. Probably not what you wanted to hear but that is what it sounds like.

 

The compressor may have squealed briefly today.The ac/heat controls had went bad thus no air, and I finally replaced them today. It had been a couple of mos since the air had worked and it did cause a rough idle then, but now it seems worse. Also is it possible to hook the vacumn lines up backwards on the back of the ac controller. I looked at them before I removed, and am 99.9% sure I plugged them back in the same way but i've done dumber things, and if they are wrong could this cause the idle problem. Thanks again

 

it is possible but I would not think think that even if it was hooked up wrong it would not be enough vaccum loss to affect anything. Just fo the heck of it i it is not to much trouble swap them and see what happens. You could unplug the compressor and see if it is still runnning rough, that may tell you if that is the problem. I would also look for collapsed lines ans anything that would make the car pull in air. It could be something blocked up and it is not allowing the car to pull in sufficient air flow. Does the recirculate button make any difference wen pushed?

 

Is the idle circuit working correctly?
To test if it is working correctly try this.
With the car running shift from park to reverse the idle should go up about 300 to 400 rpm then settle some. Then switch to neutral it should do the same thing.It will go up to about 1000 then settle back down to the normal idle.

If it doesn't do that try cleaning the throttle body and make sure it is set properly.