I bought my 99 SE-L used and from day one it has been exhibiting this behavior. When the car is at a stop and the AC is not ON then the cars idle is perfectly smooth but once I turn on the AC the idle get noticeably rougher. Is this normal? If not, is there anything I can do to fix this? Thanks in advance!

 

get an ohm meter an go to your engine bay, get the resistance readings of the 6 pin connector behind the Throttle Body. its the idle air control. i dont know the pins and specs off hand, but you should get a haynes manual. i think its 30 ohms. from 1-2 3-4 5-6. if the specs are off. get a new idle air control. if its good, read on how to check FAST IDLE CAM

 

I have this too, but it's not the ac's fault. When my rpms are in the 800 - 1000 range the car kind of vibrates. When I turn on the ac the rpms go up in to this range and i get a rough idle. I'm pretty sure this is because my flywheel wasn't balanced correctly(thanks Aamco) I didn't trust them to touch it anymore so I just put up with it.

With your car running and the ac off, try adjusting the idle up and see if it gets rough without the ac on.

 

you should be able to take care of this problem by simply turning the idle adjustment screw up some...with the AC on, the car is experiencing extra current draw, which will make the car idle rougher. i slightly higher idle should give you the extra juice needed