unstable RPM's

Hey guys,

I usually post under the grand prix forum, but I'm helping my pops out with getting him some info on his 2000 Deville DES. Apparently when he has the A/C on his RPM's are very unstable and constantly moving up and down, even when idling...have any of you seen this before? Any ideas what might be wrong? Thanks in advance.

well i have a 1996 deville and mine to is unstable when cutting a/c on, but as long as it isnt overheating i don worry bout it. does yours overheat?

when the a/c compressor turns on, the ISC raises the rpms on the engine to keep it from stalling. When the compressor turns off the rpms return to normal. also the cooling fans turn on the same time as the compressor. How high are the rpms going?

Upon further review, it's not so much that the RPM's are jumping up and down, but rather dropping. Once the A/C is on the RPM's are dropping to around 500. From my understanding, the 02 Deville should be idling at around 675rpm, so I'm not sure what's causing the drop and what is the relationship with the A/C being on. Thanks guys.

I'd suggest cleaning the throttle body and plates as a first step. If that doesn't help, consider the possibility of the ISC motor having failed.