Air Compressor - 1997 Mercury Tracer

I have a question about the a/c on our 1997 Mercury Tracer. When we started using the a/c this year, we noticed that the air wasn't as cold as it normally is. We also noticed that the idle of our car would flip back and forth when we would have the a/c on and were stopped at a traffic light. So, we took our Tracer in and were told that it needed freon and the compressor would need to be replaced relatively soon. They added freon and the a/c works great. Also, the idle flip doesn't happen anymore, but there have been issues lately with a weak idle when the a/c is not on. Here is my question. Would an air compressor/clutch affect the idle or performance of an engine if the a/c is not on and the compressor has not locked? I'm asking this because it would cost us approximately $700.00 to replace the compressor and I'm worried about getting our car back from the shop and having a more urgent problem. Any information would be greatly appreciated.

-Joseph

The idle w/o the a/c being on has nothing to do with the compressor, unless it's making a terrible screaming noise because it's about to seize up. You probably need to look at one of the standard things that go wrong with these engines as they age: spark plugs, plug wires, idle air control valve, pcv valve, pcv/vacuum hose leaks.

The engine computer is designed to increase the idle speed when the a/c is on, which masks the idle problem you have. A low a/c refrigerant charge means the compressor will cycle on and off more frequently, because the low pressure cut-off is reached more often (low side pressure drops when the compressor is running, and rises again when it cycles off).

Also, R-134 (not freon) costs less than US$10 a can (don't use the stuff with stop leak, ever!), and if this recharge lasts a month or two, you can buy an awful lot cans for $700. I'd continue recharge the system as needed (at least once a year, as often as once a month) until the compressor actually needs to be replaced. You can actually buy a recharge hose & gauge and learn to fill it yourself fairly easily.

Bob