91 Cav - 3.1 L Runs rough w/ Air Con on?

I drive a 1991 3.1 liter Cavalier RS Coupe. I had the air conditioner disconnected for about a year, over the winter and never re-hooked up till about early June this year. All seemed OK. Lately though, when I turn it on , the car will increase RPM, which I think is normal, but these last few weeks it causes the car to run real rough, and shake slightly when it is turned on. It blows fairly cold still, although I would assume it has never been re-charged. The other thing that has been happening since I owned the car is the oil pressure light dances around up and down. If it is "active" or moving around, this seems to be when the car will idle rough, etc. My mechanic says it is a "loop problem" with the gauge, whatever that means.

It has high miles 238000 kms, but runs like a top really. A solid car all in all and I have done a few things to it over the past 2 yrs. For a second car it serves me well for running around town in.

Out of curiosity, what should this car get mileage wise. I seem to get around 20 in town and 25 MPG on a trip – per imperial gallon. Should it be higher on a 3.1 litre 6 cylinder? Or with "high miles" on the motor do they tend to become less fuel efficient. Anything I can add, after market to increase the mileage? The cruise doesn’t work anymore either, are they hard and expensive to fix?

Thanks for any answers. :smile:

Regards

RossW

Regarding the rough idle; It would seem the motor mounts are showing some wear, and the idling seems more pronounced because the rubber parts are allowing too much vibration to get to the structure(s). Check that idle rpm's are on spec to make sure the tps is responding correctly.