Over the past few years, my '02 MCS seems to be idling a bit rougher. I remember a few years ago with a A/C on in hot weather, my Scangauge would read a MAP pressure of 7.8-8.0 absolute. Recently, it's been more like 8.2-8.5 absolute. Today, I had it up at Pep Boys to get an A/C check since it hasn't been cooling great. They do a vac, leak check, and recharge and say it was a little low. Then they say my compressor is getting old and it will probably go out in the near future (not sure how much I believe them on that). Anyway, with the proper amount of refrigerant in the system, it's cooling much better. I actually don't have to run the recirculate to get cold air. A couple side affects though... I heard the high speed fan running for the first time in a long time (believable if the A/C system was low and wasn't running at high enough pressures) but also my idle is now at 9.1 absolute when the high fan is on in this 95+ degree weather. When temps dropped down into the mid 80's at night, I was back in the 8.2 range and only the low speed fan was running.

So this begs the question... is this normal? I've always seemed to get lower vacuum readings than others on this site. 8.0 absolute is only 13.5 inHg vacuum.

 

Sounds normal to me. You just added freon to your A/C system and now the compressor has to do more work resulting in more drag on the engine thus lowering your idle more. It all boils down to the engine (and car) getting a little older and a little more tired than it used to be.

 

Yeah that's what I figured. It was just a bit startling to see such a big change. On a nice note, it was 97 today and I got cold A/C even without using recirculate. Nice to see the high speed fan finally clicking on and doing it's job.