Recently I turned on my AC, and it just starting pumping out hot air, and the RPMs at idle it will constantly drop from about 900 to 400 every 2 or 3 seconds. I tried driving around in a parking lot to test, and the RPMs stay up, but the engine just feels wrong, kind of like a scraping or grinding via the peddle, but no noticeable odd sounds.

Maybe spark plugs? They're about 15k old though. I'm about to do a lot of much needed upkeep for 100k, but the only thing I can think of might be the belt on the AC?
Thoughts?

 

Does it run ok with AC off? If so then focus on AC compressor and perhaps the electric fan - any chance someone has rewired it to run on high all the time? That could add extra load and drop idle.

MINI's have just one belt. No separate AC belt. You would loose everything at once.

Still, may be worth inspecting the damper and belt tensioner to make sure they work ok.

 

It runs fine without the AC, I'll check the compressor, and look for a spare. I've never had an issue with the fan, so I think that is fine. Just weird to have it bounce up and down, and almost stall the car.

 

Just for grins, I would pull the tmap sensor (on the intake, near the radiator) to make sure it is clean. Don't use any chemical cleaners on it, I use low pressure compressed air. A dirty tmap was giving me knock codes when the AC was on - I guess it was close to out of range, and whatever adjustment the computer did when you switch the AC on was making it worse. Perhaps something similar is going on here. It's very easy to remove and inspect.