BMW Repair: Ghost Misfire; Probably A/C (air conditioner), oil filler cap, vent valve

Question
I have scoured the web, and only come across 2 postings with this problem.
I have a '99 323i with 75K. The A/C gets the cabin COLD, however, it causes a misfire. I get OBD2 codes P0300 to P0306. [Random Misfire, and then Misfire on cylinders 1-6].
Additionally, when the A/C is on, I get a horriffic grinding noise from the A/C. After a few seconds, the plastic housing over the Timing CHAIN vibrates (not visibly, you can hear and feel it) and makes another horrible sound.  This only occurrs above 2500RPM {approx}. Is the compressor bad? or just the clutch bearing.

**Note, my serpintine and AC belts are shot, and I'm replacing them. (The car would squeal for the first few minutes of running.) Definite belt sound.  I HOPE (and seriously doubt) that the bad belts are the root of the problem. Is this possible?  Someone PLEASE call me. 1-310-529-4805

Answer
Mattias, I would agree that the a/c compressor is the cause of your problem. Also I have seen worn water pumps cause misfire faults. When you have the belts off check for smoothness of rotation of the water pump and play by wiggleing the pulley up and down. If any resistance on rotation or play is felt REPLACE the pump if you have not replaced it previously. They are cheap and easy to do if you already have the fan and shroud out to do the belts. The pump can be a weapon of mass destruction if the bearing starts to wobble and causes the fan to hit the shroud and radiator at any speed. Also check the tensioner pulleys for noise, they can make awful noises too. Leave the belt off the compressor and clear faults. Drive the car around as normal (with no a/c operation of course) and see if your faults return. If you do replace the compressor BMW has an updated a/c hose with vibration dampner on it that I referenced to another person with an a/c complaint (I even gave the part number). I would also would check the crankcase vent valve (PCV VALVE). With the engine idleing carefully turn the oil filler cap and just barely crack it open. If you hear a weird honking noise you need a C.C. vent valve. If this is the case also get all the hoses to it and send another post to me and I will tell you indepth how to replace it.
-Evan