Finally sorted! Rare AC / engine misfire problem.

Benvette · #1 02-06-2013, 02:25 AM

So, I was on holidays with my family over christmas and we drove over 1000ks (600 or so miles) up the east coast of Australia to visit relatives.
On the day of the return trip, we had got 150 yards up the street and my wife couldnt find her watch or bracelet or something. Instead of turning around. I stopped the car and reversed at speed. The car was doomed from here! When I stopped in the driveway the engine was misfiring. I popped the bonnet and couldnt see or hear anything out of the ordinary. Started the car and misfie was gone.
So, off we go home. Half an hour later, misfire starts again. Stop engine, restart, misfire gone. All the way home it got worse and worse untill as soon as you revved the engine it would come back. Only about 2 hours from home I discovered if the AC is off there is no misfire. So we drove home and for 3 weeks without aircon which was a real pain because every time you start the car Mr BMW knows best and makes sure the AC is on.
My wife gets home one day saying there is an awful noise coming from the engine after she turned AC on because it was 46 outside! Thats celcius. One of the hottest days on record.
The noise was coming from the compressor. So I ordered Chinas finest replacement compressor from Ali Baba for $225 delivered.
The day it arrived I got stuck into replacing it hoping it will fix the misfire.
As Ive dropped the plastic bellypan Ive noticed a piece of broken die-cast aluminium. I thought straight away this is got to have something to do with everything.
I pulled the AC tensioner off and sure enough the stopper had snapped off. This stopper stops the belt from overtensioning.
I fashioned a new stopper, put the car back together and it has not missed a beat since.

My conclusion is the stopper snapped when I went in reverse at speed which in turn caused a vibration of the belt that slowly got worse untill it was slapping about and causing a vibration in the crank angle sensor, therefore getting a misfire.

Moral to the story. Dont drive backwards angry.

BTW The loud noise was from the belt slapping, not the compressor. I replaced the compressor anyway because it was getting a bit noisy and i already let the gas out before I lifted the car.