Hi folks, I have an '08 MCS with 177,000 km (~109k miles) on it, and have been having recurring misfires since December. I've had it to the shop twice to have it looked at, and don't seem to be any closer to fixing it. (Mini dealer the first time, independent shop the second)

It only seems to occur when I am on the highway at cruising speed for a while.

The plugs and coils were replaced in October at 167,000km (103k miles), and both shops said they seem fine. Compression test has been done, all cylinders hold good compression.

Codes it threw:

P0304 - Cylinder 4 misfire
2A7F - Rear O2 signal
P2781 - Multiple Cylinder Misfire Max
2779 - Cylinder 4 Misfire Max

Codes it threw last time:
P0420 Catalytic converter system, bank 1 - efficiency below threshold.
2783 - Misfire, multiple cylinders
277B - Misfire cylinder 4
2773 - Misfire cylinder 1
2A7F - Rear O2 signal
276A - Post Catalyst Minimum
2F8F - 2F8F: Accelerator pedal module and brake pedal, plausibility
2783 - Misfire, multiple cylinders

I need some help, because I can't afford to keep paying shops to diagnose it, but can't find the problem. (Seems they are having a hard time reproducing the misfires). Any suggestions on what could be causing this?

 

I would switch my plug from Cylinder 4 to Cylinder 1, and the Coil from 4 to 3.... Clear codes.... see if the misfire follows either one... this rules out faulty coil or plug. (easy, cheap only time for that one.)
if it doesn't follow i would look at that sensor. MINI Cooper R56 Turbocharged Engine Oxygen Sensor Replacement (2007-2011) | Pelican Parts DIY Maintenance Article as it may be sending shotty information to your DME making it change the fuel oxygen mix ratio.
put your vin into RealOEM.com and pull up the sensor and you can find them relatively cheap if you look around...
I would be surprised if the problem persists after the sensor change... but start with the plugs... when looking at codes that a computer throws, I start at the top of the list and work down. If Cylinder 4 misfires due to coil or plug, its gonna mess up the fuel ratio on the next fire and send out codes to match... and generally when I had my Coils go bad I would get misfires on 2 cylinders but onyl one would be faulty, so I would swap it one by one until the new one is in the right spot, by switching to 1 and 3 you should be far enough apart to distinguish the misfiring part. (Assuming thats the one.)

 

Thanks for the comment. The plugs and coils have been checked by 2 shops and moved all around. Cyl 1 & 4 are still the ones throwing codes. I have ordered an upstream O2 sensor. Hopefully that will either fix it or narrow it down.