I have a 2008 mini cooper base N12 6 speed manual that I have not been able to figure out. Mini starts fine runs great until it is at operating temp then starts miss firing on cylinder one.
Code reader just tells me miss fire cylinder one nothing more. It is as if the power has been cut off the that coil. I have ran it in sport mode with same issue.
Compression test good hot or cold. Number 1 spark plug fuel soaked. No smoke out the tailpipe.

Things I have done:
New coils
New plugs
New injector
New thermostat housing
New vanos
Checked connections
I was going to change the engine wiring harness as I have 5 coopers in my shop and have plenty of parts.
I am not a workshop for mini's I just have sickness for these little cars.

 

DO a compression check on all the cylinders, to start.
I had same issue with my 05, after doing compression check found my problem cylinder was low, From there I had a leak down test and that
Lead to finding a cracked exhaust valve, After rebuilt head ant the cost of about 3k total all is good. I do hope that this is not where you will end up.
By the way the cause of the cracked valve was using non Top Tier gas, so i ended up having carbon build up that caused to valve to not properly seat.

Cheers

 

post some pics