Check Engine Light - P0304
 

All, Today my amber check engine light came on, and engine began running rough - same time I was accelerating to enter the freeway.
Later I stopped at the car parts store and had the code read by the tech as P0304. Mini is one-owner 2012 Cooper S with 56,000 miles and was under extended factory warranty until last year, so all factory maintenance should be up to date.

Now for the puzzler...
I always use premium fuel in my Mini, but about 2 days ago I stopped at a gas station I don't normally use and accidentally tanked up with regular. This might mean nothing?? So later today I bought some octane booster and added it to the fuel, didn't make any difference to the light or improve drive-ability.

Where to go from here?
a) Should I swap coils between cylinders 3 & 4 and check to see if the code changes?
b) Could I have a tank of bad gas? should I have it drained, and change the filters? How could I test this theory?

Any assistance is appreciated, Thanks.

Appreciate the response. I swapped the coils between 3 & 4, still ran rough, but the code changed to P0303, so starting to look electrical. First bought new plugs, all were pretty worn and need of change. #3 plug was wet when pulled. After changing plugs still ran rough, so i bought a new coil and changed it out on #3 and all is good now. I'm guessing I saved myself a few few bucks by doing this myself, plus the satisfaction of man over machine.

Thanks for the help, Joe.

Can anyone explain to me why it's always the #3 coil position that deteriorates (and not 1, 2, or 4)? This evidently is a common problem.

At 56K miles I'd be tempted to replace all coils. If one has gone bad the others might not be in that good of shape.

I always wait a few moments to let the fuel pump build up pressure before I start the car.
It's a leftover technique from the original cars that had an SU fuel pump in the boot (trunk!). Once the click-click-click stopped you knew the carburettor bowls were full.