My car has always ran as smooth as butter, just turned 43k miles. However a couple months ago it started running like its on 3 1/2 cylinders ?? The problem is perfectly consistent - always there, doesn't come and go. It's not actually missing out and running on 3 cylinders though ( no misfires ), plugs look great, compression is perfect, no codes thrown. However my manic tune won't throw exhaust codes so I wander if it could be exhaust related, so could it be clogged cat? As you see in the signature I do have an Alta down pipe ( it's used and probly has 100k on it) so it's not stock cat.
Any ideas ? Mini dealer won't touch the car due to all the mods so let me know what you think.

 

I'd hazard a guess you need a walnut blasting. Carbon buildup.

 

This is probably the best guess. Usually around the 80k+ mile mark you'll need to have the back side of the valves cleaned due to carbon buildup. The wonders of direct injected engines. They all do this, ours just tend to have more of it for some reason.

Shop around if this turns out to be the issue. Prices vary significantly in this from one dealer to the next.

 

Or get the tools from Harbor Freight and do it yourself if you're so inclined. Don't bother with fuel additives, they won't help you at this point.

 

OP has a '13 so N18. Sis not know N18's needed walnuts esp so soon. We have 68k on Countryman N18 and no issues as one data point.