Hi, I recently bought a 2011 335i 6 spd manual with 23k miles. It runs great and has no obvious issues. However, the exhaust pipes have a build up of black soot. If you clean it off, it shortly reapears. A lot of people on bmw forums say that is normal, but I have read that it may indicate a rich mixture or a direct injection issue which could destroy the Catalyst. I understand the cats on this car can cost as much as four to six thousand dollars. Should I do something to get this checked out or am I concerned about nothing?
The black soot coats the inside of the exhaust pipe. More on right pipe than left (I know about the valve that reroutes exhaust from right to left when cold - that's not what I'm worried about).