I have a 2007 335i coupe with 17k miles on it, 6 spd manual. I've noticed a thick black soot of carbon buildup on both exhaust tips, which builds up in very short time between weekly washings. BMW Seattle claims this is "normal", BMW NA claims "this is normal". I'm not buying it. I suspect this has something to do with a direct injected turbo motor, but you don't see black tailpipes on Audi A4's....so do you folks think this is "normal"?? If it looks like this at the exhaust tips, I can't imagine what it looks like further upstream.