I have a 2004 (54) Mercedes-Benz C200 CDI with 105,000 miles on the clock. A while ago I noticed that upon idling, a smell of burning diesel came through the air vents. This was resolved when I put on the recirculation but I took it to a mechanic who said that a fuel injector was leaking and he fixed it. The problem was fixed but now it has come back although the smell does not appear very often (it used to always appear). I took the car back to the mechanic who can't find anything that it could be, it's not the injectors again according to him. Any ideas as to what it could be? Thanks a lot.