Take off your air cleaner and look for the port where it is supposed to connect to the intake manifold.

If this is disconnected and not properly blocked off..your engine is sucking dirt strewn air in that hole (bypassing the air cleaner).

The system on that 240D is simple and only has 2 rubber elbows and a hose. It just blows the air from the valve cover over to the intake and burns it up. The later 5-6 cylinder models are much more complex as they capture oil/oil vapors and return the liquid to the oil pan while offing the air into the intake. (Ideally )