I'm guessing that you have a leak into the valley of the motor. All manner of stuff winds up there and if the level of 'stuff" gets too high it comes out a hole and drains down the backside of the motor.

A pressure test should show the culprit. Since this ought to be easy, it might be a good time to establish your new relationship with a trusted technician/shop. Ask a couple MB owners who they use. Look at the parking lot in a few shops. There better be a predominance of MBs there, if you wish a real professional.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician