If it's like any other engine with a PCV system, there's a spring-loaded check valve there somewhere that needs replacing.

If it's stuck open, it's pulling in oil-laden air from the valve cover or crank case. If your air flow sensor is upstream of the "tap", this is air that's not being accounted for by the ECM, which will cause driveability problems; and it'll screw with the PCV system itself.

Of course, you may not have a "proper" PCV system - I'm not familiar enough with MB engines of your vintage to say for sure...

__________________
A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.