In my quest to troubleshoot excessive oil consumption (1 qt every 500-600 miles) here on the forum, I keep running across discussion of an "oil separator". My '81 240D has a hard plastic pipe that runs from the valve cover directly to the intake manifold. Nothing else is attached.

A couple of others in various threads seem to have something similar, but most seem to have some type of separator that magically cools down the vapor to extract some oil.

I'm still trying to find a good manual for the car as the ones I purchased off of eBay appear to be pirated copies and won't even install.