I have an oil leak in my 1989 240DL. My husband found where it was coming from. It is coming from the back of the engine where the camshaft is. He said when he felt around that area (He couldn't see around there), it felt like there was something missing. He felt a metal disk with a small hole in the center. Is this normal or should there be a cover there?

 

There is supposed to be a circular round plug back there. There isn't much to hold it in other than friction...number one reason why it blows out is excessive crankcase pressure from a clogged oil separator. IPD sells a nice little item to keep it from blowing out...
Rear Cam Seal Retainer