Has anyone heard of this problem on MB Diesels? I have a 1984 300D with weak compression (200psi) on the #3 cylinder. The other four test above 400psi. The car lopes at idle. It runs smoothly at speed. I hear a ticking noise, like a tappet, at low rpms. Burns no oil, and gets 25-26 MPG.
The mechanic who did the compression test rechecked the #3 cylinder after adding oil. It came up slightly to 250psi.

The mechanic who did the compression test suspects that there is something the matter with the valve train, probably a loose guide.

I spoke to another fellow who informed me of a "common" problem where the piston ring grooves collapse, making a mess of the piston, scoring the cylinder wall and requiring a new motor. I have not yet talked to anyone else who thinks that this is a common or likely problem.

Has anyone had any experience with a motor suffering from piston ring groove collapse? How did such a problem present itself? Are there symptoms that I should look for to tell whether this is the problem, or a loose valve guide? Any advice, comments, etc are greatly appreciated.