I need some advice. I bought my first Mercedes-Benz last fall. It is a 1984 300D with 204,000 miles on it presently. It was meticulously maintained at a reputable local shop by the little old lady that had it before me. Shortly after I started driving it, I noticed that it loped at idle. I have replaced the fuel injectors, the fuel filters, and the injector pump idle dampener screw. I had a professional replace the timing chain and tensioner, adjust the start of delivery on the injector pump, and adjust the valves. The problem persists.

Frusterated, I took it to another reputable local shop today hoping to get to the bottom of the problem. A compression test was performed which indicated that four of the five cylinders tested at over 400psi. Cylinder #3 only showed 200psi. The mechanic put oil in cyl. #3 and checked it again, which showed slight improvement at 250psi.

The mechanic that worked on my car believes that this is a valve train problem, probably a valve guide and recommends pulling the head.

Big repairs make me antzy, and I contacted an outfit that sells cylinder heads for MB diesels to check out worst case. I spoke to the owner who says that quite often the ring grooves on the piston collapse, making a mess of the piston, scoring the cylinder wall, and requiring a full engine transplant or rebuild.

Are there any clues that would give me an idea about how big of a problem this is? Do I need a valve job or a new motor? Thanks.