smoking 1992 300SD

valve guides and seals

the case, minus a few cans!

It's much worse than you think. The 3.5L diesel has a well known reputation for bending its connecting rods, more often than not those of #1 and #4 cylinders. The bent rod allows its piston to score the cylinder wall with the result being the problems that you describe.

Many engines were replaced on this account. Some died young - at say 65K miles and others ran a long, long time, yours appears to be one of those so in a sense you "lucked out" since many diesels need an overhaul starting at about 250K miles.

Replacement connecting rods have been modified from the too weak original ones, I suggest you have a chat with the local M-B parts man as to their availability.

This won't be cheap to repair.

Doing a proper compression test including a wet leak down test will tell you if the leak is the rings or the cylinder head... This should be done before you take anything apart (obviously?).

If the wet leak down shows that it's the pistons, then TGs scenario is most likely. If it shows it's the head then pool man is probably right...

Good luck, Marty

I agree.

A compression test is normally done in cases of reduced performance. In this instance, oil consumption, it will be useless.

Wrong.

very wronge!!!!!!!!!

the case, minus a few cans!

Not so. How will a compression test reveal worn valve guides or bad stem seals? And oil control rings have little to do with compression. Finally, the engine's using oil, so it's already wet for your dry/wet test.

Poor compression will indicate whether there is a leak or not... poor compression means a leak which then leads to leakdown test... and listening for air hissing via the intake manifold or via oil cap.

Basic diagnostic of engine internals are compression test... and then leakdown test.

It won't reveal the stem seals or valve guide issue, that's true(because the valves are closed and seal), but it might help eliminate the scored piston/bent rod scenario which TG brought up. Doing a dry compression test / leakdown followed by a wet one will show if there is significant damage in the piston bore (or broken rings) in my experience.

The amount of smoke the OP seemed to describe doesn't seem to be enough to create a automatic wet test.

It is true that the leak down test MIGHT not prove useful, but it is potentially useful and an inexpensive diagnostic step that can provide critical information. Certainly worth trying IMO before deciding on a new/rebuilt engine or head work.

I kind of tend to Poolman's valve seal/guide theory as a guess.

Marty

Given the high milage on the car I would agree. Digging down to the stem seals is not a large job, once there the condition of the valve guides can also be assessed.