I own a 300SD - the finest MB that I have every driven or owned. Every thing works on this car. It has 144K miles on it and I purchased it with complete service records. There are no obvious leaks on the ground- have removed all encapsulation panels. I can see small leaks from bottom of intake manifold - oil running down from manifold toward bottom of engine. I am not sure how much oil it is burning but I am keeping accurate records and will need to put more miles on it to determine amount. I suspect it is burning approx 1 quart /per 600 miles. I want to keep the car and I am willing to put money into it. But before I go over the deep end I would like to diagnose actual problem for oil usage.
The car starts immediately , runs smoothly and I have driven it at over 100 mph without any problems. The engine sounds good - no knocking or unusual noises.
After reading these threads I am aware of connecting rod problems and cylinder out of round problems from bulletin boards. Oil burning could be leaking turbine seals or cylinder out of round. What I want to do is find out what actually is the problem before I attempt a solution. Also what would be the most economical way of fixing. For the time being I will keep oil level
correct. If cylinders are out of round would that show up in a compression test , you would think that would impact starting (lower compression).
If problem turns out to be out or round cylinders(how do I determine that) would an engine rebuild with new connecting rods and cylinder liners be the way to go or a replacement rebuilt short block be the way to go.
Any help . opinions etc would be appreciated. Thanks in advance. Mbdiesel
p.s. the exhaust appears clear only smokes under very hard acceleration.

Welcome to the forum.

It's an unfortunate fact that these engines have a predisposition for bending a connecting rod. When it occurs, the forces in the rod begin to ovalize the cylinder and that cylinder will have reduced compression and increased oil consumption.

So, before anything else is done, it makes perfect sense to get a compression test done and look for a cylinder that is significantly lower than the other five. If this is found to be the situation, then you can be reasonably sure that the engine is suffering from the typical malady. Naturally, no confirmation is available until the head is removed.

There are a few schools of thought on replacing the engine. All are costly. Getting a brand new short block is certainly an option, and rebuilding the engine with new rods and liners is also an option. The costs for each need to be carefully weighed. I'll not make a recommendation because I have not done it.

The least costly option is to find a fully functional engine (preferable a 3L engine from the '86-'87 vehicles) and install same. This can probably be accomplished at one-half the cost of the rebuild or the new short block.