I have a '91 350SDL at 127K with no significant oil consumption at this time (change oil every 3K using Rotella 15w-40) but am confused about what I'm reading here. Is the common problem of high oil consumption cylinder connecting rod related or valve push rod related or something completely different?

 

My understanding of the 350 engine is that the cylinder walls become deformed, or oval, rather than maintaining round. As this process become more and more oval, oil gets by the rings and thus burned. Eventually the piston begins to bind and at some point a rod will bend or break.

The reason explained from one tech I read in another forum is that MB went beyond the safe bore limits of the 603 to develop the 350 engine. There may be more to this than I am aware, but I'm just telling you what I have read. Excessive oil usage with these 350 engines is a sign of bad things to come in short order. Time to overhaul, or replace.

 

To amplify a bit on Benzbob's reply, the problem was with the design of the original connecting rods. They were too week, and could bend. So instead of the pistons moving up and down in a straight motion, the bend rods would now exerts a slight lateral force causing the pistons to push against the side of the cylinder. This eventually lead to the cylinder walls acquiring an oval shape. Oil consumption would than increased suddenly and dramatically.

No amount of preventive maintenance or type of oil used would minimize the problem. However, Vehicles that were driven mostly on the highway, would have less of a chance of developing this problem. This is because when cruising at highway speeds there is less stress in the rods. On the other hand, city driving tends to put more stress on these week rods. Especially when accelerating from a stop.

Apart from that, there is no way to predict which of these series of cars will develop the "Bend Rod" Syndrome, and certainly not all did. In fact most did noy.

Phil

 

Thanks for input. Sounds like my '91 350 SDL at 127K miles has a limited lifespan even though it is not burning any significant amount of oil, has never been overheated, and continues to run great. Are there any tests I can have done to birddog any potential problems with this engine? I was hoping to keep this magnificant road car for at least 300K but it doesn't sound possible from the entries in this forum and another forum that I frequent.

 

If your engine is still in good shape, and you do plan to keep the car for a long time, than you may consider replacing the rods BEFORE they bend. The new replacement rods from Mercedes, have been redisgned, and don't have that problem.

Not sure however of the economics of doing that.

Phil