What does the EGR valve do? I have seen this before and the gist of it seems to be that it doesn't do anything that is important.

Exhaust Gas Recirculator.

It has to do with reducing emissions.

Tom W

If slightly too much liquid enters the cylinder.........the engine will fail to rotate. You can't compress liquid. Your scenario works only for the one specific situation where the perfect amount of fluid is sitting in the cylinder..........not too much (the engine won't rotate)...........not too little (the rod won't bend).

What would the effect be of reducing the combustion chamber until the compression ratio were lets say 60 to one instead of 20? or 80 or 100? all of these possibilities exist up to the point of a full combustion chamber.

Tom W

I understand what you are saying. I just don't agree that that automatically disproves the theory.

Tom W

The engine would simply push the liquid out the exhaust valve on the first (or third) stroke after the driver turned the key. The rod would not bend in such a scenario and the driver might get a bit of a rough start.........but, just for a brief moment.

It can only bend at the point where the combustion chamber is just slightly overfilled.

Are you thinking that the rod will bend under a compression ratio of 80:1? I haven't pondered that one..........but, I think the rod is strong enough to survive that.

If you agree that the available cylinder volume must be just slightly overfilled with liquid to bend the rod, the theory is largely disproved because of the statistically remote possibility for this to occur with no other symptoms.

However, if you believe the rod will bend at 60:1 or 80:1 compression, that's a completely different scenario. I'd need to study it a bit more.

One other point of note to completely dispel this theory:

M/B changed the design of the connecting rods for the later engines. Clearly, if they identified a head gasket leak as the culprit, they would have addressed this problem. The fact that they redesigned the rods tends to point to a fatigue issue in the rods rather than a failure caused by external influences.

Furthermore, a slightly redesigned rod won't dramatically reduce the failure rate caused by your theory. Sure, it's got a bit more capability, but if it can't withstand 80:1 compression, the new rod is not likely to withstand 90:1 compression.

I don't think we've got sufficient data on the new rods to make any conclusions as to their success or not........however.

Several folks have reported bent rods here with the replacement blocks with the "upgraded" rods.

Tom W

That would definitely help your case. Who are these folks.........I'd be curious if any symptoms of head gasket failure would be evident?

Yes, that is my question. At what point does a rod bend? Does it take a 101% full combustion chamber or would 99% do it?

Tom W

If the head gaskets fail due to excessive flex in the block, then a redesigned gasket would not likely solve the problem.

If benz did not know for sure what was causing the problem the upgraded rods may have been an easy to do solution with no assurance of success. We do know that all 3.5 versions suffer the bending rod syndrome, I believe, so benz eventually solved the problem by going back to the trusty 3.0 block with four valve heads.

This supports in my mind the theory that the 3.5 block flexed too much.

If there were nothing wrong with the 3.5 block why would they go back to the 3.0?

Tom W

You'd need to conclude that M/B was completely in error in the conclusion that the rods needed to be stiffened and that they had no clue as to the true nature of the problem. While this is certainly a possibility, I'm not willing to believe it without some data.

There is no question that the 3.5 block is not stiff enough when the proximity of the cylinders is observed. In fact, the 3.0 block tends to violate the typical spacing distances that are considered "normal".

Whether this lack of stiffness translates into a head gasket failure (no data) and that it further cascades into a connecting rod failure (no data) is a real stretch, IMHO.

Who spoke of a rod bearing failure? I did not, but if a rod is bent I would want to replace the rod bearing too.

And as far as data, is there any data to support any of our theories about the cause of rod bending?

Tom W

My error........I edited it to read "connecting rod".

Nope..........all gas.........no data.

But your theory is especially weak because of the lack of any anecdotal evidence to support the head gasket. If the head gasket fails, there are symptoms to support that failure..........you have none.