Babymog. I did not know you were an automotive engineer with some engine experience. I am not. The cause of the bending 3.5 rods may be proved now.

Simply if we can prove the spread of the pressure flame front is slower on north American fuel than on the European fuel. Plus there were no failures of this type on European fuel as is rumoured and we verify this.

Basically I suspect when the crank journal is at 90 degrees in relation to the cylinder. Is there a chance that the loading of that power stroke is greater at that instantainios moment on our fuel because of a lower but longer overall cylinder pressure burn profile? I would expect the better fuel might produce much less burn duration. Or one way or another when the piston is halfway down the bore the instantainious pressure is less.

If we can nail this down. Suggestions like get the idle speed up if say the car is in the driveway warming up in the morning. Or adding celane boosters as a regular diet. But at this point I am over my head.

I do not own one of these engines but if we can nail it down we can then help other members perhaps in advising how to reduce the risk.

In a way it would be similar to the advice given me to not let our nine liter v8 international truck engine sit and idle years ago. Or if forced to. Get the idle speed up.

We religiously followed that and when the truck was eventually sold the engine in my opinion was as good as the day we purchased it. Yet there were a lot of owners who did not like that engine.

We had no trouble over the years other than rust perforation of the oil pan once. Design flaw with the choice of material utilized for noise suppression under the oil pan. Combined with our rustbelt location..

The 3.5 liter even though a different casting was employed. To me is just a stroked and bored 3 liter engine essentially. The shorter skirted pistons bothered me a little but in themselves have never seemed a problem. The massive reduction of bore spacing in comparison to the 3 liter block did distract me as well.

After some consideration I felt that hydro locking may be not much more of a risk than the 3 liter as well. Also we were not getting any indication of the 3 liter suffering hydrolocking damage.

In other words what was bending these rods was more dynamic in nature. Once again at this point this is just an unproven suspicion and I am not really challenging all the others made over time. When you stroke an engine the angle of the connecting rod becomes greater at the point I suggest than the same unstroked engine. Plus provisions in the piston have to be made as well. You can build a new rod but probably not strong enough without increasing the weight. Impossible for an existing engine without a balancing rework.

I just guess I just fell into this approach as a result of my examination of the 616 engine. In my opinion a massively overbuilt engine that was having troubles that where not logically a result of the prevailing thoughts. All too soon after I could have even absolutely proven the issue.

Not wanting to ruffle any feathers as I wish no hard feelings or other plausible effects with members. I declined to do so and post it.

In reserve is I also want to conduct a serious examination on increasing the fuel milage of the older 616 and 617 engines. For the last few years on and off I question my sanity for even considering attempting this. Yet there to me are indications that it is not totally hopeless. The effort it would require at the same time seems massive. I also feel I owe the attempt to members for their good treatment and tolerance of me over the years. I still hope I might get on it.

I was also going to post how the lack of pretty simple stuff may be seriously reducing the lifespan of the 617 and 616 engines in detail. I thought through earlier examinations they should be going till at least 400k. Some are but not the majority perhaps. I find it impossible to factor in the odometer spinning and failure of the odometers. There is a need to simplify some things as well.

With no dis-respect. If anyone here can resolve the bend-rod problem, or at least determine the cause of it then MBZ would hire you are the Chief Technical Officer ( CTO ). The bend rod problem, as I understand, is more prevalent in North America, not so much in Europe. I am sure American, as well as European, would neglect their cars at some point. Some engineers suspect it is the diesel fuel here which causes the bend rod. Once the cylinder is oval'ed or rod bent then the oil consumption would be high and engine smokes. I am no mechanical or auto engineer but I think it is a combination of factors yet to be determined. I don't think MBZ is interested in resolving it any time soon.

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Certainly I cannot resolve it. Still there is a chance that simple evaluation of the difference of the fuels charactaristics in burn may provide a really strong clue. Anyways what I suspect is not rocket science and I think Mercedes has known this even before the replacement rods where produced.

But apprehensive perhaps about liability as there was not a really good sound fix within the limitations of the problem. How can a company state we built a decent engine without making sure it would be durable on your fuel. In many ways we would not be making a discovery of anything new. Just a verification approach so members can cope better.

This lower 3.5 block and reciprocating components are far more stressed in operation than the 3 liter engines is also a reasonable thought.

Things point at the same time to weak rods for our fuel perhaps. The cylinder damage is just a logical result of the rods bending. If the block was stronger the final effect may have even occurred earlier. That being the cylinder damage.

I only really posted this in the hopes that the automotive engineer that posted would have the tables on the two different fuels and verify the comparison I think he would find. I have no ideal of where to get my hands on them myself. You need this material for engine design work.

No disrespect felt incidentally . It is just one avenue that should be examined in a way that makes it far more probable. We perhaps cannot eliminate the issue but we may be able to come up with ways and suggestions to reduce the number of occurances.

I try to keep things simple as my mind is that way. The detrimental factor of radically increasing the rods arc with a stroked engine. The offset of the rod accelerates faster and is more stressed is pretty obvious.

Someone on the ball got the pistons right though it seems. Even when reduction of the skirt was required to stroke the engine. Otherwise we would have been hearing more from these engines.

If I am on the money. Several things that have a high potential of bringing on a first cylinder rod bend. Running with low fuel supply pressure in the base of the injection pump is going to automatically increase the loading on the first cylinder and it's rod. As these cars age this may become a real killer.Although I think it probably has occurred a lot already. This also is not productive to the integrity of that oil passage on the front of the block as more stress is added to the front of the head gasket. Only a tiny amount but that area is a little weaker to start with. Plus has about the same amount of sliding movement stress as the last cylinder to start with. When the head is heating or cooling.

So frequently change the fuel filter and periodically make certain the fuel pressure is at least as high as Mercedes requires. I thought I heard of at best several first rod bends with the oil passage assumed to be a contributor. How could you absolutely establish this if the cylinder was already processing a lot of base oil up the bore? So there is already reasonable prevention possible. Not a fix by any nature but a possible reduction of failures.

I never really wanted to dive into these engines as I do not own one. At the same time I felt I might contribute something worthwhile. I can almost forcast what the results will be if running low fuel pressure in the base of the injection pump on a 3.5. How long they will tolerate it is unknown.

Just let the fuel filter become restrictive to the point of reducing fuel pressure in the base of the injection pump. Or the lift pump seriously age. The engine will run the same with some reduction of fuel pressure but at the same time the loading on the first cylinder is increasing. Drive these engines until totally restricted or fuel filter change is physically needed as an operating practice is really threatening fate. I have absolutely no doubt about at least this.