theories
 

brian, first, the engines differ in bore and stroke, and have the potential to affect head sealing and/or block rigidity. second, yes it is possible for coolant to be lost to the combustion chamber and not into the oil.

larry, i like the sound of your statements, but could you humor me and tell me a little more about the turbo failure and how exactly it bends the rods?

t
 

t if you want to be humored go read the phantom benz post,i wrote a story for it because there was a post about humor.
seroiusly,i give my opinion and dont expect all to agree,seems as most folks always want to be right in their thinking,i for one really do keep an open mind,seem to learn more that way.
for instance,your statement about coolant in the cylinder but not in the oil,well if you open your thought up a bit you would see that to be impossible if your cylinder has piston rings with an end gap.as small as it is that opening leads directly to the motor oil,hense the statement of all diesels do have blow-by is correct.(tell me if i am wrong)
ok now to my theory of rods bending,as i rebuild these engines i see build up on the head and the piston,some is at least .010-.015"thick,most is black and hard,i figure this is caused by a leaking turbo,its lube oil is introduced thru the intake which gets it to the cylinder in droplett form,not atomized enough to burn good.pull your egr valve and look inside the intake manifold,i have never seen one with a leaking turbo be clean,better still,pull the intake manifold completely off and take a look.you get time and intrest go to an open house at a coal fired power plant,look in the furnace,the coal aint throwed in there in large blocks,its ground to a powder and blown in to get the best combustion.
you see the mercedes and most independent shop plan is to get a crate engine and replace not disassemble and rebuild,so do they really get to see the insides of these engines? just my opinion,hope this helps,sorry about waking up on the wrong side of the bed this morning, have a good day.
larry perkins
72 old cars

fluid
 

yes i see larry's point about if there is water in the combustion chamber it will get into the pan past the rings. but while the engine is running most of it will go out the exhaust.

brian, of course the water will leak while running too. but while it is running it just goes out the exhaust pipe and (obviously) cant bend a rod that way, only if it has built up enough while the engine is stopped, under residual pressure, to fill 1.6 cu inches of fluid, or more.

larry, yes we found a tremendous build up of carbon and junk in the crossover and the intakes. i was under the impression that this is more or less normal in this series of engines and is related to the egr. some say it is from using non synthetic oil. what do you think?

Exactly my point. There would be a noted coolant loss. Many (all??) of these engines don't have loss of coolant prior to the "event" that results in the bent rod.

Wonder if all internal build up on head present when just changing a bad head or head gasket on these engines? If not generally that may be a clue that the turbo bearings and seals should be replaced as preventative measure from time to time on these engines. If buildup signifigant I suspect compression ratio slowly climbs up and that could bend a lesser rod easily. That same rod would of course be fine at designed load paremeters..Maybe this engine should be diesel purged every six months as a possibly preventative item? Have to wonder if there is anyone out there that has purged this model engine regularily since new and has accumulated high milage with no difficulty? What will the purge hurt? Perhaps a scoping of the cylinder to see if deposits are there and building may be a future requirement. This may also be an indication of too much heat transfer by aluminum head aiding residual buildup as well. Mercedes designed this head for high power levels demanding rapid heat transfer after all. Only fly in the ointment is the failure of some engines at extremely low milages but even that could be explaned by poor quality control or inneficiency of the seals used in turbo perhaps. A more conclusive suspicion could perhaps be made if the seal part numbers are different from the earlier turbos. But even that is subjective as well. Perhaps will send off a note to germany as mercedes may have some new thoughts about these problems. Pretty sure all my thoughts must have been mentioned before. Maybe mercedes has already released the diffinitive cause but really cannot absorb that as you fellows would know it.Thanks for your thoughts Larry as any increase in the possible knowledge pool is needed on these engines. Oh yes, pick the above apart as it is about only way to eliminate or support any possible direction with these engines as it seems to be stalemated reciently to some extent. No offence taken at all as i would like to own one of these but have to wait until some reasonable certain explanation or prevention of the difficulty comes to pass. :) I want to experiment with your engines first as well. :D Or is it a case of buy the car remove the engine and install a stronger set of rods and have no nagging concerns? :)

My 350SDL Engine
 

This has all be most interesting and informative since I started this thread and today, I drove my 350SDL to/fr Dallas (2 hrs one-way) and how sweet it was. I wanted to stretch her legs after being laid up having had the trans rebuilt. It performed flawlessly. I took it to Dallas to look at a '97 S320 with 30K miles and once I found out what they were willing to give me in trade (really not too surprised), I realized that this car, even with the possibility of blowing the engine one day, was worth much more to me in comfort, style and performance. I will continue to pursue reasonable preventive action including replacing the connecting rods and making sure that the engine is never overheated and will watch my oil consumption with an eagle eye, etc. In the event of engine failure, I look forward to doing a failure analysis since I am a Metallurgical Engineer and perhaps that might dispel some of the unknowns that we're struggling with here.

Would you be able to utilize a bent rod from another 3.5 L engine to perform some analysis?

It might be possible for Don Richard to get you one from his.

Failure Analysis
 

Actually, a broken rod (both parts) would work best but that would only be the beginning. But, a bent rod and the 2 pcs of a broken rod would be helpful. They would have to be expendable as destructive testing would have to be performed. I might even be able to get my hands on these parts locally but in the end, an entire failed engine would have to be analyzed. It would also be dependent on how much 'government' work I could get down in my company and/or a customer's lab as a complete failure analysis is very expensive. The key is to find a Motorhead at my Tech Center.

Wondering if a compressive failure is even detectable by grain structure with so little movement on failed rods. But then again the electron microscope can deal with very small grain movements. Still have a nagging suspicion that mercedes would know the exact cause but is holding it in house for whatever reasons. Another thought is why did they keep an engine that was problamatic in production year after year. Mechanical design at that time was no longer rocket science. It was very damaging to their corporate interests and they must have known it. If they were strapped for money at that period could perhaps partially justify it but they were awash in cash during that time. Stops a lot of people going out and buying a new type diesel engine in this years mercedes cars I would think as now their product reliability cannot be taken for granted. Not for a pretty long time in fact.

Stonewalling
 

Which brings up a very interesting point--the fact that many car companies stonewall known, major problems that have high visibility but also high liability repercussions--such as the VW Bug engine dislodgement on frontal impact and Audi spontaneous reverse ! See any similarities here? But I guess this is another Category of discussion.

mbenzer
 

to satify my curiosity,would you tell me why you dont consider completing the inspection and possible repair of the oil baffel plates that i have experienced destroyed engines from?? i dont know what a shop would charge to complete this but i know it should be about an 8 hour job and even at 100 per hour and essentially no parts cost,what maybe 900$ total.i am asking this to determine if you dont believe it happens or what.
it wont offend me at all,your answere.
larry perkins
72 old cars

What failure rate sets off the 'recall' campaign? If only 1% of the engines are 'rod benders' is a major problem? 10%? 50%?

Remember, at a 20% chance of failure, 8 out of 10 cars will NOT fail. :D

Best Regards,
Jim

I don't believe any failure rate sets off a recall, unless it is safety related. If 100% of the engines experienced a rod bearing failure, outside of the warranty, M/B would not issue a recall for all of them. You might get some consideration from the dealer if it failed just outside the warranty, if you are a very good customer, but, that is about it.

To answer Larry P
 

I have not forgotten about the oil pump bolt/washer issue that you presented. I figure if I go ahead and replace the connecting rods, bushings and bearings that will be attended to. I'm still exploring that approach. I ran the oil pump issue past my MB Guru and he just shrugged his shoulders and said he never heard about it but he was deep in thought about what I proposed to do about the connecting rods.

precautions
 

back to your original question of how to prevent or head off the bent rod thing. we have four theories going that i know of on the cause of the problem:
1. there is a theory that i will call brian carlton's since he talks about it a lot. this theory is that the rods fail because of metal fatigue and just bend when the metal gets tired. brian will correct me if i have misstated this theory.
2. another theory i will call larry perkin's theory because i heard it from him. this theory holds that carbon builds up on the top of the piston and perhaps on the head too until it gets thick enough to bend the rod. the source of the carbon seems as if it could be from two sources:
a. failed turbo seals letting oil into the intake stream.
b. from the egr. why these engines put so much into the egr i am not really clear on but i saw the evidence in my engine when the head was off. there was so much carbon in the intake i dont know how it got enough air to run.
3. my mechanic/ auto machinist believes that it also could be because of a leaking head gasket. this theory holds that while at rest the gasket bleeds oil and or water into the cylinder. when the amount is more than 1.6 cu inches (the size of the combustion chamber) upon startup the rod bends.
4. another theory is that some of the large blobs from the buildup in the intake break off and get in the combustion chamber and bend the rod. there is a small possibility of broken glow plugs getting in there but i havent heard of this on a mb engine.

now, the question is how to prevent the problem from occurring. i am thinking that an oil analysis would reveal small amounts of coolant in the oil if there is a small head gasket leak. someone can chime in if i am wrong about that.

if it is larry's theory would the compression be bumped up by the carbon buildup? on a gas engine you would get pinging when the compression got high enough.

brian is there any early warning signs if it is metal fatigue?

and of course if the fourth theory about carbon blobs is correct, it would be fairly easy to remove the intake and clean that out. i think that cleaning the intake ports on the head with it on the engine would be tricky at best.

these are thoughts about preventing a bent rod. the next question is what if the rod is bent? a compression test should show a low compression reading on the cylinder(s) with a bent rod. also if it is bent enough to lower the compression a miss will occurr when it drops below about 14 or 15 to one, i believe i heard. so if any of these things occur it will be prudent to pull the head, i believe.

it is my understanding that when rods bend they usually twist too. this will cause a misalignment and premature bore wear leading to excessive oil cousumption.

after the head is off i believe i heard it is possible to measure from the tops of the pistons to the top of the deck and see if any are low, indicating a bent rod. now if the cylinder is not too worn it may be possible to replace just the rod and piston. but probably at that point it would be cost effective to just do the rebuild.

if i am at that point i will be thinking about what to build. it seems that the 3.0 liter engines dont often bend rods so i probably will look for a three liter block to put in my 3.50.

anybody seeing something that i have missed?