As long as you never, ever, never over heat it. Of course if the PO has already over heated it, that sucks too.

Yeah I don't overheat my cars, most engines don't like that.:D

I did see an S350D that was overheated so bad the temp sender melted!:eek: I'm sure the head was cracked but that was the least of their problems since the pistons melted to the walls.:D Top hose came off, the guys wife was driving it...kept going at 80 until it stopped!:eek:

The 300SDL is a dog compared to a 350SDL when it comes to low end acceleration. You really have to stretch the engine out on a 3.0 liter to get the power you need. Whereas a good 350 could chirp a tire.

It takes some adjusting...The 350 is a Hell of an engine. And by that, one can easily anticipate the irony in that statement.:D

Yep nothing the old checkbook can't fix. I don't know if a bit more low end power is worth $5k-$13k to me...hmm I'd have to drive one maybe it is.:D

IMHO the older 603 is worth a possible $1k-$3k head or headgasket job; so is the larger one that much better?

What did you finally do with yours?

I don't recall any conclusion posted.......??

Traded it for a Pathfinder?!:D;)

Don't you 2 ever sleep? It's 1:00 on the east coast.:D

..........soon.........

350's not that bad
 

As a proud owner of a '91 350SD I strongly disagree with the above allegation! There at least three of us on here who own 1990/1991 350SD's or 350SDL's that currently exceed 500,000 miles. Quite an accomplishment when you think how few were built in comparison to all of the 240D's, 300D's, and 300SD's put together. But in my seven years on this forum, there is a non-stop extolling of the virtues of the 240D, 300D, 300SD, my favorite of course "the bulletproof engines". If you put all of the hundreds or perhaps thousands of 240D, 300D, 300SD, owners on this forum together I don't think you have three in the whole bunch, that exceed 500,000 miles. With that huge expanse, of time from the first 240D to the last 300D, if these vehicles were so spectacular, so reliable, so bulletproof, wouldn't you think between all of these owners with such widely available vehicles there should be huge groups, in each category (240D, 300D, 300SD) with mileage meeting or exceeding 500,000 miles? Where are you? Where are you hiding? It's the limited production, mighty '90/'91 350SD/SDL's that are the true reliability, high mileage champions. Rod benders, I've never disputed it, nor do I do so now. However it would appear that among the small group of '90/'91 350SD's and 350SDL's owners on this forum, we own the most reliable long mileage vehicles here. The 350SD's and 350SDL's apparently have had their connecting rod failures, but what exactly is the major engine failure in all of the thousands of 240D, 300D, and 300SD's, that's sending them to the crusher long before they reach 500,000 miles? Enough of the constant bad mouthing against the 350SD/SDL's. I really can't take it any more. Lets concentrate on finding the causes of all the catastophic engine failures on the 240D, 300D and 300SD's that's sending most of them to an early
(pre 500,000 mile) grave!

'87 560SL 150,000
'94 SL500 40,000
'91 350SD 515,000 that's right 515,000 miles not kilometers

The only thing that kills a 617/616 is improper maintenance or just plain wear. The 617. for example, does not have a re-occuring issue with breaking internal engine parts. Infact, every 617 failure I have heard of is due to lack of maintenance (think oil cooler lines, radiatior hoses ect ect)

I am still up....and its 3:36am :D

My question about the rod benders is how is it that some are out there with 200k+ or 250k+ on them with original engines and no signs of failure. Seems odd that these "fluke" engines could have made it so far. Are those just the lucky ones? There's a 350SD for sale near me here that just had a "new engine put in 15k ago" according to the seller. Its a mint, beautiful car, and they only want $6500 for it. Seems fishy. Not that I could (or would) buy it....just wondering why not all of them seem to suffer the fate. ???

I think maybe you should go eat worms.:P

Tom W

So.....you have no evidence to support any other theory.

Tom W

No evidence has been found to support any theory. The only people that know the real reason(s) are MB's R&D engineers.

If they put in stronger rods and that did not solve the problem and the next engine they built had 3.0 instead of 3.5 liters that suggests that the displacement was the problem.

That is evidence.

They are not saying what the problem is for liability reasons, obviously

Tom W

Mercedes has upgraded rods for these engines, I priced them for $200.00 @ rod.

Again, the discussion was your theory.

In your theory, there should be evidence to support it. The fact that there is no evidence of head gasket failure prior to the rod bending event tends to discount your theory.

My theory is the theory of the rock. I'm going to put forth that theory in the future if you continue with your theory. I have no evidence of the rock internal to the cylinder and you have no evidence of a head gasket failure.

We both have poor theories.

By Tom's theory, we must conclude that the engineers do not know the true cause of the problem. If the bending of the rod is caused by a head gasket leak, it hardly makes any sense to strengthen the rod slightly.

So, to accept Tom's theory, one must also conclude that M/B is clueless to the problem.

My theory is possible and there is good reasoning to support it.

You have not proposed anything better but have decided to attack my theory for some unknown reason.

The rock theory is impossible and you know it.

Tom W

Sorry guys, I couldn't help myself :huepfenic

http://www.peachparts.com/shopforum/diesel-discussion/217481-rock-theory-vs-gasket-theory.html

No, sleep when your dead!:D

I have posted "my" theory on this several times.

I bought one of these, a taupe 1991 350SD with a cream leather interior in pristine condition with about 75,000 miles on the odometer. I loved the car. Fit my family, was safe and got reasonable mileage. And it was a Diesel that I was certain would last forever. The rod bending phenomena was already known to MB and to the small group of owners posting on various sites on the relatively new internet car forums. I was blissfully ignorant.

At about 155,000 miles it began to have a loping idle and would blow a puff of smoke out, that would sometimes turn into a smoke ring. I took it to the dealership and assumed I was going to have to rebuild the head. I was not happy about this but figured the aluminum head demon had bitten me. The service writer at the time tells me it is more likely a bent rod. I ask why and he sort of shrugs and tells me they have seen it before. More often than failed heads or failed head gaskets.

The dealership service head cheese calls me and tells me they took the head off and I have a bent rod on two cylinders. I don't recall which two. I tell them I will be over to have them show me how they made that determination. I went over and they showed me they measure the height of the piston at TDC and compare it to the others. 4 are good, two are low. Bent rods. I ask about the head. No problem. They even suggest that if I have it repaired the head is likely good to go as is. I throw a **** fit, and ask how I could have bent a rod. I am told there is nothing I could do to bend a rod. The car won't let me. I ask how it happened and they say they don't know, but it does happen. I am an MB Club member and at the time they gave 25% on parts and 10% on labor. I get an estimate, something under $7k. I authorize the repair.

I examined the head and the head gasket while I was there. Nothing I saw, and the mechanic doing the rebuilding confirmed there was no sign of any head component or head gasket failure. I tell them I want the guides and seals changed and the valve seats reground. This adds a few hundred bucks to the repair. So it is around $7,500 with new all kinds of stuff.

I get the car back. It has a 12,000 mile, one year warranty. At about 13,000 miles it is smoking again. I now go back in and ask what, specifically they did. They try to tell me it is likely the turbo seals. I tell them until they can explain how the 7,500 dollars I just spent fixed the rod bending problem, I am not going to do anything. The service manager tries to calm me down, saying all kinds of meaningless crap about how the problem was rare and they are sure they fixed it even though he can't tell me what changed. I tell him the guy who wrote up the original repair order told me it was bent rods before the head came off and that sure makes it sound like more than a rare problem.

I go home and start searching the internet. I find MBShop.com and a link to a bunch of links about this problem. It is clear I am being screwed by an old man at the dealership who runs the service department. I go back with about 50 printed pages of stuff I gleaned off the internet about this problem, including groups forming to press MB with a class action suit, and go to the sales department. I start a loud discussion about the subject, and likelihood I would be better off buying a VW Diesel, with the sales guy I now deal with, who reminds me he did not sell me the car. He gets up and says he is going to speak to the management.

He comes back and tells me the management has agreed to give me full Blue Book value for my car (about $7k back then, before considering it was smoking and had a rod bender engine in it), the full $7,500 they charged for the repair as a trade on any new or used car in inventory. I find a silver 1998 E300D TurboDiesel. We work a "deal" out and I go to a Benz dealer in New Haven to see what kind of deal they will give me. Basically a plain white with black MB Tex car, no significant options, slightly higher asking price and they offer me $3500 for my 350SD. I took the Carriage House of New London deal.

Based on much reading I did at the time the issue, as I see it, is the rods don't have an infinite fatigue life, as built. This is due to a combination of issues, many interdependent, some not. But the production methods seem to be producing borderline rods. Not all are good, not all are bad. Due to dimensional tolerances of finished parts, their starting material properties and their as heat treated properties, all of which have tolerances. You get bad ones and the rods suffer fatigue failures at something over 65,000 miles. Mine failed in all likelihood around twice that mileage. After that the rate of failure is lower meaning if you survive the first 200,000 you likely have next to zero chance of a fatigue failure because the rods met the design specifications with a bit of margin.

Redesigning the rods to be heavier may help, until you stress them at full speed. Higher mass means higher stresses. This is not, in an already settled and now dimensionally controlling environment, a path to success.

I have heard all kinds of theories about hydrolocks, ingesting chunks of the scum that collects in the intake manifold downstream of the EGR valve, flexible blocks and so on. I see no hard evidence those are the problems. And, the displacement selection of an engine in Germany is made to comply with tax codes. You will notice all car companies have offerings at the break points. When designing a new engine the manufacturer always designs around the "bread and butter" models in the line up. 3.0 liters is a break point for turbocharged engines. Insurance is paid, by the way, on hp rating regardless of body style. They figure 200 hp has a fixed potential for damage liability, regardless of whether or not it is in a coupe or sedan or station wagon.

As for those bulletproof W123's, well, I have only ruined one mechanically (my son ran it without oil after it had well over 300,000 mles) yet I have retired several due to body rot. Better rustproofing in later cars makes them last longer. And, a 240D is so loud to drive it 500,000 miles averaging 70 mph or so means you will likely suffer a hearing impairment. A W126 or W140 is another story altogether. But I don't think you will find W140s and later cars in use in Beirut or other rough cities twenty five years after they have gone out of production, in significant numbers. The bulletproof label is one that has been earned, literally. I have never been stranded by a W123. I do not think I could say the same for the W140 - I doubt it would run without any electricity, for example. A W123 will. And an ordinary Joe like me can keep a W123 running indefinitely.

Jim

Very good post, Jim. Where did you find the information about the rods suffering from fatigue?

Tom W

To be absolutely clear, Tom, I made it up. After looking at my car and reading about another 30 to 50 cases posted on the internet at the time, I concluded that is the only way some can fail and some can be good. As I noted on the other thread, relatively few can be bad and make a large number of MB customers very unhappy. I believe the design is sound analytically, but not compatible with the production methods used to make rods. Not enough margin and that was likely due to the competition for real estate inside the engine when everything got tighter as the displacement was enlarged. Jim

Almost every other scenario makes the likelihood of any population of engines being immune as miles are built up less likely. Jim

The only fly in the ointment is that folks with the uprated rods have reported bending as well.

Tom W

I tried to point out that making the rod heavier is not a 1 for 1 type of gain. At speed the loads are higher and it is not clear the heavier rod is the answer - a different configuration with the same net result - higher loads and not enough margin to the fatigue limit. Some still fail. If you don't drive over 4,000 rpm, maybe the odds are far enough in your favor that they won't fail.

Bigger is not always the right answer. Jim

Fatigue is to do with crack initiation and growth - were the old rods cracked?

If the rods have bent, but have not cracked, it's not fatigue.

It's possible for struts loaded in compression to fail via a buckling mechanism - in engine connecting rods, this compressive buckling is aided and abetted by the sideways inertial force as the big end accelerates side to side, and the moment caused by bearing drag. These two extra factors can make the rod bend elastically, and then as the compressive load is applied as the cylinder fires, the critical buckling load is reduced.

This is all fairly well known info, and I would be very surprised if MB didn't take it all into account. However, did they try to stretch an existing design beyond a sensible limit - was the margin against buckling for these rods as high as on other engines?

I've never even seen one of these engines, so, a few questions spring to mind;

In which direction are the rods bent? If the block were made of glass, would you see the bend looking from the front, or from the side?

Does this engine tend to knock more than most MB engines?

Are these the same rods as used in 3 litre engines mated to a longer stroke crank and larger pistons? Was the rod section increased? - the stiffness will go up with thickness cubed, so even a small increase in the "beam" section will make a big difference.

The idea that fatigued rods would have cracks seems right. That i show forged steel fails by fatigue, right?

The momentum of six rods and flyweighted crank moving in an engine a 3500 RPMs will be hard to stop. If you can get enough fluid into the cyclinder/combustion chamber in one stroke then........

I inspected Jack's car again yesterday. There is so much oil in the exhaust system that a few drops formed at the tail pipe.

I believe once they bend the loads go down significantly. Lower compression, poorer firing conditions, more blow by and they move out of the fatigue life threatening loading environment. Thus only mass and stroke, thus the speed remain the same.

In most cases the oil consumption rate quickly rises to alarming levels and most owners stop driving them. I have yet to hear of one failing by cracking. The failure is a twising and bending, I believe, although to be honest all I have seen is the different heights of the pistons at TDC as the engine is rotated by hand with the head off.

I also do not think the space is available for thickening where it would be most effective. The added stroke in the same basic block as the prior 3.0 liter engine makes it a challenge to get the section thickness you want where you want it.

"Fatigue" may be an inaccurate engineering term, however, it seems that the phenomena takes place only on engines with a substantial number of miles, and then, if you get past about 180,000 or more, the engine is immune to the problem. This leads one to believe the problems is not a first order design failure.

The issue does seem to be related to loads and cycles. And, some of those who have seen and posted shots of the old vs. the new rods have said they are visibly stouter in the new configuration. There are rumor there have been several iterations of design changes, and it has been reported that at least one new rod design didn't fix the problem. Jim

PS: This engine was smooth as butter and had next to no knocking when it was right. When it went bad it had an obvious idle speed variation, and would rock the car, visibly. Oil dripped out the back of mine as well. Might have had something to do with a quart every 100 to 200 miles going out the exhaust as mostly vapor and then sort of condensing near the outlet.

In the background of this shot there is a beige 350sdl. The new owner got a smokin' deal (pardon the pun) and is looking for a new engine for a perfect, (I mean really perfect) body. I suppose the 3 litre from another SDL would be the best replacement. This is at my indy friend's shop. I got to start it up, yikes! She still moves, barely. You can tell from the discoloration of the back tail light how bad its gotten.

http://i251.photobucket.com/albums/g...0/DSC01453.jpg

Bad 350...This is what they run like when they're ailing...

http://www.zippyvideos.com/6161441694710296/350klocker2/

Good 350...This is what they're like when they're only burning 300 miles per qt, as opposed to 100.:D

http://www.zippyvideos.com/2773151204709756/350toprun/

Any diesel I drive receives no mercy from me.
:cool:

Yeah, nobody knows. But at least one can confirm, the 3.0 liter was a solid design.

Maybe if the timing was altered, a 300 wouldn't lag so much compared to the 350? I actually happen to have a 350 pump lying around, from the first car in the first video (block makes a nice paperweight). Planning to have it modified one day.

Eventually, the problem seems to parallel a head gasket breach...Except it gives you 10's of thousands of miles of forewarning before it needs attention. Save up some $$$$ in the meantime.

If the bending were from liquid in the cylinder the lowering of the compression ratio would lessen the likelihood of it happening again, and after wearing for a while the compression is less too.

I find it hard to believe with all of benzes experience in engineering diesels that the straight engineering problem of making the rod strong enough would be what they miss on.

The block flexing too much seems a more indeterminate problem and easier to miss on.

I would love to see the two rods side by side to see what the difference is.

Tom W

>>the straight engineering problem of making the rod strong enough would be what they miss on.

Didn't Cummins make engines with dodgy pushrods for years? (The story I was told was that they were told, in no uncertain terms, to sort it out while being considered as an engine supplier for a European truck, the Ford Transcontinental)

The more serious point I would make is that when designing parts, there are margins allowed - one of the contributing factors in this margin is uncertainty - uncertainty over the exact load the part will bear, and uncertainty over the strength of each individual part. It's entirely possible that MB had under-estimated their ignorance of either or both of these uncertainties. These errors are usually picked up during a development program, but, if these failures occur sporadically, and at higher mileage, it's entirley possible that they would be missed, even by a rigorous testing program. If the 350 was a development of an existing design, it's also likely that the development program was not exhaustive, as many of the parts could claim prior development heritage.

Buckling is actually an unusual case to design for, as the yield stress of the material is much less important than the elastic modulus. In other words, as the Young's moduli of most steels are very similar, there's no point in spending lots of money on expensive steels with high yield stress values if buckling is the only load case under consideration. Of course, a connecting rod bears loads other than the compressive or buckling case, and for these, yield stress is important.

What is your thoughts on the flexing block? Are you an engineer? Automotive? It sounds like you have some expertise here.

If you are a rod designer, I would like to ask some questions:
1. What are the design loads for compression that is created by the compression stroke of the engine?
2. What are the design loads of the firing stroke?
3. What are the design load of the piston thrust?

Now the next questions is, if you are a rod designer, how do you increase the strength of a given rod?

My understanding of structures is from building design and we typically use a pretty large safety factor on columns, often overdesigning them by a factor of 2 or 3 simply because it costs very little and weight of the column is inconsequential.

A connecting rod it seems could be strengthened by increasing the web or the flange or both. This can be done without increasing the overall size by increasing the section toward the inside of the rod structure. I recently held 616 and 617 rods in my hand and it is not readily apparent the difference. I took them to my favorite machinist and asked him to look at them. He spotted the fact that the wrist pin is larger on the turbo pistons and the area around where the bearings fit is slightly thicker. At casual inspection they look the same. The outside shape looks the same, they are the same distance from center to center too, I think.

The extra mass in the 617 turbo rod does not seem to affect the operating rpm of the motor, IIRC.

The operating rpm of the 3.5 603 on the other hand is kept lower by design, presumably because of the heavier (presumably) weight of the rods and pistons and perhaps the crank.

I would love to see the differences between the 603 3.0 rods and the 3.5 rods and then the "upgraded" 3.5 rods as well. Does anybody have information on the differences?

Tom W

If I understand it correctly they bend straight down and twist. The twist creates a binding situation against the cylinder walls since the top of the rod is no longer parallell to the crank. This wallows out the cylinder wall and brings poor sealing on the rings and smoking, etc.

Tom W

I have never heard this idea before.

Tom W

I have been pondering this more and have a couple things:

1. You don't seem to know what they did exactly when they "fixed it" the first time. They would have had to replace the two rods as a minimum.

It would follow that they would at a minimum also replace the liners, and install new pistons fitted to the block. If this is the scenario then one must conclude that they did not think that the rods were the cause of the problem. One would also have to conclude that at the time upgraded rods were not yet available.

If upgraded rods were available they would have done all of them, right?

If the upgraded rods were heavier they could not have just replaced the two bent ones because of the balance problem.

If they only replaced two of the rods then the explanation of the failure in 13K miles is because the other rods failed from being too weak. (?)

2. If they replaced all the rods with upgraded ones, then why did the engine redevelop the problem in 13K miles?

The first scenario seems possible if the rods were inherently weak but they did not know it at the time.

The second scenario would support (perhaps) the hydro lock or partial hydro lock theory. If this were the problem and they believed nothing was wrong with the head (it could have been slightly distorted above the place between two cylinders and leaked into both cylinders. There is that expansion slot between the cylinders which is open to the cooling jacket, IIRC) and they simply put the head back on it would not last long before leaking into the cylinders again. (This seems unlikely as automachine shop 101 would dictate that you always skim a head if it has been removed to assure good sealing, con't you?)

Tom W

The acceleration and top speed look weak to me, my 300SDL is faster than that :D (no EGR, no CAT, no ALDA)

The stress/strain on a rod during operation of a diesel engine are pretty well known. By the '90s, good modal finite-element packages had become available in P-Method analysis, very reliable for determining the design, material, hardness/temper etc. of the rods. I find it hard to believe that the design of the rod was inadequate for use in this engine. It is possible that something such as a resonant frequency that created repeated osclilations in the rod during operation lead to their fatigue and was not predicted, but again IMO unlikely.

I'm not trying to punch holes in anyone's theory here, but being in on the process of designing and engineering parts for automobiles (including Mercedes-Benz) for over 20years there are some things that we go through each time iinvolving analyzing the design, including likely process variations, and creating a design and process that can operate in the worst-case tolerance stack-up. This includes everything from the clips holding vacuum lines to the brake calipers. Critical parts (determined by good sense and FMEAs) are closely controlled and often real-time X-rayed in the production.

It seems that, in theory, if the rods yeild due to fatigue, and being that it is nowhere near 100% failure rate, it cannot be a design failure or it would affect 100% (all being of the same design). The attrition being more random and a smaller percent indicates to me that it was a process variation, possibly in the material, or in the heat-treatment process, or even in hand-finishing of the part, or outside influence (hydrolock and rock theories for example).

If it were a hydrolock, IMO again, it would have other indicators. If it were a hydrolock due to a head gasket etc., are we to believe that it occurred during the cranking/starting phase? If it were, is the starter capable of creating enough force to yield the rod(s) on top of the infinite compression in that cylinder that would precede impact with the liquid or would the high-compression slow and stall the starter? Would the rod be the failure mode of a hydrolocked cylinder? I've had this problem in a gas engine, cracked pistons, didn't hurt the rods, high-torque starter in a high-performance engine (jet boat).Different engine design, but the question remains and we haven't done any analysis, anyone want to donate a bad 3.5 so we can hydrolock a cylinder and crank it to see what fails?

If the hydrolock were to occur during running, it seems it would require a lot of liquid to enter the cylinder in a very short period of time. Again, possible? If such a substantial breach were in the gasket wouldn't the car be belching white (steam) smoke from the exhaust?

Are there any other possibilities? If we had a couple of "bent" rods, a couple of good ones from the same engine, could do X-ray and metallurgical analysis and compare it to the original Engineering specifications, could run a couple of 3.5s with bad head gaskets etc. we might be able to solve it. I've spent many hours trying to fail parts in a quality lab until the lab techs were tired of me, I know how hard it can be to reproduce a failure in a lab envronment, even harder on the internet.

You really need to read up on fatigue and understand the very wide statistical population that results from fatigue failure testing.

It's quite typical for two identical parts to have failure rates that differ from each other by a factor of 3.

A rotating part in a jet engine is designed to survive 60K cycles. The part is removed from service at 20K cycles and destroyed. This provides some understanding of the nature of fatigue and the impossibility of stating a fatigue life with any certainty.

Fatigue life is quite similar to bearing life. One bearing will survive 5K hours and another bearing of the exact same type will survive 25K hours. The statistical population is huge.

When parts in a jet engine fail from fatigue do they bend? Or do they fracture?

Tom W

Jet engine components are usually flat discs or hubs with a very stiff outer structure, and, therefore, bending of such components is highly unlikely. They cannot be compared to the relatively weak construction of a connecting rod with an attempt to draw some type of analogy.

Jim, unless your dealer replaced ALL SIX of the original rods, it's pretty likely the failure 13kmi later was due to one of the other four original rods failing. You did not specify if they did replace all six or not. If they did, and you have a receipt for the six new rods... I'm stumped.

:confused:

Didn't someone else post something about this in which all the rods WERE replaced and it still failed again? I vaguely remember reading something to that effect.

Not that I know of. I can't recally any 3.5L engine which failed after receiving six of the new, updated rods AND also had the cylinder sleeves repaired or replaced. Putting a new rod into an oval bore isn't going to fix the oil consumption. You need round bores, good pistons & rings, AND new/updated rods. Times six.

:stuart:

I understand fatigue, the failure modes, the causes, the statistical analysis of such, the point is that a design shortcoming affects all parts in all engines. A bad design affects 100% of the parts. Again looking at statistics and six rods per engine, you would need to have a fairly high percentage of failed engines to indicate a bad design of the rods.

For a process or other variation to be the problem, any number from 0% to 100% is possible.

If an engine suffers the failure of 2 rods (using Jim Smith's example), they are replaced, and the engine again has failures, this would indicate to me that I am looking in the wrong place for my failure cause or variable, and I should tear into that engine to find out what is different. It could still be the rods, but why two in one engine, or six, was this from a bad heat-treat batch? Bad forgings? Bad material? Maybe it is another variable in the engine.

Engineering as you know is a fairly repeatable process, and every part on the CAD screen is exactly the same when you're finished, within fifteen or more decimal points depending on where you have it set. You can produce a "perfect" part and run it through analysis that can exceed design limits by whatever you wish to find the weak point (don't know if you've ever done FEA but I'm guessing you're familiar with it) and strengthen it or determine that it is well within design limits. The variables come into play in the production process, this is very hard to predict and control (especially these days when everything is out-sourced in the automotive world). By the mid-'80s CAD was the standard for Engineering of such parts.

I feel that it can be a failure of the rod due to inadequate rod strength, but my opinion based on my experience and training tells me that if so, this is due to part production & process controls rather than design. I'm confident that (from M-B track record of diesel engine design) the design was completed properly and to normal M-B standards and practices to perform for the normal M-B diesel engine life and the normal testing was completed also.

Back to the Jim Smith example: Is it also possible that there is an issue with these blocks? Possibly with the crank? How about the piston? Could there be something else that allows the piston to travel in a non-linear path and put abnormal side-loads on the rod? How about the cylinder walls?

I'm not saying that the rods aren't the problem, but there's precious little data to go on so my normal analysis path is to eliminate or table the ones that can be considered less likely and focus on the remaining possibilities first. In the absence of any real data or parts to analyze, to look at the whole assembly, the engine in this case, and see what is unique to the failed engines, or to the failed engines' families (the 3.5 or the 60x in this case). It doesn't seem to manifest itself in the 3.0L, only the 3.5L 603, so what parts are unique to this engine: block? pistons? crank? rods? balancer? ... no theory can be proven without data that we don't have.

Next question is the hydrolock (and rock) theory: If it is a hydrolock, and the early 60x engines had a known leak problem with heads and head gaskets, why don't we see a high rate of bent rods in these engines when we find bad gaskets and bad heads? This might already have been addressed but I haven't seen it.

This is a dead-horse as far as I'm concerned, and I'm disappointed in myself for spending so much time thinking about it, mainly because no conclusions can be drawn without DATA. We have none, can theorize, but we must then accept others' theories as it is all that we have. Discussion, theory, debate, but no data, this is the process that we go through so that we know what data to gather, we have no data.

Jeff,

All the data I've seen points to the original rods being a flawed design (i.e., not strong enough to deal with the additional load from the modified bore/stroke). The updated rods are significantly stronger. And in almost all cases, when the engine is rebuilt (note: rebuilt!) with six new/updated/stronger rods, the failure doesn't re-occur. If the block was the problem (or the crank), the new rods would fail again. 99% of the time, they do not. Also, Mercedes engineers did not re-design any other parts to cure this... just the rods.

For the record, I don't subscribe to any of the theories about carbon, EGR, hydrolock, or anything else... these would affect all other OM60x engines as well, yet they have no problem (ok, at least it's like <1%, if not zero... statistically nil, anyway.) Only the 3.5L has the extended bore/stroke compared to the 2.2L, 2.5L, and 3.0L versions.

:deadhorse:

Perhaps I'm an idiot then, ... but why, if all of the original rods were of a flawed design, does a fairly low number of them fail? All are stressed to the same point, some have gone several-hundred-thousand miles, others fail before 100k. This is where I feel that the rod design is not as likely as the rod. The difference being the production of the rod and variations therein.

Just as some rods are statistically on the low end of the scale, some will be on the high end. Those engines with high mileage could have gotten six rods from the 90 percentile in strength. Thus the design could have been flawed, yet still give some good examples.