Rod Bending ~ weak rods /poor fuel/ bad injector timing
 

For some time have been looking over information on the 350 'rod bender' & other information on high speed diesels that have suffered rod bending.

It is generally accepted that the primary cause of the problem with the 350 was weak rods.

One issue that is often raised by MB & others is why did it only happen to a proportion of the motors & only in the USA? There are still 350's running with the early weak rods & not in need of replacement. Both in the USA & other parts of the world.

In other parts of the world the 350 motor with the early style rods were used without problems & during MB testing in Germany & other parts of Europe prior to the release of the motor MB did not see the problem.

The answer appears to be related to the diesel sold in the USA & the statistical variation of injector timing and rod strength that occurs in the manufacturing process.

The fuel;
The main property determining measure used is the Centane Number.
USA produced diesel is known to have a lower value than the fuel produced in the EU/ Middle East/Africa/ S. America/ Asia, basically the rest of the world.
What does Centane Number measure?
The following is from BP & explains it in simple terms;

What is cetane and why is it important?
Cetane number is the key measure of diesel fuel combustion quality. The number relates to the ignition delay - the period that occurs between the start of fuel injection and the start of combustion. Good quality combustion occurs with rapid ignition followed by smooth and complete fuel burn: the higher the cetane number, the shorter the ignition delay and the better the quality of combustion.
Conversely, low cetane number fuels are slow to ignite and then burn too rapidly, leading to high rates of pressure rise. These poor combustion characteristics can give rise to excessive engine noise and vibration, increased exhaust emissions and reduced vehicle performance, with increased engine stress. Excessive smoke and noise are familiar problems associated with diesel vehicles, particularly under cold starting conditions.

Basically what we had with the 350 was a motor that was fine with high Centane Number fuel & potentially a recipe for failure on low Centane Number fuel. That is the main reason that the 350 motor did not fail in other parts of the world.
You will note that the lower the Centane Number, the slower the fuel is to start to burn, some ill informed individuals argue that one should advance the injection timing to compensate for this. The problem is that is only half the picture, along with delayed ignition, the fuel burns many times faster. This is more significant. The flame velocities with low Centane Number fuel puts the combustion of those fuels into a category of explosions. Even though the fuel may be a milli second or so later to start burning, its combustion is complete many milli seconds before a higher Centane Number fuel. So we have a situation that the fuel may be burned even prior to TDC. This combined with the PVT effect of the compression, results in higher combustion temperatures & extream loads on rods/pistons /rod & wrist bearings. we could replicate this problem with other diesels by simply advancing the injection timing a bit.

If you wish to kill a 61x, 602 or 3ltr 60x thats one way of doing it. Probably the rods will survive but something else will fail. Could be the head/head gasket/rings/ rod bearings/pistons. Those of you in the USA have the fuel that will do it. Interestingly the latest Porsche & VW top end diesel 4WD's call for higher Centane Number fuel than the norm in Europe. It is likely that a detuned version would be needed in the USA or we may see a 350 type repeat. BP are marketing a high Centane Number fuel in the EU for them.

What we all should learn from this is that if you advance the injector timing on any diesel motor including those made by MB, & use low Centane Number fuel like that sold in the USA, you greatly increase the risk of premature motor failure.
If you set your injector timing for maximum combustion temperature, you are probably having complete combustion before TDC some of the time.

The second part of the 350 story is more intellectually interesting.

Why didnt all 350's in the USA fail?

There are two main reasons for this.

1/ the rods are manufactured with normal manufacturing tolerances. Half are going to be lighter (weaker) than the median, the other half, stronger.
The rods are graded by weight so that any motor tends to have rods of very similar weight. This results in some motors being more susceptible to bending than others. You may note that MB fixed their problem with the 350 motor in the USA by using stronger rods. If some one was lucky enough to get a 350 with the original rods that were on the heavy(stronger) end of the production range, they may have not experienced the problem.

2/ Injector timing;
The motors leaving the factory typically had a tolerance of +/- 1.5 deg on the injector timing. This meant that some motors ex the factory were much more susceptible to the ill effects of low Centane Number fuel. Those motors leaving the factory with retarded injector timing may be less likely to bend rods.

Putting these 2 together; If you had a motor with the heavy end of the rod variation & on the retarded end of the injector timing, you were luck & probably would get many 100k miles with no problem. If your motor had the light end of the rod variation & 1.5 deg advanced ignition then you may have been one of the first back to MB with a problem.

To try and vary from manufacturers recommendations with regard to injection timing for what ever reason is a very foolish practice, particularly in places like the USA that have low Centane Number fuel.

If any one has any credible information that contradicts this please provide a link.
I know that we have a few members with university training in reaction kinetics, thermodynamics & combustion theory. I note that when this sort of thing is discussed, they stay right out of it. Probably they realize that many of those commenting do not have sufficient technical back ground to be able to make proper informed comment.

Makes you wonder why Mercedes did not know the above information and why they did not build the Car to function reliably in the market they planned to sell it in.

Or perhaps the issue could have been avoided by Mercedes by simply timing the US Cars differently to allow the lower Catane Fuel to burn properly (I have not read anywhere that that has been suggested).

Is it also possible that the Rods simply made by more than one Factory or Supplier?

I am not sure how many different foundry's made the rods. I suspect that back then it would have been all in house.
I have not read of any intent to overcome the problem with timing adjustments. probably would have killed the cars performance.
My initial interest in the rod problem was not for a 350. It only came up when looking into problems with another non MB motor.

As layback says, the "rod bender" is only known in the US, it simply didn't happen anywhere else.

The other factor is that apparently if your "rod bender" didn't bend a rod in the first 75k, it wasn't going to bend a rod ever.

What follows is anecdotal, I haven't been to the states myself.

I *have* seen first hand a lot of diesel plant and equipment that has run in the states, and then run elsewhere, e.g. ship engines, generators, power packs, tractor units, and a fairly large number of these things came with operators.

US diesel fuel always had a very bad reputation, on a par with India for example, for consistency and quality, "***** runs sweet as a nut on euro diesel" was a phrase I have heard so often I lost count.

I have also heard stories that experienced marine engineers could sit on the quay stateside and tell which fuel company supplied the boat pulling out based solely on the exhaust note and smoke.

I have also heard stories from ex-pat americans that the mandatory strict annual euro vehicle testing regimes meant that there were no clunkers, while back home people would literally drive the vehicle into the ground.

What follows now is not anecdotal.

I used to run motorbikes as my only form of transport, being english that means Triumphs, BSA's, Nortons, etc, plus the odd HD / Guzzi / Ducati etc.

British bikes back then had a reputation for being utter ****, the british bike industry had just died, japs were everywhere, and everyone *knew* that british bikes were crap.

Which used to boil my piss.

Because yeah, sure, some british bikes were true piles of crap, but other weren't, just a succession of careless owners for 20 years who either neglected or bodged everything, is it any wonder that 20 years later that bike was unreliable?

Which brings us back to MB, back in the ponton era (we had a 220 SE) anyone who bought a new merc was serious money, they cost more than a family home, but they were built.

The W123 was built and financed by the german government of the day to promote exports and bring in foreign currency, competing with the French govt with the early 305 series Peugeots which you also saw all over Africa and so on, and it did that very well indeed, but is was never a "Mercedes Benz" in the old sense of the word.


The W124 was built in a new era, no longer did manufacturers assume that the product was as likely to end up in a wadi in Iran as it was a farm in rural Gloucestershire, and the science of CAD was also in its infancy, no longer did people make a world-wide product, then grab a supplemental spares bag labelled "Iran" or whatever and throw that in the shipping crate.

The lines of communication from the field back to the design offices had gone, completely, in the name of rationalisation and streamlining, there was serious talk of not producing the W124 in RHD at all, and ceding the british / australian / new zealand / south african markets to british car makers, and made under licence holdens, which meant ceding it to the japanese with their nissan sunny's coming off the ships by the tens of thousands.

In the USA the separate marketing department there didn't even acknowledge the W124, still selling off the W123 until well after production of the W124 had started.

Lots of W124 components, from engines to complete cars, were made in south africa, and shipped via europe for sale everywhere except europe (my car is 100% Stuttgart) or sold locally, I know of at least 3 cars that made it to the US and got sold there.

I have just learned that the US W124 has a completely different cab heater / ac control system to our models, I can only assume this is down to some licensing or manufacturing agreements involving patents or market segmentation, none of which applied in the ponton era... back then the product was eligible for import / export or it wasn't, period... and if it wasn't, well, sod you, your dictator could still get his fleet of 600 limos.

The only thing that is certain is that there is no valid engineering rationale for these differences... just ask henry ford.

So the problem that layback is trying to address is HUGELY complex, and I doubt we will actually find a single smoking gun, it will be more a case of finding which of the 20 wounds was the fatal one.

I'd like to address two points/authors:

W124: There were many differences between US bound cars and other markets' cars. Most of them due to marketing. We could not buy a Mercedes-Benz automobile in the mid-'80s and beyond without: Automatic transmission, automatic climate control, tinted glass, alloy wheels, power windows, power sunroof (for example). There was no cloth interior option, no stickshift even in the SL, no 300SL in the '80s (would have loved one), our options here were quite limited (I sold Mercedes-Benz new in the '80s before returning to college). The reasons were the same as Audi, BMW, Porsche, et al used; market placement and US buyers' expectations. The marketing of a luxury car that had a cloth interior, stickshift transmission, steel wheels and hubcaps, manual windows, and no sunroof would have been too much of a challenge for our salesmen, and too confusing to the general public here to grasp such a concept. In the early '80s, Mercedes-Benz outsold Volkswagen in the state of California, it was the hood-star that has been selling the majority of Mercedes (and Lexus etc.) cars since, and marketing is more important than Engineering. I was an Engineer in the mid-'80s through the end of the millenium, SAE/Automotive, Mercedes was my favorite customer, ... my next-younger brother was Product Planner, All New Vehicles for Lexus, I couldn't understand the decisions that he made but the market loved them. Off my soapbox.

Layback:
I agree with some of your assessment, but don't agree that it is all of the formula that was cooked into the demise of the OM603.97x. I had one. It didn't use oil, but became an organ donor for a project of mine (which is why I bought it). Didn't exhibit any "rod-bending" symptoms. However, when I tore the head off at 237,000miles, I noticed that all cylinders were scuffed significantly, along the sides radial to the wrist pins. The pistons were all level in the bores at TDC. I still have the block, just haven't found the energy to scrap it, but the point is that IMO, the design was inherently less robust than the 3.0L engine due to the longer stroke causing higher crank angles, the lower wrist pins causing still higher crank angles, and the side-forces from these two changes forcing the piston skirts against the walls (both sides BTW, not just compression or just power stroke sides). So although the lower cetain theory has merit, I believe that there are other factors, and as much as I respect your opinion I have to call you and mention that there is no supporting data that proves the cetane and timing relationship conclusively. I don't believe that you intended it to sound conclusive, but it kind of did (to me anyway). Unfortunately, there really isn't enough available data to make such a conclusion. Also, if timing and cetane were the only causes, it seems that there would be many of the 601/2/3/5/6 .5l/cylinder engines around with the same basic design that could be timed a couple of degrees early, and thus suffer the same failure mode.

Yes I agree with you mog.
Interestingly you observation of scuffing in the bore is one of the symptoms of explosive detonation of fuel. That is low CN & excessively advanced injection timing.
The exact same 350 motor was used in other parts of the world without the problem.
There was not a change to the rod length to "fix" the problem. So the angles were not the issue when it came to overcoming the problem. The same motor component dimensions have been used in other applications through out the world without the scuffing problem.
I put this thread out there as I had received some supporting information when I was recently looking at another motor with bent rods. Outside the USA there is very little known about bent rods & the 350. MB did a good job of keeping it quiet at the time.


I like the last comments in your sig!! Your comments are always appreciated by me including this one!!

Stay warm !!!!:D

I believe that the crank was also used in the 3.6L gas (E36) engine also, mine went to build one.

Irregular combustion could certainly cause some scuffing, and I would expect much more noise than a properly timed/running engine, you'd think that it would be noticable.

Jeff,
If the timing is way off you would. Just on the end of the allowable limit may not be that noticeable. I bet there would have been no MB repaired 350 that were not timed spot on or 1/2 Deg retarded.

cylinder scuffing occurs in all long stroke engines, side loading is a fact of life. I would assume that more damage is caused to these engines by their owners running high rpm than has ever been caused by timing, rod weight, or fuel quality. No manufactures put out engines that are so delicate that within tolerances they bend rods (atleast not the germans, i would suspect low quality work like that from the japanese). I would suspect it has something to do with our speed limits and commute-fueled society. just my .02

Hey Josha,
The 350 was one motor that MB got wrong. Do a search about "rod Bending". There is a ton of stuff about them. I guess in all fairness, MB is allowed 1 bad one. Look at the ricers, they are having recalls with ever increasing frequency & lots of cars, isnt the lates like 2 million Toyota's.

The 350 was a completely different motor with fuel that had a CN>45.

Comparing toyotas troubles to mercedes is hardly fair, toyota has yet to get much of anything right. If it was such a a small change in fuel quality they would be having problems in other parts of the world from abusive behaivors. And i would suspect that people have problems with rod bending that dont beat on their cars and i dont think any of us will dis-agree that during fuel throttle operation cylinder pressures are much higher. It just seems to me that cylinder pressures from changes in fuel quality or injection timing are not the main cause of the problem, but rather a function of sustained high rpm loading. I could be totally wrong but i really think the parameters you are adressing are really miniscule when you take into account what is experienced under normal combustion events. Im not trying to be arguementative i just dont agree.

At any rate it comes down to mercedes using a rod that was capable, barely. so... bad germans!

Josha,
Being down 5 points on CN is a substantial difference in the behavior of the fuel.
The increased detonation force from low CN fuel is substantial. It is basically extreme shock loading on every second rev of the motor. These shock loads are far more damaging than a full throttle load. These motors were not in sports models or interstate heavy hauling, just gentle limo's.

There was a ship tanker load of USA winter diesel arrived in Australia a few years ago. The difference in the way motors ran was very great. Many large trucking companies refused to use it. I sent 5000 gallons of the stuff back to my fuel supplier, my machinery ran like C##P on it. I think in the end they blended it slowly in with good diesel over a long time.


I dont find you at all argumentative Josha, I find your forth right nature very refreshing.
Please remember I never see a question as at all stupid. My answers generally reflect the question.
People like you that want to learn from others & challenge, refresh the forum. :)
You are one of the members in here that is unlikely to ever get a complaint from me.
I believe the forum needs more guys like you who tinker & are prepared to do what some of us may think is a little off beat.
I have done some horribly stupid things to cars & motors & I am not that pig headed as to not admitting it.
You strike me as a guy that has an old beater & are having a lot of fun trying things on it. If it went bang tomorrow, you would probably conclude not to try that again & think of something new.

Good luck to you!;)

Beating a dead horse, anyone?
 

The 350SDL is an engine to avoid if at all possible, period. This engine will self destruct even if you do everything right. There is exception, of course, just like everything in life. But why chance it.

I had a 91 SDL for a couple of years. When I got it it had some issues with the head IIRC. When tearing it down we found the intakes about 60% blocked with black gooey stuff which had hardened.

Before taking it apart I had noticed on occasion when driving down the highway big puffs of smoke occasionally which I took to be hunks of goo breaking off and going through the combustion chamber.

Mine had a leaky head gasket IRRC.

My favorite machinist and I pondered the problem with the rod benders a fair amount and we came up with a theory about it being caused by seeping head gaskets and partial hydro lock....or from large pieces of carbon goo getting in the very compact combustion chamber and bending the rod.....or gradual carbon build up inside the chamber until the rods were bent.

We could not imagine that MB would build rods so weak they could be bent from simply the combustion process.

As far as timing most that had the problem would have been running the timing set at the factory.

We felt the overbore to create the 350 engine weakened the block enough to let it twist and that along with the aluminum head was enough to compromise the head block seal enough to create the seeping.

There are a couple of long threads on the subject from when I first came on the forum looking for information about the 350 motor.

This the first I have heard they sold the 350 diesel motor anywhere beyond the US.