The pump teeth where jammed with small metal piece. The customer drove another 2 miles before engine lost power and she pulled over....no oil pressure. #1 rod journal totalled and 3 pistons siezed.

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[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Ok... I was beginning to get the impression that an engine is said to be experiencing severe service in city driving ONLY due to the excessive fuel addition to the oil and related stuff. On the highway, there's maximum stress on the rods due to the high rpms. Now, from what you just posted, are you trying to say that the stress experienced by the rods from the load and mass of the vehicle, say, in those few seconds during which a city driven car goes from 0mph (like from a stop light) to 40mph is greater than that experienced at high rpms? I know this may seem like sidetracking, but I just want to learn.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

The rods are overstressed clearly as they wouldn't bend otherwise. The CAUSE of the stress is questionable. Most of the failed engines seemed to have suffered bent rods but continued to operate for many months or years until the very slightly bent rod caused the cylinder to wear/deform and oil consumption to increase first slowly, but at an ever increasing rate as wear progressed. NEVER a hint of coolant or oil loss (that might cause hydrolock). There is certainly the possibility that hunks of EGR oil/carbon residue that accumulates on the intake manifold of OM60x turbo engines (especially if largely city driven) and should that break loose and be ingested, it MIGHT cause the problem. But then why wouldn't the 602.96 and 603.96 engines be displaying the same failure (if perhaps somewhat less often)? They exhibit precisely the same buildup.

There is one major difference in the engines. The bore/stroke ratio (89/92.4) of the OM603.97 engine IS less than ONE (0.96). That's different from the ratio for all the other OM601/602/603 engines (87/84) which of course is more than ONE (1.04) and perhaps because of this, ingestion of hunks of residue MIGHT cause the rods to fail when rods in the other engines tolerated the "overload." If that's the case, then strengthening the rods (which is what Mercedes is said to have done in ALL of the rebuilds) could be all that's required to solve the problem (which it did - ther rebuilds are NOT failing at the same rate or way).

Marshall

Aptly put, Marshall, and thanks for the clarification on MB's position on rebuilds with/without warranty--first or subsequent owners.

Marshall Booth has also provided an estimate of fifty percent failure rate on the hundred cars he sampled. Do not know if this would be typical or not but interesting. Other posts speculate on many possible causes as well. Generally it seems the original engines succeptability to the problem will not be minumised unless the rods are changed out. Guess if I owned one would try to get a set of used rods out of an upgraded engine or the complete upgraded engine used out of a wreck. But only install it if and when required. If I had known the replacement engines were good I would have bid on one car this summer that had 75k with a new dealer engine. But at that time my knowledge was lacking. Just seemed too difficult to pay a fair price for the car and find the engine was on the way out a short time later. Anyways this thread has certainly increased my understanding, thanks all.

marshall, do you have any info on how the bent rods affect compression? my engine is using a lot of oil. i had compression tested and came up with four at 300 psi (at 700 feet above sea level) and one at 290 (#5) and #6 at 280. what are the chances of stuck oil rings causing the oil use?

also on the replacement engines are the heads left alone? the head gaskets?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

There is little doubt that the connecting rods experience more stress when the motor is accelerating as opposed to simply maintaining a constant speed. More fuel is exploding in the combustion chamber and is pushing harder on the piston. And then once a speed is reached, the amount of fuel going into the chamber levels off, momentum takes over and does much of the work (kinetic energy). This includes a reduction in the stress on the rods and crank as well. If the rods aren't strong enough, then they will fail more often in an engine that is used where there is more acceleration and deceleration cycles.

If there is less space in the chamber due to a build up of carbon from whatever source, then that could lead to problems, but might be measurable with a compression test. My intuition says this doesn't seem the likely culprit in the many engine failures, unless you posit that there is a greater carbon build up in engines which go through a larger number of accel-decel cycles. (city driving)

Did MB use the same rods in the 3.0 liter 603 engine as those installed in the 3.5 liter engine? Simple concept really that a larger bore and stroke means you'd better build a more stout rod and maybe the piston too.

If the failure rate is really 50% then we know that MB isn't holding information about this especially knowing that their fix is a redesign of the rods. That they didn't respond by backing up these incredibly expensive machines, and correcting the problem in subsequent years, is really the pits. I'd be plenty pissed off if I spent $70k on a car and the engine failed in 100,000 miles.

DS
87 300D
Northern Iowa

There is quite a bit of doubt.

Do you know the stresses of accelerating the rod to 80 feet per second and stopping it within .007 seconds?

If you do, then you might revise your opinion on the subject.

The forces at 4000 rpm are considerable. They may be significantly higher than simple combustion pressures.

You are engineer Brian, Do the math.

"As we all know" gasoline engines have far higher RPM service specifications AND much lighter duty rods than their Diesel counterparts.

If the reciprocal forces are truly significant please explain why gasoline engine rods don't seem to bend in high RPM service?

The math is complicated. It depends on the force on the rod and the angle of the rod when the force is applied. If the throw is 90 degrees to the axis of the cylinder, the force on the rod is minimal. When at TDC, the force on the rod is at its maximum. However, the maximum burning in the cylinder takes place at some point after TDC, so, I can't calculate it without knowing more about peak pressure in the cylinder and when it occurs.

However, "if we all know" that diesel engine rods are stronger than their gas counterparts, this would conclude that the combustion forces are likely to be greater than the inertia forces created by stopping the rod in a very short time interval. But, the fact that the weight of the rod is greater results in higher inertial forces, in and of itself.

In thinking about it a bit more, a diesel engine that puts out 220 lb.-ft. of torque should stress its rods the same amount as a gas engine with equivalent torque.

"We may know" that diesel engine rods are stronger than their gas counterparts, however, "we may be wrong" if we compare engines with equivalent torque output.

My 2 cents........ How well balanced are our MB diesel engines? A couple of hairs off balance at high rpm will increase stresses.

BC said:

"Do you know the stresses of accelerating the rod to 80 feet per second and stopping it within .007 seconds?

If you do, then you might revise your opinion on the subject.

The forces at 4000 rpm are considerable. They may be significantly higher than simple combustion pressures."

I was trying to describe that the downward pressure exerted on the piston and rod after combustion occurs is greater during acceleration since the earlier argument suggested that the rods bend from downward force.

The forces at high speeds are considerable, but you can still increase the fuel/air flow into the engine to a certain maximum point before the engine fails suddenly (say 8000+ rpm for the sake of argument). What happens when the engine fails? What breaks first? Hard to know, but it probably is the weakest point. Many variables, I know, but interesting stuff.

I will follow this thread, because I really want one of those beauties with the 3.5 liter engine and you guys can make me feel better about buying it.

DS
87 300D
Northern Iowa

The compression in MOST failing engines remains pretty well within acceptable limits until the engine is very far gone and oil consumption is VERY high (qt/few hundred miles). There is a chance that you oil consumption could be from some other cause, but I wouldn't make book on it.

I've been told that most of the replacement engines supplied by Mercedes were crate engines, but recently many people have used long or short blocks (much cheaper - but requires more labor). There is NOTHING wrong with the OM603.97 heads (they are ideal to replace the flawed heads used on '86-'87 OM603.96 engines).

Marshall

marshall, thanks for the info regarding heads and such. are the mb replacement engines the same casting on the block? any evidence that they are making the blocks stronger?

comments regarding bottom end. gas engine rods and pistons are lighter because they have a compression ratio that is about 40% of a diesel. also the diesel combustion is more violent. hence heavier rods and pistons, hence lower operating rpm. lower operating rpm allows smaller valves because engine cant pull as much air through at lower rpm, smaller intake ports, etc. so stress of diesel is much more for all drivetrain components, hence rubber donuts at both ends of driveshaft, more rubber isolation parts, hence thick rubber fuel line where it attaches to engine... all to resist vibration caused by the more violent diesel vibes.

mercedes engines are balanced to a very high standard at the factory. i have had several balanced and the balancer commented "not much needed".

since compression and combustion counteract tension at high rpm i believe that the highest stress occurring at high rpm will be in tension, hence NOT bending... hence i am thinking that the bent rods are not from stress failure but from things in the combustion chamber that are not designed for such as water, oil, carbon or carbonlike stuff. occasionally metal stuff, not often i would think.

i am coming to believe that the most likely culprit is the carbon build up that larry perkins advocates. i dont know why this is more a problem with the 350 than the 300 engine... less gunk? stronger (relatively speakng) rods?

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Most of the replacement crate engines were rebuilt rebored/sleeved blocks from failed engines! The few NEW blocks I've seen were not new casting (don't know about more recently).

Marshall