I had one with a broken head gasket, and cracked head because of overheating. (broken hose) rods were fine.
I also saw one that had the 10mm bolts that hold some plate under the crank come loose. the bolts swirled around in the pan until one managed to get through the screen. it went into the oil pump and busted a hole in it. engine lost oil pressure, rod bearings didn't get proper lubrication, engine failed. rods were still fine...

Posts here and other sites before the world wuz round...
 

Indicates that some of the rod bender engines were caused by the EGR valve. Little balls of tar like substance are formed with an active EGR valve and any leakage of oil from the turbo. Also mentioned is any pump back into the intake of unburned diesel fuel. Anyway these small tar balls apparently go/went into the cylinder to be lodged between head and piston resulting in bent rods. Sometimes allowing antifreeze/coolent into the cylinder also due to head lifting/cracking. IF so, how is Ma Benz gonna piss and ***** about a mandated "green" device mandated by the government?

This is all conjecture at this point, but a good precaution would be to inactivate the EGR valve via a plug in the line to that device.

Regards

Run-Em

A guess at the very most.

This theory has been around for awhile. But, to be valid, one would need to explain why the 603.961 engine suffers a small fraction of the connecting rod failures that the 603.971 engine suffers. If it was strictly an EGR issue, the failure rates should be comparable.

I agree with what was said above, I'd yank the head before assuming its a bent rod. Most likely its a head gasket or an issue with the head.

When you compare the pressure at full-throttle/boost on the piston to what it would take to crush a small ball of tar, doesn't make much sense.

I honestly feel like the W123's were the last cars produced by Mercedes Benz that were built to a standard and not a price. It seems like even though these cars retained their reputation, they became more stylish than engineered. :( I can't imagine the old Mercedes Benz company not doing their research and homework and letting an obvious flaw get mass produced like this!:(

????? How did he get it covered under a recall? Is there a recall I don't know about?? The last time these engines were available in the US in a car from the factory was model year 1995.

-Jim

What exactly is different about the engine? Is it just longer stroke? or is the bore larger as well?

And what about that flexible hose in the exhaust manifold of all OM603 engines. That looks like and expansion joint to me. The block and the manifold should have nearly identical thermal expansion. But the head's expansion rate is probably about 10 times larger than the iron parts. So maybe we get all of the issues with the heads squirming around on the blocks in addition to the bonus of longer, possibly more heavily loaded rods.

If MB knows what is going on with these engines it would help if they would tell us now. There could well be ways to adjust the excessive stresses out by de-tuning(via fuel timing), rpm limiting, or something.

Bore and stroke are both slightly larger, and longer, respectively

Nah the W140 is built better, they spent $1B and 8 years of R&D on that one. Lost money on every one they sold until the mid 90's. MB expanded the 603 and it didn't work well, I guess it was a stop gap until the 606 came out. However all the other engines the W140 was fitted with are supurb in all respects.

The 606 was out before the midpoint of the 3.5's life. The turbo 606 powered the W140's outside the USA.

I had a 3.5 603. It had a bad head gasket when I bought it.

We have had long and passionate discussions about the cause of the rod bending.

My favorite machinist and I have had long discussions over st pauli girls and we think the bigger bore and stroke of the 3.5 weakened the block enough so that it flexes too much and eventually causes head gasket failure. The head gasket fails and if you don't get it fixed quickly enough fluid can leak into the cylinder enough to cause a partial hydrolock and just bend the rod a little.

The combustion chamber holds only about 1.5 teaspoons before becoming full and anything from that on up will bend the rod a bit since liquid is not compressible.

A rod that is bent just a little will also twist and cause a slight binding in the cylinder and eventually will cause loss of compression, etc.

Nobody knows for sure what happens and if the benz engineers know (I suspect they do), they are not saying.

The replacement engines with (supposedly) stronger rods have also been known to suffer from the bent rods too, so making the rods stronger is not the solution.

I would only buy a car with the 3.5 motor if I could get it cheaply enough to put in a 3.0 liter block. The 3.5 heads are great.

When running correctly the 3.5 motor is a sweetheart...smooth powerful and very torquey.

Tom W

The problem with the theory is as follows:

1) A "partial" hydrolock can occur if the volume of fluid in the cylinder is about 3 ml greater than the available volume. Any fluid amount less than this and the engine will rotate without restriction. Any fluid amount more than this and the engine will stop rotation.

So, if the fluid is leaking into the cylinder at random rates while sitting.........there must be a situation where too much fluid leaks into the cylinder and the engine fails to rotate.

To my knowledge, none of the 603.970 owners ever reported such behavior on the engine.


2) The owners of the 603.970 never reported any signs of a failed head gasket during the period leading up to the issue. The engine runs perfectly...........no signs of rough idle or smoke until the oil consumption gets very high.

So, for the theory to be plausible, you'll have to find some answers to the aforementioned questions.

I disagree that the logic that there MUST be an example in which it fails to rotate.

The leakage only occurrs as long as there is differential pressure between the coolant system or oil system. As soon as some fluid drains off the pressure equalizes and the leakage stops.

This will not be a large amount of fluid.

Tom W