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Old 02-01-2011, 03:39 PM
C Sean Watts's Avatar
C Sean Watts C Sean Watts is offline
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Join Date: Mar 2006
Location: NC
Posts: 1,318
What we know vs what we don't

Lot's of questions so I'll try and find all I can 'from the book.'
If we look at it in the engineering matrix, it gets a little clearer than mud, hopefully.

1. State the problem. 3.5L engines in USA bend rods.

2. Redefine the problem. Do USA 3.5L bend rods more than other regions?

3. Identify the constraints/specs. They tend to bend before 75,000 miles or not at all.

4. Identify solutions. Redesign head gasket, Set max RPM lower.

5. Select most viable alternative. -see four-

6. Readdress problem. Are rods still bending?

1. The main reason the US versions bend more rods is the 3.5L was made primarily for the North American market. The majority of them sold in USA, in S class cars. Comparatively, not very many went to other regions before the rod bending was addressed.

2. The 3.5L 603.970 (350SDL W126) debuted in the USA market 03/90 and that specific production run was until 06/91. NOW let's look at the 'outside USA' production...3.5L 603.971 (300 SD W140) ran 07/91 - 05/93....keep these numbers in mind.

3. What happens BEFORE 75,000 miles?....Are the first 16 months on the road within the first 75,000 miles? Sure, unless assembly lines are making used cars...keep this number in mind, and include 'warranty coverage.'

4. The company actually DID issue a TSB about rod bending and went through the motions of a recall without actually issuing a formal recall. IN THE 75K (or less) range, they DID identify; the head gasket tended to erode in BOTH 3.0L and 3.5L near the #1 cyl. The gasket was redesigned with more material in the front. In the 3.5L, sufficient oil had been found in the cylinder(s) to cause a rod to be bent on compression, in some test engines - some, not all. A few didn't have any oil in the cylinders, they just bent rods. For THIS problem, the solution was to reduce the max regulated RPM, this is why the 3.5L had lower peak horse power than the 3.0L but higher torque and better 0-60 times for similar weight cars. Documents are closely guarded still but there were even bent/broken/thrown rods in 3.5Ls shown to be from oil starvation...from windage tray screws vibrating loose and being pulled into the oil pump - there's a post about all this somewhere on another forum but I don't remember where, someone even found the TSB number. I'm sure someone will find it before I do. Back to RPM, injection timing changes with RPM when the timing advance gear works correctly. Stands to reason that at high RPM.. with full advance.. on a longer stroke.. the rod is under exponentially MORE stress [or is it strain? ] than a shorter stroke... other things equal. BEFORE 75,000 miles MB USA replaced many engines that failed and corrected many that had not failed. They even replaced some failed engines out of warranty for original owners. What they did to correct the problem does acknowledge the problem but it still wasn't 'officially' a recall. Did it really need to be?

5. Repairs, before or after failure, included both solutions. These were being done before the release of the OM603.971 model, outside the N. American market. Makes sense that the factory WOULD implement the new changes on the assembly line. Whoa - I think we got it!

6. "Rods still bending?" No. "Would we keep looking into why?" How much do you want your Christmas bonus?

--Now if somebody can find that TSB number....
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