Thread: diesel350?
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Old 02-11-2003, 06:13 PM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596

Unfortunately for me, I experienced one of these bent rod events and one of the worst features of the experience was the inability to identy what caused the problem. MB still insists there was no class design problem.

I am not a believer in the ingestion of carbon chunks theory as I have seen the intake runners on a few MB Diesels and do not think there is something especially bad about the intake conditions on this car than existed on the W123/W124 and non-350 equipped W126 Diesels. I am convinced if there was such a problem, unique to the 350 turbo Diesel engine MB would have issued directions to simply have the manifold cleaned or the problem fixed, rather than make good customers live through the experience of bending rods and being ignored by the manufacturer. I am also convinced if there were a global susceptibility to bending rods caused by ingesting intake carbon chunks, then the other MB Diesel engines would have exhibited a similar susceptibility.

I am more a believer that the increase in displacement from the six cylinder 3.00 liter turbocharged Diesel used in the 1987 300SD, for example, to the 3.50 liter size was not carried out correctly and the connecting rods were left susceptible to a combination of in service stresses and manufacturing tolerances that lead to a limited fatigue life on select engine builds. Once the cyclic loads approach the finite fatigue life on these connecting rods, they fail.

I am not sure that the highest load on the connecting rod span between the crank bearing and the wrist pin is necessarily experienced when the piston reverses direction at the top of the exhaust stroke. I agree this is the highest bearing cap fastener load, and it may be the defining design condition of many other items in the crankshaft/connecting rod/piston/wrist pin assembly. However, the failure mode of a bent rod shows up as a shorter piston travel in the particular cylinder. This can be caused by a number of potential combinations of stress patterns, however, I consider it most probable that the rod is bent in a high cycle fatigue type failure in the compression and power stroke cycles.

The issue is apparently complicated enough that MB had to take a few passes at the fix, as there are repeated part number changes for a number of parts, including the connecting rods for these engines over the near decade they were in production. Given the number of attempts to address the problem that failed, none of which were directed at intake manifold configuration changes or suggestions to have intake manifolds cleaned, I am convinced there was a more difficult to diagnose combination of manufacturing tolerances (dimensions, heat treatment, chemistry, etc.) for the key parts, as well as potentially peak stresses due to driving styles and typical performance variations from one example to the next.

When I took delivery of my car it had 74,000 miles or so on it, and it could spin the rear wheels at take off without much coaxing. When it succumbed, it was still a pretty good performer for a Diesel, but it was notably less eager to respond to throttle inputs. It isuddenly dled roughly and abruptly began to consume oil, demanding about a quart of oil every 800 miles in highway driving or significantly less in city traffic with a lot of idling. I assumed I had a valve seal and guide issue, so I arranged to have the head rebuilt. When the head came off, I was amazed to find the valves and rest of the head in near new condition and nearly fell over when the mechanic showed me the two cylinders with pistons that did not quite make it to the same height in the cylinder as the adjacent pistions with unbent rods. I had the engine rebuilt by the dealer, and only the valve seals were replaced on the head. I would expect carbon chunks of the size to damage the rods to have inflicted some damage on the valves or valve seats.

Given the bad news of a pair of bent rods, I was concerned I had done something to the car to cause the failure. I asked the service manager and the technicians and was told there was nothing I could have done to cause the problem on my engine. All this lead to me finding this site and becoming a member nearly two years ago now. I found out more about the problem here and used it to negotiate a fair trade in value for the car towards the 1998 E300 TurboDiesel I have now. So, while the MB corporate organization failed here, the local dealer I have known for twenty years owned up to the problem, and made a very real attempt to correct it.

The point of all this is to note the problem is a hit and miss situation, and the later the model car or the engine build in the production cycle of these engines, the greater the chance you have one that has a combination of new parts that actually work. While I don't think the intake plumbing becoming coated with gunk causes this problem, I agree it is good to get that stuff out of there, so go ahead and follow the recommendation to clean this area out good every so often. Good luck and I hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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