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dkS350TD 02-10-2003 11:21 PM

I have '95 S350TD.Someone has informed me that I will have a problem with the 3.5L engine?
I'm clueless what they are talking about?Can anybody explain it? :confused:

turbodiesel 02-11-2003 12:00 AM

How many miles? These engines are known for failure of the connecting rods.. some symptoms are excessive smoke, loss of power and engine grunting. Failures usually happen under 100,000 miles, and with your car being a 95.. you may be ok. They updated the design of the internals of the engine in 95-96 and they did not have as many failures.

dkS350TD 02-11-2003 12:13 AM

Hi,thank you for the info.She has only 68K miles.I've not observe any more smoke than usual.
If any think goes wrong, what will this cost me to repair it?The warranty has just ended(Feb.01'03).
She is very strong and has not shows any problems yet.

turbodiesel 02-11-2003 12:59 AM

If you don't have any of those problems or oil consumption, you're fine.. if the engine does go, its around $12,000 for a new factory crate engine installed.

Just keep an eye on the oil and monitor how the car runs.

rwthomas1 02-11-2003 12:30 PM

I may be way off here but I believe the bent rods have been attributed to chunks of carbon built up in the intake from EGR valve operation breaking off and getting sucked into the engine. Connecting rods don't just bend unless something gets in the way. If a connecting rod is going to fail it usually fails in tension (stretch) not compression. That being said there are a few things you can do. 1. Clean out your intake manifold. Remove it and have it hot tanked. Check intake ports for carbon buildup too. 2. Switch to synthetic oil. Some here have remarked that there is much less carbon buildup with synthetic use. 3. After you are sure there is no carbon present remove/render inoperative the EGR system. This will require a little work. A member here that goes by "OldSouth" did this, I believe, on a 94model with great success. You might want to ask him about the specifics on this modification. RT

JimSmith 02-11-2003 06:13 PM


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

rwthomas1 02-11-2003 06:52 PM

Excellent response Jim. I had the theory about the rods only thinking that the rods are likely "on the edge" of their design limit anyway and the chunks, even small ones would cause them to bend. Either way its a moot point as the bending seems to occor anyway. I have seen the replacement "upgraded" rods and they are MUCH beefier! Its russian roulette owning one of these and I guess the million dollar question would be is it worth it to address the issue with a preemptive rebuild or wait until it goes boom? RT

dkS350TD 02-12-2003 12:53 AM

'95 S 350 TD
Greetings to you all,
Thank you for your time to respond my question.
Looks to me,that its a lot to learn about diesels.As time goes on I will (only with you help).I'm thakfull for it.
Jim thank you again.

JimSmith 02-12-2003 01:03 AM

Given the old proverb "Don't fix what ain't broke" I would just be aware the problem announces its presence by a change in oil consumption and a rough idle, and try to catch it as soon as it happens, if it happens, then fix it. Afterall, like some others that have these cars and are running them trouble free, you may have an example that is not susceptible and openning a perfectly good engine to change parts that are fine as is would be a costly waste, and you never know the rebuild will not suffer from some of its own ills. Jim

rdanz 02-12-2003 01:13 PM

Wouldn't a compression test tell if the rods were bent?

JimSmith 02-12-2003 01:37 PM


Yes a compression test will tell the rods have bent, and it gets real obvious as the cylinder wall gets ovalized using this measurement technique. I do not believe it will tell you they are going to bend though. I was responding to the question of what to do if you have one of these cars and have no symptoms, yet. Periodic compression testing might be a path, but a compression test at a shop or the dealership will cost a few bucks, and the end result if the test says it is time to rebuild/replace the engine is no different than if you just wait until it makes itself known by more obvious means. The change in oil consumption and the knocking/rough idle are not subtle. You have to decide if an investment up front to assess the present condition, which is likely ok if the visual/audible and olifactory signals are not present, is of value. This is a personal preference decision, not necessarily one that is technical or likely to bring you a significantly lower rebuild cost.

I hope this helps, and good luck, Jim

TimFreeh 02-12-2003 03:48 PM

Great response Jim , thanks for taking the time to compose it.


rickg 02-12-2003 04:03 PM

You guys have addressed this very well. :) I had started looking for a 350SDL when I first started shopping for another diesel Benz a year and a half ago. I came across the "rod-bending" syndrome in a couple place I was doing research to see if it's what I wanted. In addition to what has been mentioned, I read one article that said it was usually the front cylinders that went. Ever heard this one? And one theory had it that is the problem was contibuted to by the fact that they took the 3.0 liter engine and just bored it out to 3.5 liters. This resulted in thinner cylinder walls, and an uneven cooling of particular cylinders for some reason. Wish I could remember where I read this. I don't remember the details now. But it sounded good anyway.
It was enough for me to stay away from the car.

turnne1 02-13-2003 02:35 PM

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Head design was also completely changed in the newer designs In my car rods 1 and 6 were bent...MB WILL help you out if your car has under 80-90K miles and it has been dealer serviced...but they typically won't if you don't have a dealer backing you up
Most cars were rebuilt with a new short block(one year warranty) car however got a complete new long block(valve cover to oil pan) with a 48mo 50K warranty
I was prepared to pay the difference in the two procedures(as I was very opposed to having a new bottom end and an old top end)......but as it turns out I didn't have to pay the extra charge for that procedure

Jim did you get the money back in trade from the dealer that you spent on all that engine work?

1992 300SD 160K
Columbus Ohio

Beech 02-15-2003 12:58 PM

3.5 TD
My god. are we still theorizing what may be wrong with the 3.5 turbo diesel engine? Its junk, most all had problems, Mercedes let everyone down by not admitting it had a problem with the engine. Go to and query S350 or 300SD.

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