Sounds like my '91 350 SDL at 127K miles has a limited lifespan even though it is not burning any significant amount of oil, has never been overheated, and continues to run great. Are there any tests I can have done to birddog any potential problems with this engine? I was hoping to keep this magnificant road car for at least 300K but it doesn't sound possible from the entries in this forum and another forum that I frequent.

My understanding of my readings about this engine is that not every one of them is a "rod bender" and some actually do last a long life without exhibiting the failure. If yours shows no signs of trouble at this point why would you worry? There's nothing you can do to prevent it except perhaps driving a little more conservatively to reduce the stresses on the connecting rods and hope for the best. If you start to see symptoms of impending failure (ie rapid oil consumption) then you'll know you have to deal with it, but given the number of these cars out there with 200K+ on them I think having made it to 127K you may have one of the good ones. Also, I heard that MB was replacing the short blocks which started failing in these cars...how do you know your car doesn't have a new, improved block in it? Are you the original owner?

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

A correllary to these engines would be terminal cancer.

The odds of getting it are reasonably low for the average individual, and, there is very little you can do to improve those odds.

In the case of the engine, I can't think of a single thing other than to keep the revs away from the top of the range. And, this presumes that we know the rods bend due to high cycle fatigue. It's a good theory, and I ascribe to it, but there are those who believe a foreign object gets into a cylinder and bends the rod.

It is not a certainty that any one engine will suffer bent rods.

I remember a thread on this forum giving a figure of 30% or so of the engines were thought to be so afflicted. Of course, if it is your engine, this jumps to 100%.

As others have said, don't obsess, just do the regular maintenance, drive it and enjoy it.

I think my OM603 with 258,000 miles has the equally problematic #14 head. It will probably die of rust, or someone will crash into it, before the engine dies...

Best Regards,
Jim

i dont have the experience that some of the folks do, started collecting these older diesels about 3 years ago,so anything i say has been learned thru my personal experience.
tell you one thing for sure, you would have to hold a gun to my head before i would move a 3.5 engine without knowing that the oil pump didnt have lose washers from the hold down bolts fastening the oil baffel plates to the block. i say this because i have two 1991 350sdl cars and they both had destroyed engines when i bought them that the 10 cent washer had locked the oil pump. cars had about 208k and 280k,i think they come lose from vibration so how could one predict this failure?
if you are going to keep the car,wouldnt it be wise to pull the oil pan and secure these bolts with red locktite?
larry perkins
72 old cars

Mbenzer,

I really wish I could honestly advice you to keep your car and hope for the best, but with these cars unless you can confirm that the rods or the block have been replaced with defect-free one(s), I'd say your safest bet is to sell the car. I've come across one of those engines that only threw its rod at 280K. Very high mileage by any means, but in these cars you really can't tell when it will happen. I have heard of some that went through the same thing at 70K, some at 120K, etc. I currently own a 1995 S350 with the same engine that bent its rods at 171K (check out this thread: Came across a rod bender that had bent its rod today. Basically, you can't tell when it will happen and unless you can verify that the problem has been solved in your engine, you can't tell if it will happen to your car. Enjoy the car for now and start planning to sell it as soon as you find an MB equipped with a diesel engine that meets MB's engineering reputation. For instance, if you insist on a W126 diesel, then that would be either a 300SDL (1986-1987) or a 300SD (1981-1985).

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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

Larry is your experience general knowledge? If not is there any way you could post it as a potential and easily preventable problem with this engine. This kind of knowledge is priceless. Know i will never forget it.
When you found the oil pump jammed was the chain broken? Sprocket keyway sheered? Just curious as if the pump is jammed there is no upside at that late point.

Braverichard,

How is your project '95 350 SD, coming along? After reading your commentaries on the experience, I decided to go after my dream S600 V12. Life is too short to go through the stress and frustration of the 350, and also the new series BMW 745's. The 600's, may be worse than those two combined and I'll probably wind up with a MINI or a Cavalier!

My project is in the preparation section right now, thanks for asking. I haven't really don't anything other than research what parts we are likely to need and get their prices and sources. I'll hire a kid from my neighborhood to video record the entire process. Many people from this website have expressed interest in seeing what we find when we start tearing the engine apart. I guess many people really want to find out how MB got to damage its reputation as a builder of bullet-proof diesel engines with the 3.5L I-6. It will be a big project. Some of the items involved:

-rebuilding the engine, with the so-called "updated" rods, probably new head as the current one was overheated for a while, etc
-replacement of A/C evaporator
-replacement of most engine components like glow plugs, water pump, the usual stuff

I guess I'll go bankrupt once the car is done.

Hey, those S600s are a great value right now. In the past three weeks, I've seen models made between 1994 and 1999 with less than 100K and in great condition with everything working sell for less than $16K!! Sounds like this is the time to get one. I'd definitely love to get one but the money for that has been allocated to the S350 project, fixing up certain things on my 300SD and doing other things. However, I might have to change plans or even finance the S600 or something.

Hatterasguy here advised that the key to owning the top of the line Benz with the V12 is to fix things once they break and not to put things off. I don't know how good of an advice that is, but it makes a lot of sense to me from a logical standpoint.

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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

Phooey, just because some engines fail and others don't so what do you do?

Nothing is guaranteed in life. I've gotten 2.5 years out of my 350sdl, truely a great car, and the apex of mb technology. Sorry, but I wouldn't trade it for anything.

I regularly do an oil analysis to check on the wear metals. To date, everything is within low norms, and I test every other oil change. I guess if wear metals spike, I will start buying parts, but its a great ride, and how many days do we have left to enjoy great cars like this???

Don't worry, be happy!

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71 220D 169K wrecked
83 240D 118K sweet 4 speed
91 350SDL (one of the 60% good engines) 156K
84 300d (loaner to my sister) 189K
79 300SD (partswagen)
86 420SEL partswagen
70 220d (partswagen)
68 280s GASSER!!! under construction now
85 300sd 310K miles winter beater car retired
93 300d 2.5 turbo 168K wife's car
83 280SL euro 5 speed 155K
69 250S newest project 54K

I am most grateful for all the constructive advice about my '91 350SDL.
I will begin tomorrow by contacting MB to see if my car's short block was
replaced since I bought it used at 72K privately. Depending upon that answer, I will then explore my altenatives and options. But, I'm tempted to look at a '96 320S with only 37K miles at a dealership--Smoke Silver. But conservatively speaking, there's an '86 300SDL with only 150K said to be the
'best one around' for $6K. That might be the better way to go.

It's a ticking time bomb! You may remember me from over in BenzForums "BusyBob" and my advise is to get Tripple A (AAA) and just keep on driving it until the first big money problem presents it's self. Then get rid of it for what ever you can get!

my mechanic and i have a theory that suggests that the rods are bending from failed head gaskets. it goes like this: the gasket fails, and leaks water or oil or both into the cylinder when the car is shut down, and if enough gets in the cylinder, specifically more than 1.6 cubic inches, upon startup the rod bends. the engine still runs but may have an occasional miss, and eventually will oval out the cylinder and cause excessive oil consumption.

i believe this theory but unfortunately dont know how to prevent it because i dont know how to detect an early failure of the head gasket. it seems a little too much to just replace head gaskets at random.

we have replaced my head gasket but i still dont know if any rods are bent because i was not aware of the possibility and didnt do a compression test before we tore it down and didnt measure the distance from the top of the piston to the top of the block while the head was off. i have since done a lot of reading on this web site and know a lot more than i did a month ago.

my 90 350 is in the shop at present getting a compression test. i have an occasional miss and am using too much oil.

one idea i have is to install a higher rear end gear to reduce engine speed on the highway. the 300d's which dont seem to bend their rods very often have a 2.65 gear and the 350's have a 2.87. this doesnt seem very significant but i know that the chevy 6.2 diesels from the eighties tended to break main bearing webs in trucks with 4.10 rear ends and didnt much if equipped with a 3.73. another idea is to convert to a five speed stick which would be a lot more expensive and i am not sure about parts compatablility.

in the mean time i would watch like a hawk for any sign of a head gasket failure and replace it at any sign. if that is the case i would also do a compression test before tearing down and measure the piston to deck height and see if any come up short.

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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.

Now, this is the mechanic's theory, and not yours.

Ask the mechanic why very few 603 engines with the 3.0 L engine suffer from bent rods, however, a good many 603 engines with the 3.5 L engine suffer from the same malady.

Do you believe that the head gasket is different on the larger engine; sufficiently so that it fails significantly more often?

Where is the evidence of coolant loss on these engines? AFAIK, there is no coolant loss that goes in conjunction with a bent rod. Do you believe that coolant only leaks into the cylinder on shutdown and not during operation? Do you believe that the head gasket fails in such a manner to never allow coolant into the oil?

There is very little evidence to support this theory. I don't ascribe to it whatsoever.

my experience if from disassembling and reassembling these old diesels,my nails use to be clear but now there is a permanent dark tint so i suppose one could call it practical experience.
when the pump locks the only common problem from both engines is a scoured crank shaft,the one with 280k took out the chain and the front timing cover,the second one just snapped the drive assy shaft on the oil pump,but again both damaged the crankshaft.
i have already posted my opinion as to preventive maint for this engine,baffel bolts reinstall correctly,rod bending is caused by turbo failure,new rods are stronger but one has to maintain the turbo or the engine gets too dirty to operate.
larry perkins
72 old cars