350 Engine

[PeterinCO]

Others here have said that it depends on what you own and I agree somewhat. I happen to have a 1991 350SD--I've had it about 5 or 6 years and it is great. At the time I bought it I looked virtually all over the country and bought the best one I could find (black/grey, about 80,000 miles). I remember calling Mercedes-Benz and researching the numbers of various models and I think there were only about 600 of these cars brought into the U.S.

I have never heard anything about the "rod bender" engine until reading this forum. Can anyone provide any REAL information on the problem, such as technical articles, mechanic's experience, etc.? What rod is it? Is it possible that maintenance and improvements in lubrication can deter it? And what exactly does the rod replacement entail? Thanks a lot.

[Rick Miley]

How about mechanic's experience? Benzmac is the owner of this site. Also take note of his credentials at the bottom of the post. Click

[Rick Miley]

Here's another one, this time from M.B.DOC. If his signature doesn't show on the post, click on his name. Click.

[Grdenko]

wow, really only 600? my parents have a black/gray '91 350SD too. theyve owned it since new and it has never bent a rod, but it lost its timing chain at about 50k miles. its been rebuilt twice (first rebuild was no good), new turbo, new a/c and it runs and looks great now. maybe itll be worth something in 10 years or so.

[Hatterasguy]

Rod bending has been discussed to death, do a search. Most seem to fail at lower mileage's. Actually two forum members in the past few months have had this happen to their cars. Mapleleaf (sp?) and dslbnz I think? Mercedes messed up they didn't make the connecting rods strong enough.

[PeterinCO]

Thanks Rick and others. I realize those without owner's concerns are probably sick of it, but much of the discussion isn't really information you can act on. I'm glad to find it out--I flat out had never heard it before. I understand benzmac's technical explanation only partially and am very much looking for corroboration and/or remedies. I don't quite see how heat in this area would cause a piston rod to bend. And, I wonder if better lubrication or cooling would help.

Specifically--and selfishly--I wonder if a particular concern of mine is related. I live about 15 miles west of Denver and a main highway in my direction goes from from about 5,500 feet elevation to 7,500 feet in maybe eight miles. On a warm day I can see my car's temperature guage rise as I climb it. I've changed the antifreeze, the radiator looks fine, I think the thermostat is fine, etc. The car has never overheated but I wonder sometimes what might happen if I'm out in the desert in the middle of nowhere. It would seem that overheating could be related to, or even a precursor to, rod bending.

I have always put a Slick 50 type additive in my cars and I think this one is due for a renewal. (Some, e.g., a '67 SL and '76 BMW sit for periods and I think it is particularly helpful for those.) I wonder if that might help.

I will search through those two member's names. Thanks. Yesterday I searched some other sites and could find nothing. I'm looking for any info I can find.

[Hatterasguy]

Don't put Slick 50 into your car!!! That stuff will clog the oil passages, do a search their is a whole bunch of links somone posted awhile back. Dupont even says teflan (sp?) has no place inside of an engine! Basicaly Slick 50 can no longer claim anyhting on their box I think they were sued for false advertising. Good oil is the only thing that should be in your engines crankcase. Mobil 1 15w-50 or Delvac 1 5w-40 if you want to use the best. With an OE german filter.

Now back to the rod bending problem, the connecting rod is the part the connects the piston to the crankshaft. When MB designed the 3.5 they bored out the previous 3.0 603. However now that the connecting rods are longer they failed to beef them up enough. So the rods would bend, the first symtom is very high oil useage, the car will usually run fine. This is because the rod is bent and ovaling out the cylinder. Eventually the rod will go threw the block. Now the only fix for this, if this problem bothers you is to pull apart the engine and replace the connecting rods with the new updated ones. However not all engines fail, their seems to be a 50-50 chance. Do a search their are some really good posts awhile back.

[Alan Hamm]

Originally Posted by PeterinCO

Specifically--and selfishly--I wonder if a particular concern of mine is related. I live about 15 miles west of Denver and a main highway in my direction goes from from about 5,500 feet elevation to 7,500 feet in maybe eight miles. On a warm day I can see my car's temperature guage rise as I climb it. I've changed the antifreeze, the radiator looks fine, I think the thermostat is fine, etc. The car has never overheated but I wonder sometimes what might happen if I'm out in the desert in the middle of nowhere. It would seem that overheating could be related to, or even a precursor to, rod bending.

Look at this thread. Temperature fluctuations under load appears to be normal on these cars. 1987 300TDT Overheats on long hills

[tjohn]

Originally Posted by Alan Hamm

Look at this thread. Temperature fluctuations under load appears to be normal on these cars. 1987 300TDT Overheats on long hills

Yeah, all diesels exhibit this characteristic to some degree or another - they generate much more heat under load. On the other issue, both Turnel1 and Jim Smith (the current Diesel Disscussion moderator) had 350 engines that failed the same way. There really is a problem, its just that the number of those engines in this country all small enough that Mercedes wasn't forced to deal with it in a public way.

[fj bertrand]

There were only 875 91 350sdl's imported in '91. So, I wonder how many 91's failed. maybe mine will fail eventually, but it's one heck of a great vehicle til then...

[PeterinCO]

Hattarasguy will have a fit but I'll come out and say it--I put Slick 50 in again today. The 350SD flew up the "hill". Go to my website if you like (www.theResearchSource.net), and check out the pictures of the BMW 2002 and the bio on me if you want. I have spent some time in marketing, product claims, litigation and the like. Three different brands at Wal-Mart? It is very hard to survive making claims others can openly attack. Change the oil every 3 months or 3,000 miles? I'm lucky to start some of my cars that often. Personally, and honestly, I've never had a problem.

[rwthomas1]

I have a tough time believing that connecting rods just bend from normal use. Seems far more likely the rod would fail from stretch than bending. Usually a connecting rod has to have the piston hit something for it to bend. Someone here theorized that the EGR system in the 350 produced chunks of carbon that were routinely ingested by the engine. A piston crushing carbon chunks seems like a more plausible explanation. A member on this board, OldSouth, has a 94 SD350 that he removed the EGR system from and converted the wastegate to a standard pressure type from the electronic version to get around the computer issues. Haven't seem him post here for a while but the mod seemed quite successful. RT

[gsxr]

Constantine, a member of the MBZ diesel email list, just installed a factory crate motor into his 350SDL after the bottom end took a dump. He's in the process of starting up the new motor for the first time as I type this. Add that to the list of people with blowed-up 350's! As to the exact cause, who knows. I don't buy the carbon chunk theory one bit. I do agree with disabling the EGR and ARV, if your engine is so equipped, but this is a LOT harder on 1990-up engines.

About the Slick-50. I don't mean to sound offensive, but you are a fool to continue using that junk. I used to use it myself, and the last time I put some in any of my cars wa back around 1997. (I used it from about 1990-97, and only still own one of those treated cars now.) Then I learned the truth about Slick-50, and other oil additives. It's absolutely a terrible idea to use it in ANY internal combustion engine. I'm annoyed that Slick-50 (and other, similar products) are allowed to be sold at all. PLEASE do some research about it before you use it again. PLEASE. After much personal research on the topic, I strongly recommend using a top-quality, Group IV/V synthetic oil such as Mobil-1, Delvac-1, Amsoil, or Red Line... and not using ANY oil additive at all, none, zero, zip, zilch, nada.

Some links:

http://skepdic.com/slick50.html

http://www.thegsresources.com/garage/gs_additive.htm

http://www.smartsynthetics.com/articles/facts_about_aftermarket_oil_addi.htm



Best wishes,

[PeterinCO]

I'm sure you're right, the stuff is worthless if not bad. The package says very little--they seem to have toned down statements about engine wear and gas mileage, at least this brand. But the business about oil dripping down and stress at cranking makes sense to me and perhaps this will help just a little with that.

Maybe my situation of having some nice cars that sit most of the time while I drive a Jeep Wrangler is unusual. Means superceding logic. I know it is terrible for cars to sit like that. But old cars with low mileage? It is almost inevitable.