I was at my indy yesterday getting my newly aquired 190D worked on and my mechanic had a 1991 350SD in the shop getting the head rebuilt because the chain broke at 86,000 miles! That's awful

I asked what the cost was going to be for a job such as that and he said it will set the owner back somewhere in the area of $3K. Ouch!

I just thought I would share that with everyone and maybe get another "chain" thread going.

Kevin

There was a TSB for certain engines in the early 90's that had faulty chains:

http://www.meimann.com/docs/mercedes/OM60x_Timing_Chain_TSB.pdf

I wonder if MB would have covered any of the repair under warranty. The sad part is that the bottom end will probably fail not long after the new top end rebuild, since those 3.5L engines used in the W140 have weak connecting rods.

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Dave
Boise, ID
Check out my website photos, documents, and movies!

How in the world did Mercedes ever put the 350 on the market,being the world leader in durable auto diesels it seems to me that they must have run test after test when they replaced the wonderful engines that proceded it............

William Rogers.........

All of the OM60x engine made between July '92 and April '94 (used in all models of diesel cars sold from '92 thru '95) were randomly equipped with a small number of defective timing chains. A technical service bulletin was issued (TSB 05/93 dated 11/95) listing the range of seriel numbers of cars that were imported to the US (models and serial numbers of cars sold in other markets were NOT included in ther TSB). Most of these cars were serviced (new chains) under warranty. None of the 126 "S" class chassis diesel were equipped with these defective chains.

There is ALMOST no excuse for the timing chain to snap from wear as it is relatively easy to check for chain stretch. Indeed, the number of snapped chains remains VERY low. It only takes a few minutes to check for chain stretch if the encapsulation panels and valve cover have alreeady been removed. Removing/replacing the panels and valve cover on a 126 requires maybe half an hour or a little less. The chain should be changed whenever stretch exceeds 4 degrees, but there is little chance of the chain snapping until stretch reaches almost 10 degrees.

Chain wear is usually an inverse function of oil change frequency - change the oil often (keep teh oil clean) and chain wear rate diminishes (everything else equal). Apparently the quality of the oil can influence this too. In every OM60x engine that I've changed to synthetic oil (Mobil 1 or Delvac 1), chain rate wear dropped by half or more.

If the chain tensioner (very rare) OR the vacuum pump (less rare) should fail catastrophically, the chain can snap.

The cost to repair following a chain snapping can easily be $2000-$4000.

Marshall

Seems to me after reading several of these timing chain threads, and after changing out a couple of them myself, that the primary culprit in a catastrophic failure on a 617 engine are old and probably neglected gudes. For sure that can be hard to pinpoint as there is so much damage when these let go.

Clearly damaged or worn guides require replacement, but if the chain is replaced when it reaches the accepted limits (more than 5-6 degrees on an OM61x engione - more than 4 degrees on an OM60x engine - well within the capacity of the tensioner to maintain tension) the guides will NOT be worn 99% of the time. They usually need only to be replaced when the entire engine is rebuilt. Guide life will exceed engine life as long as chain strech doesn't much exceed accepted limits and the tensioner doesn't fail.

Guide replacement on Mercedes diesel engines that I've examined (unlike MOST other engines) is almost NEVER required. Same with the engine, timer, oil pump and crank sprockets, they almost never require replacement before the engine is rebuilt from the ground up. The exception is of course when something actually fails or routime maintenance is neglected - then they need to be checked and if they ARE damaged they require replacement. If you change the chain when it's worn to the limit, it would be VERY rare for the guide to need replacement. Of the chains actually changed, only one or two from hundreds or reports exhibited sufficient wear to require replacement (more than a 20 were replaced, but in most cases the change was NOT required and I would not have hesitated to put most of those guides into a used engine and expect them to last many hundreds of thousands of miles more).

Marshall

Marshall, I DO NOT disagree with your analysis. However most of us are running these old high mileage cars, and just the age can be of great concern with those parts. Especially since we are often buying a pig in a poke, with scant records.
Last time I got into mine it had 200k miles and when I compared the old banana slide with a new one it was more than obvious that it was well worn. At that time I took pliers to a corner of it, just to see how brittle it was. Busted like glass.
That was with an engine I have come to believe suffered in the hands of fools too long before I took over. Lots of indications that it was not well maintained. Unfortunately the same plays out with a lot of other members here on these old deisels.
With that in mind, and with the consequences of having a timing chain take out an engine, I will always recommend at least taking a good hard and objective look at changing the guides with any timing chain install. Not that expensive and not that hard to do. (With the exception of the lower guide)

My point was, that based on the reports I have accumulated over the years, if you change the chain when it reaches the wear limit and do NOT bother to examine or replace the guide, you will have a less than one or two chances in 100 of the chain failing prematurely. Guide failure is VERY rare.

Marshall

While were on this topic, how difficult is it to replace the guides on a 1987, 603?

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The top one in the head is fairly easy. Not sure about the other two - I know the lower is awful, the long/banana one may not be as bad. The lower requires front timing case removal (ugh). My top guide with 230kmi had nearly zero wear, but I replaced it anyway since it was cheap ($5?) and it had to come out during the head replacement anyway. I didn't touch the lower guides and don't plan to. I have a new timing chain as well.

All I can say is I hope they do better testing on their aircraft engines..............

William Rogers.........

MB made the same mistake with timing chains on the 380 series gas engine. They installed a single row instead of the double row that was in the 350, 450, 500, etc.

the timing chain on my dads 350SD broke a couple years ago, and at about 45k miles! MB paid for most of it.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

Thanks gsxr I want to do mine soon, and was wondering about the guides. I'll just do the chain and the top one.

One more question, my lifters are ticking I'm hopeing Mobil 1 Delvac will fix it when I put it in during the next oil change. However if it doesn't do I have to fix them? Could I leave them for a year or so? How difficult is it to get the cam out?

Thanks.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The chain replacement is a little tricky and I prefer to use the proper crimp tool, which you can rent. Definitely try synthetic oil before replacing lifters. If you do indeed have a bad lifter (or several), you can test them per the factory manual procedure. They're ~$25 each so you may not want to replace all at once ($300). Getting the cam out is easy if you follow the factory manual, if you don't, you'll break the camshaft in pieces. You can probably leave them alone for years, they mostly just make noise. Synthetic fixes most cases though.