If money is an issue, yeah just wait for the rattle then you have to change it. It could last for a few more years.

If you have the money I'd do it right away, since chances are they are original...

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

what about my 84 300SD w/ 210K and my 85 300SD w/ 240K? do they have the same issue?? are u saying i need to have them torn down to have the chain, tensioner and guides replaced as well?

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

On the 617 its a good idea to check the chain for stretch but they usualy don't break. After 300k miles it probably would be a good idea to roll a new chain on to regain performance.

Overhead cam V8's have very long chains, two sometimes four actualy, inline engines don't.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I bought a 91 420SEL a few years back and gave it to my son to use while he was going to law school. It had 200,000 miles on it and the timing chain had never been replaced so I order the parts and was waiting to take it into an independent shop to effect the change.

Before it was done, the car was driven by my wife (a school teacher, no less) and she returned it to the garage. It sat for a couple of days before I tried to start it and it wouldn't. It turned out that the LH exhaust valves bent on the startup attempt and it seemed to conform to that described elsewhere in this thread. I pulled both valve covers and found the inner upper chain guide on the LH side broken and the exhaust valve rockers out of contact with the cam. There was no damage to the RH side.

The timing chain, tensioner and upper guide rails were replaced and the LH head reworked by an independent shop. The car ran perfectly for 15,000 miles. Then it failed to start when my son ran into a bank for 15 minutes. After it was towed to a local MBenz dealer, the failure was determined to be the ignition control module (by substitution) and it was replaced with a used one from eBay.

Two weeks later the 420 failed again while running. The failure occurred while running at moderate acceleration (34-40mph) with the engine warm. A clunk was hear, the engine began to lose power and it stopped completely and would not start. This time, I had it towed to my home. After checking the obvious, I pulled the LH valve cover and found the exhaust valves not returning with the rockers dropped off for the exhaust valves. I also saw a small chard of plastic (about 3/8" polygon) on the ledge of the chain cavity and took a picture of it.

Could one of the new guides have fractured? Or, could a piece of the old guide rail been left in the engine? With a mirror and light, I could see no evidence of any missing parts or damage to the new upper chain guides. I called the independent shop, got sympathy, but no plausible explanation of what could have happened.

Over Thanksgiving weekend, I disassembled the top end of the engine. The timing chain guides were indeed intact, the timing chain was still in mesh. I turned the engine to TDC using a socket on the crank. The RH cam was in time, but the LH was off, late about 12-15 degrees (the tower mark on the left cam tower is somewhat ambiguous). The LH cam had indeed jumped time as suspected. I checked the distributor rotor position as well and it was in time.

Why? I removed all three plastic rails. They were in perfect condition. I removed the hydraulic tensioner and rail. The plastic tensioning rail, too, was in perfect condition. In fact, everything that had been renewed a year and a half earlier was in perfect condition and installed properly. I removed the LH head to have a valve job done on it. But, the problem of why it occurred keeps nagging me and I began to remember the admonition of the definition of insanity. So I went to this website.

I've read the speculative posts, most of which don't make sense to me. The lack of oil pressure on start up doesn't apply here because it failed the 2nd time while the engine was running. Besides, what engine designer in his right mind would rely on holding hydraulic pressure through a check valve for extended periods to avoid catastrophic engine (interference) damage? Too, the tensioner is preloaded with a mechanical spring and this failure wasn't at start up, but at moderate running speed. (I noted that Mercedes has increased the mechanical tension on the replacement hydraulic tensioner such that a longer bolt has to be temporarily used to draw it up). Worn sprockets? No evidence at all. Brand name disloyalty which should be punished? No, replacement tensioner supposedly was Mercedes; likewise for chain and plastic upper guides. Low oil? No, it was recently changed and was less than a half-quart low. Failure of the oil pump? Could be, but it would most likely be catastrophic, not intermittent; dropping the pan shows no apparent problem with the pump or the oil pump chain.

Improper repair? Well, there was some evidence of some minor things done poorly like two o-rings in an injector holder due to improperly cleaning the old one out of the cavity, but nothing material to the fault of jumping time. The car passed emission tests shortly after the first repair and subsequently without problems. However, I don't know that the engine was in perfect time after the repair, but I doubt that it was marginal.

This timing chain problem with the 420/560 discussed herein has far too many instances for anyone to have found the true cause and it seems endemic to this engine design. (I have a 76 MBenz that has over a million miles with the same time chain; likewise a 300TD with 375K miles with the same timing chain and I have never had a GM engine timing chain fail. Replacing a double-row steel timing chain every 100,000 miles as preventive care sounds ludicrous, because it is. Speculation about oil check valves preventing chain slap on start up may be true, but if it is so, the engine designer should be shot. I suspect something else is going on that preconditions this engine for timing failure on startup or while running. At least in the instant case, I know it is not "chain stretch" or "broken chain guides" or "worn sprockets" or the other transplant diagnostic, whip-the-messenger-boy catchalls.

As coach Bear Bryant remarked, there's just too many failures on this side of the field.

Why just the LH side? It's the first in the series, for sure. But if it were due to a worn crankshaft sprocket (which has one half the diameter sprocket of the cams, thus less teeth in contact, and would be the first sprocket to encounter a slack loop upon failure of they hydraulic tensioner), all three (LH cam, RH cam and distributor) would slip time - not just the LH side. There was no evidence of bearing seizure on the LH cam or towers and the cam turned freely as it should.

Now, I have no reason to advance this new conjecture other than that the others fail to satisfy the instant case fault phenomenon, but here goes: What if the so-called timing chain/guide "failure" was a symptom rather than a cause? No post that I have seen in the past two or three years offers anything but speculation about how this failure actually occurs. I did see some pictures on one site whereby a reconstruction of the broken guide could run under the LH cam and act like a broom handle in a bicycle chain. But has anyone been able to reconstruct simulation of this catastrophic failure?

The theory that a piece of a broken rail is carried up over the top of LH sprocket works against gravity since it would be on the inside, not the outside, of the chain. The break of the tip of the LH inner chain guide is plausible, but such was surely not the instant case. Besides, this instance does not have any damage to the upper LH guides, inner or outer. So how did it slip time on the LH side and leave the intermediate mechanism of the failure intact?

I've looked into the dynamics of chain design and it may be ignition related. The load on the chain varies (pulsates) as it pushes down the valve springs and it reacts to change in load. I'd like to see a strobed picture of a chain running under load whereby the ignition is interrupted to see if failure could be simulated. (Bear in mind, the MB inline diesels, which use mechanical injection and have no immediate shutdown, don't have this problem, though they use the same type of hydraulic tensioner and chain. Sure, chain "stretch" will affect diesel valve and injection timing, but they don't have any where near the incidence of catastrophic "chain failure" of the 420/560 gas engines. Could it be that such an event as an abrupt ignition cut off creates a "traveling wave" that presents a whiplash loop that invites the first loaded cam to skip a tooth and/or also breaks the plastic guide rail (if it's embrittled with age)?

This engine ran for 200,000 miles on the original chain, so why did it fail with only 15,000 miles on renewed OEM parts? What could the repair shop have done wrong (or failed to do) that could have caused this second failure? What quantifiable specification was violated?

I'm going to take off the front cover and take photos while I'm doing it to see if I can forensically determine the cause, but would appreciate any insight those that have been there may have.

I have seen a few of these problems on 420s and 500 engins. I have never seen a broken chain but have seen broken cams and cam towers and many bent valves on the left head. In every 6 cases I have seen, there was at least 1 broken guide. I believe when you pull the front cover you will find that one of the lower guides is broken. I personally believe that when a chain jumps timming that the front cover and oil pan need to come off and all guides replaced. I think that part of the problem is that the chain changes direction so many times, the load is constantly changing and the chain starts to vibrate and fractures an old guide which causes all the problems. I will agree that the chain is the weak link in the V8 engins but with proper maintaince, they will last forever.

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84 500 SEL (307,xxx miles)

This timing chain problem with the 420/560 discussed herein has far too many instances for anyone to have found the true cause and it seems endemic to this engine design. (I have a 76 MBenz that has over a million miles with the same time chain;
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I don't recall ever seeing any posts on this site saying that the chain broke. It is always the plastic parts that cause the problem. That being said, why would MB design an engine using plastic in the timing system? This is a huge problem just waiting to happen. I grew up driving and racing GM vehicles and never saw a timing chain problem on any street car. Only at the drag strip with highly modified V-8 engines did chain failure ever occur. When I begin driving MBs I was told about the 10 year/100,000 mile rule. I couldn't believe what I was hearing. Again, why would any car manufacturer design an engine with plastic parts in the timing system?

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1986 560 SEL (159K miles)

You can't compare a GM V8 to a MB. The MB engines are overhead cam so the chains are much, much longer, most of the new engines are quad cam so thats even more complex.

On a GM V8, like the 5.7 the cam is in the center of the cylinders, so the timing chaid is very short. Their are no guides or tensioners to wear out.


Different design's you can't compare them.

Plastic guides and rails are used to keep the engine quite, and they last a very long time. Even the metal ones wear.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Plastic guides and rails are used to keep the engine quiet, and they last a very long time.
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I don't consider 10 years or 100,000 miles a "very long time" when you are talking about an MB. People used to buy these cars because they were built to last. If the main engine can run for 300,000 plus miles without any major overhauls then the timing chain and parts should also last that long. To me the timing chain and related equipment is not a service item like brakes and filters, etc.

I still own an '86 560 SEL and had the timing parts replaced at 10 years/100K miles and I am going to continue to drive this car until it stops running with no more replacements for timing pieces. MB is losing the long haul buyers. The newer models are just not made to last as the older ones were. And MB is probably correct in their marketing. Many folks today won't drive the same car for 5 years, let along 10 or 20 years. So they could care less about long lasting durability. They buy a new model and dump it when the warranty expires. As they say, if you are buying a used late model MB be sure to get the longest factory warranty you can buy.....you will need it. The main reason that the newer used MBs are so cheap compared to years past is that people know they are going to have to spend a lot of money just to keep the car on the road. An '86 560 SEL in 1996 was selling used for around $22K. Now in 2007 you can buy a used 1997 S500 for $12K. The 1986 model sold new for around $55K, the 1997 model sold new for around $75K as I recall. Do the math. The used '97 is much cheaper for a ten year old car than the '86 was at ten years of age. And if you consider the difference with inflation it makes the '97 model worth even less compared to the '86 at ten years of age.

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1986 560 SEL (159K miles)

What model?

Seems from reading these boards that for every m117 failure, there are 20 m116 (420) failures.

Never hear much about 450 or m100 t chain failures. or at least i don't.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

[QUOTE=fahrgewehr2;1686557]What model?

300SD, Stuttgart edition, special order with leather seats.

1976 300sd?

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

Don't know how the "S" got in there.

It's a 300D, naturally aspirated.

That chain was really the only major downfall of the V8 engines in the W126 models, even the 380SE model. That was exactly why I sold my 1985 380SE. I loved the car, but the date/mileage of the last timing chain/tensioner/rails/guides was unknown, so I sold it. It was too old and had too many miles to sink hundreds of dollars into for this repair, especially since I had developed an occasional stalling, which I was informed was bad injector seals.

Too bad MB did not make these "non-interference" engines, or have designed the engine to have not had the "crumbly with age" rails. I think alot of that junk could have been eliminated. Take Lexus for instance... from 1990-97, the 4.0L V8 was a non-interference engine. If the timing belt was to break, no engine damage would occur. I had a 90' LS400 with 252K and AFAIK, it still had the original timing belt - engine was still super smooth and quiet and very powerful. I would have to look down @ the tach to see if the engine was still running, it was so quiet.

However, in 1998, they (Lexus) switched to a VVt-i engine, which was an interference engine, as was the 4.7L V8 in my old Toyota Tundra truck. Why they did that, I have no idea. I mean, why unnecessarily add risk of major engine damage if its not necessary?

To the guy that also has the SD models... unfortunately, from what I have heard, these cars have interference engines as well. If something in there breaks, the engine is likely ruined.

Good luck!

m110 owners don't defer maintenance. Ever.
And as for the cast-iron block 3.5 and 4.5's, the early ones had aluminum-backed rails. Mine has 145k on the original rails and I will NOT ever replace them at this rate. MAYBE I will have them professionally recoated, but they will not snap unlike the plastic ones.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Yeah well go buy a BMW or a Jag and you will run into the same problem. Its an overhead cam V8 and V12 thing. Go take a look at what the chain on that kind of engine looks like and how it is run compared to your old GM V8's, it will become clear.

Inflation adjusted my 300SDL costs about what an S320CDI would go for.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200