420SEL Timing Chain

[Rich126]

I know it's recommended to replace a timing chain in the 420 engine every ~100k, but I recently purchased an 88 with 46k on it and am considering having it changed. The chain was never changed before, the car is 13 years old, and I know sometimes the plastic guides can dry out with age, break off, and drain out your savings account.

This happened already to me with my '87, so I'd like to avoid a flashback. Am I being overly cautious, or should I just go for the (relatively) cheap insurance plan and change it ASAP. The car has had oil changes regulary, from what I can gather out of recent service records. Has anyone seen a chain break on a 420 with less than 100k on it?

Any comments are appreciated.

I think that I would at the very least have the guides changed out .and while they were doing that I would ask how much more to slip a new chain in while they are at it . On the other hand maybe have a shop just open the valve covers up and take a look and see what they think. But I dont think you are being overly cautious. better safe than soory .I hope this helps with your decision.Goodluck

How many miles did your '87 have on it when the guide broke? I have an '88, 420SEL, with 75K, and just had the chain and tensioner replaced, but not the rails (the mechanic's decision). This was done at a Mercedes' service center, and the service consultant said the guides (rails) are not actually plastic but a composite of some nature which I don't recall, and that the material is actually very durable. He said, further, that the guides on my car showed no wear. I'm still worried. I'm the third owner, and I supposed the guides might have been replace earlier but I have no record of it. All that I have read indicates that the 4.2, V8 is at high risk for chain failure so, as other have said, replacing the chain and guides is cheap insurance. With only 46K miles on the car, though it's a shame to have to lay out that kind of money. This sort of thing ought to be on a Mercedes recall and done at their costs, in my opinion.
Jerry

[Rich126]

Lakelover,

My 87 had 130k on it when the chain broke and bent the valves on the drivers' side of the engine. I'm guessing the guide broke off and caused it, but I'm not sure - it happened at about 35mph, not at idle (which is probably more common).

I agree about the recall, but it's my impression that Mercedes doesn't care about cars they produced more than 5 years ago.

I'm wondering why your mechanic didn't change the rails when he did the job, they seem to be pretty inexpensive. What's the going rate on this repair?

The 560 engines don't seem to have this problem. The chain is different, it must make a difference.

I think you might be looking at about $700.00 give or take . To do timing chain, and guides and tensioner. I doubt that they will save you much money if they dont install the chain I've seen them basically fish the new chain in thru the top of the motor,with out taking apart much more than the valve covers but after all we are paying for there experince could be deadly to the pocket book if that chain fell inside the motor while performing this operation.yikes

[Rich126]

I just called my mechanic, he gave me a price of about $350, including parts, to change the chain, tensioner, and upper rails. The lower rails would be much more difficult to get to, and he's never seen them break off. He said with 46k, I probably shouldn't bother.

Another tip I got was that after the oil is changed, the lack of lubrication causes the tensioner loosen and release the chain onto the rail somewhat, and when you first start up the now-dry engine after refilling the oil, the chain slaps against the rail, which can cause it to break off - potentially causing a problem. He suggested disconnecting the ignition wire after oil changes, and cranking the engine first without a spark to build up oil pressure, then reconnect and start.

Now that sounds like a deal I would have it done . and that makes sense about the tensioner I did not know that they were affected by oil presure thanks for the tip

I paid $738, for timing chain and tensioner replacement. I also had a quote of $1,100 to $1,200, for the same job plus rails. So, your quote of $350 is an incredible deal. One warning I've read on this forum or perhaps on the Mercedes Club forum is to be sure that a genuin Mercedes chain is installed. Apparently, after-market chains have been know to break shortly after installation.

Thanks for the oil tip. And, maybe that makes a case of synthetic oil with a thin viscosity that supposely reaches moving parts faster at start up. Also, the tensioner is partially activated by oil pressure. So, a faulty tensioner could result in a chain "slap" and broken rails.

[Rich126]

I'll make sure he uses an OEM chain, thanks for pointing that out.

I've heard negative things regarding synthetic oil in these old MB's, by the way. I never tried it.

I now realize what a deal I've been getting on service when I see the prices people are paying on the forum. If anyone lives around NY, I'll be happy to refer you to my mechanic (he's MB exclusively).

Rich, reference the synthetic oil, I know several people here on this forum do not advocate the use in an older vehicle. I, however, have had good success the last two oil changes in my 177k mile '81 380SL. The mobil 1 has worked super. But, again, that is just my one vehicle experience.

[psfred]

Some notes on 420 timing chains:

First and foremost, it is a stretched chain that causes the problems, not a broken guide. The guide breaks when the chain "hops" a tooth on the right head cam gear when it is too long, or it is too loose and "slaps" too much on startup or at idle. The piece of the guide then falls down and gets under the chain and ...... bent valves on the right head.

The cure is to get the stretch checked reguarly -- the "cheap and cheerful" way is to set the cam to TDC and see where the crankshaft pointer is -- more than 8 degrees out means change the chain. Find a mechanic who knows how and check it at least every 30,000 miles or if you are feeling insecure.

The tensioner on all these cars leaks down every time you shut them off -- they do not hold oil pressure. Hence the slap on startup, and the broken guide when the chain is too long. The newer (60x and 20x) engines use a ratcheting tensioner, pushed out by oil pressure but held out by a ratchet, which is fine until you forget to re-set it when putting in a new chain and promptly overtighten the chain and break it......!

Check the stretch on the chain, change the oil frequently, and the 420 will run almost forever -- it is almost exactly the same as the 500/560 other than the single roller chain instead of the double roller chain.

Synthetic oil is only a problem in an engine that already needs an overhaul -- it will leak more, both from the engine and up the cylinder walls past worn rings, and it will clean up all the dirt out of everything, including varnish filling up the clearance on hydraulic lifters, etc. If the engine is in good mechanical shape, wear will be nearly nothing so long as you change it once in a while!

My favorite synthetic oil story is the Ford lubrication engineer who had to use some during a constant run test, found that engine wear stopped while the synthetic was in the engines, so had his new 1965 Lincoln filled with it when the break-in oil was drained. He ran it for 100,000 miles with only topping off after filter changes at 5,000 mile intervals, then had the engine dissasembled and inspected when it didn't break down and still only used about a quart every 2000 miles or so -- for all intents and purposes, NO wear -- lifters looked like new, still had new looking honing marks, no wear ridge, etc, and HE NEVER CHANGED THE OIL! Published in Popular Mechanics or Popular Science in about 1972 when Mobil first started marketing Mobil 1. I don't actually recommend never changing the oil, but synthetic oils really to what they claim.

Peter

Peter. . .

Thanks a million for your info on timing chain/guide problems and on the use of synthetic oil. My chain was stretched 10 degrees when I had it replaced, so I guess I was lucky to catch it in time. I've been of the understanding that rail breakage was the primary concern but now I understand from your perspective that it is chain "slap" that results in rail breakage which in turn breaks or jams the chain causing valve damage. So, I guess I should feel confident that the rails in my car are in good shape as the mechanic reported when he replace the chain and tensioner.

Also, I'm going to switch to Mobil 1, at my next oil change. I'm hoping and planning on having my '88, 420 SEL for a good many trouble-free years and a couple more hunder miles. It's a great car.

[psfred]

Change the oil and filter again at 2500 miles after you first change to Mobil 1 -- all the crud it will wash free in the engine will nearly plug up the filter! Once you get the crud out, change it as often as you feel you need -- I run 5000 or so miles between changes (factory recommendation for the Volvo is 7500 with a filter change every 15,000! -- I change the filter every oil change.)

The 420 is a wonderful car -- decent milage, plenty of power, and that lovely W126 chassis!

If you start to get a puff of blue smoke on startup and some oil consumption, get the valve guide seals replaced -- oil isn't good for the combustion chamber! They usually get hard and start to leak at about 150,000 miles or 10-12 years.


Peter

Psfred has given some good insight into the chain stretch experienced by some engines. I would add only that many vehicles experience a chain breakage or jump upon initial cold startup. This is what happened to our old 420. The reason is based in the fact that upon the previous shutdown, the engine momentarily rotated backwards, (compression, backfire, detonation, dieseling or what-have-you) leading to a jumped tooth or two. The tensioner does no good in these cases since they are on the other side of the engine travel. Of course, this is only a problem when you have excessive stretch to begin with.

I'm a believer in frequent oil changes, perhaps more often than necessary but I figure it's cheap insurance. I've change oil at 3,000 miles in all my cars, including Castrol Syntech 5W-50, in my motor home (Ford 460 engine), which I sold recently. Castrol is the only synthetic I've used, but since Mercedes and all the owner's I've seen post messages recommend and use Mobil 1, that's what I'll use in the 420. In fact, I may go ahead and switch soon. I've just owned this car for about 700 miles, and had the oil changed during the pre-purchase inspection. The mechanic said, however, that the drained oil was quite dirty, so I'll consider that oil change as a "flushing."

I'm also wonder if Mercedes has redesigned or upgraded the chain tenstioner since the original production. In another words, is the new tensioner, I have, "better designed" than the old one. Also, whether synthetic oil will improve the performance of the tensioner. I guess those are questions without easy answers. It's doubtful that the ratcheting-type tensioner is adaptable to older engines. The best advice given on these posts is to check the chain stretch about every 30K miles.

Jerry