So I've got a 1988 420SEL that has a rattle on cold start. Lasts maybe 1 second at most.

Sounds to me like a timing chain rattle, though the car only has 75k miles on it - isn't this supposed to happen at 120+ on these engines?

Could this be a broken guide, rather than a stretched chain, or do they fly off (into the valve gear!) as soon as they break?

Anyway, I'm going to go and pop the valve covers off and see what it looks like in there, but what am I looking for? How do I tell if the chain is too loose?

And what the hell is it that is actually making this noise? Chain banging the guides around, guides slapping the valve cover?

Any advice from those of you who have been in there greatly appreciated!

Thank you.

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1988 420SEL

That sounds like the classic signs of a sloppy chain and or tensioner.

Timmy2, if you rarely start the engine in this 1988 420SEL the timing chain tensioner likely loses oil pressure which results in a slightly slack timing chain at start-up. That would cause the brief rattle you hear. A new timing chain tensioner, or a rebuild (if that's even possible) should resolve the problem. I am unfamiliar with MB 8-cylinder engines, but I doubt that removing the valve covers will expose the timing chain. As far as I know, you would need to remove the upper timing chain cover to view the timing chain and upper guides.

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Fred Hoelzle

Yes the top drivers side guide can break off and fly into the crank sprocket and make a huge mess.

you can easily check it if you remove the drivers side valve cover.

also check the inside of the valve cover for chain marks.

he's right. the lh cover will be scarred on the inside. chain breaks at the wrong speed and it'll break through that cover. 75k for a loose chain is not unheard of. in the old days, you come in for an oil change 4 times a yr, we'd charge extra to pull that cover at some point and check the chain. seen them break at 90k but more often 100k to 120k.

replace the tensioner with mb part only, do the upper guides and chain at the same time. iwis for the chain, febi euro, if you can find it, is ok for the guides. there's some febi coming out of china and i wouldn't trust it.

Thanks. That's the kind of perspective I needed to hear on this. Much appreciated.

It is an intermittent sound: like maybe 1 in 10 starts.

Ferdman says no oil pressure on start up in tensioner: first time I heard this was after an oil change, ie when there was no oil in the top of the engine for a moment(?)

So do tensioners need TLC from time to time? I don't know how they work but I am imagining some kind of spring loaded plunger?

Thanks again.

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1988 420SEL

Due to age, even if the chain is good, consider replacing the upper chain guides, camshaft oiler rail/tube plastics and valve cover gaskets as preventative maintenance.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Timmy2, the timing chain tensioner does have a plunger and a spring. If you remove the tensioner, be sure to follow step-by-step instructions for reinstallation using the proper torque values.

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Fred Hoelzle

Do you have to take the sprockets off the end of the cam shafts to get the guides in? Seems to me that once you've done that, you might as well put new sprockets on and once you've done that, you might as well put a new chain on. And once you've done that... does it ever end?

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1988 420SEL

No you don't have to remove the sprockets to replace the upper guides and the tensioner rail/guide. With your mileage I bet the sprockets are fine. Measure how much stretch you have on your chain. If it's not too much you may even re-use your old chain because the chain breaks when the guide falls into the sprockets and rarely because the chain is worn out.

Sounds like a smart move to change the 30 year old plastic bits!

So I think we're talking about replacing (a) one curved rail that the tensioner presses against to tension the chain and (b) three straight guides (one passenger side, two driver's side). Is that right?

Question: are the three straight guides identical? Looking at the Pelican site I see only one kind of upper (in the head) guide.

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1988 420SEL

I can't answer your question about the specifics of each rail, sorry. I do reccomend replacing the plastic bits on the camshaft oiler tubes.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

All good. Thanks!

There are some really good resources on this - can I post links here? - and this looks like a very doable job, either including the timing chain or not.

The thing that is still mysterious to me is why did they build this great engine and put it in this great car, and cheap out on these totally essential parts? Could they not have been made out of something more durable?

Anyway. I'm going to start by replacing guides, oilers and gaskets. I'll check the internal timing while I'm at it and take it from there.

Thanks for the help!

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1988 420SEL

Not doing the chain is a waste ive done several of these engines, yeah you can do the upper guide rails but whats causing the noise is the chain has stretched and then the noise is the rails getting smacked, to do the chain is approx. 4-5 hours, pay a Mercedes shop to do the chain if you cant, its easy
basically you connect old to new and roll it in, the lower guide rails aren't necc but now with the age I would do those, but then its a major job, but the piece of mind, my relatives 420 sel hit over 500,000 with just upper rails and chain at 220 k

these new sissy oils, maybe the problem, newer oils are made for mileage,not protecting the engine

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1999 w140, quit voting to old, and to old to fight, a god damned veteran