I'm trying to troubleshoot a car that has had a new timing chain, all rails, tensioner, the works done 2 years/6000 miles ago but MAY have chain slap (see my other posts also questioning if the alternator). At any rate, during my searching I came across the following post from Steve the moderator which seems to have the most relevance to what I may be seeing. He wrote:

"As to the tentioner, there is a check ball inside that retains the oil to maintain pressure. Even brand new tentioners can have that check ball leak occasionally and the short rattle that then occurs is what gets the rails. The rattle CAN NOT be permanently prevented. The ONLY prevention is to remove and replace the rails before they become brittle so they can do their job and contain the rattle when it happens."

So he says that even brand new tensioners can leak. I wonder how that happens? I've ordered a new one and will probably replace it, but I'm wondering what the story is there. What Steve describes is what I think is happening with my car because I have a new tensioner in there. I wonder if the tensioner cares about viscosity or if oil can coagulate around the tensioner and this can disrupt it sometimes. To test that out, I've drained and refilled the car with Mobile One 10W-30, I previously had 20W-50 Castrol in there. Anyone have any thoughts on new tensioners that can't hold oil to maintain pressure? It looks like a pretty simple device, I wonder why it would have problems like that even when new.

Which model and year are we discussing?

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1989 420 SEL

It uses a hydraulic tensioner. Looking at the MB manuals, Steves insight is interesting. It is very reliant on oil be pushed through it and there being pressure. I am very curious to see if my change to mobil one 10w-30 will affect it from thicker 20w50. On paper it looks like it could I think. I also wonder what affect sitting for days has on the chain sound versus overnight. It seems to only do it sometimes and lightly after sitting for days. A new tensioner wil arrive monday but i think I might wait and observe a bit more before I change it myself, still havent decided. It looks like an easy job but I have learned to not touch this car unless I have to. On the other hand, any chain sound at all is scary. The tensioner in there is 2 years old and has 6000 miles on it.