All,
I have a quick valve clatter when starting after sitting overnight. Lasts only a second or two. I have had the timing chain inspected, I assume with a borescope and was told it and the tensioner is fine. I found a TSB about this issue but cannot discern if it applies to my engine. The TSB says to replace the oil check valve; I have found no other reference to such a thing. TSB L105.10-P-056435.
Engine 276, engine 278, engine 157 as follows:
Engine 157: Up to engine 1579xx 60 022333
Engine 2768: Up to engine 2768xx 30 001280
Engine 2769: Up to engine 2769xx 30 406603
Engine 278: Up to engine 2789xx 30 103675
The 642 engine is not listed here, as 642 use a much more simple distribution system without any adjusters.
In your case the "stretch" of the chain should be checked, not the tensioner.
You can try this attached procedure with the help of nearly cheep special tool...but in 642 simple timing chain system exchanging the chain and the camshaft sprockets will be enough...as the intake manifolds and the oil cooler will be like "accessories" on this challenge.
Here are some pictures that may help visualize what's going on.
The tensioner has three dimples in the face of the housing that stop the piston at the larger diameter shiny part of the piston. When the chain stretches, I'm guessing that it allows the tensioner to hammer against those dimples and eventually wear them down such that the piston is free to extend beyond its design limit. Once it wears past the dimples the piston can easily be removed.
There's a check valve in the tensioner that keep it from collapsing when the oil pressure drops to zero, but when the engine sits, the tensioner can slowly collapse because of pressure from the valvetrain or whatever. That's what makes slack in the chain and causes a momentary rattle on cold start.
You might find some posts where folks removed their tensioner and then struggled to reinstall it. I think this is because of the check valves. The new tensioner won't have any oil in it and will compress pretty easily, but an old tensioner full of oil will be very difficult to install and might wind up putting too much pressure on the guide.
If a new tensioner is installed with the old stretched chain, it will be restrained by the dimples and won't be able to extend far enough to fully tension the chain and it may rattle all the time. That's why it's important not to just try installing a new tensioner. I read one account where a check valve apparently failed in the guy's tensioner and was causing his cold-start rattle. So apparently it can happen, but I would make real sure the chain isn't stretched before 'just trying' to install a new tensioner and see if that stops the rattle.
I don't know how far a tensioner can extend before everything blows up, but apparently there's a pretty generous safety margin. The tensioner will extend substantially beyond its design limits and thankfully give a very clear audible warning on cold-start that indicates chain and tensioner need to be replaced.
The picture below shows my 2011 gl350 with 150k mi, and I'm pretty sure my old tensioner would extend beyond the point shown in the pictures below once it got oil pressure. The tensioners apparently relax somewhat when oil pressure is removed, and the oil has time to seep out around the piston and allow it to collapse a bit. Then at some point the internal spring prevents it from collapsing further.
My old chain was stretched by about 14mm. When this this picture was taken, the car had been sitting a few days, so this is the state in which it would have rattled on startup. I wish I had stuck a prybar down there and put some pressure on the guide to approximate how much more it would have extended with oil pressure. With a new chain, the tensioner should only extend about 1/4".
On my 2008 grand cherokee (not shown), there's a nice clear view down the oil filler hole and I can easily see the state of the tensioner. That car has almost 200k and as far as I know has the original chain and looks very similar to the picture below. It's not rattling on startup (yet...) but is apparently at end-of-life also.
In the later engines, there's a baffle in the oil filler that blocks visibility down to the tensioner without a borescope.
How many miles did you have when you replaced the chain?
I had 181,000 miles at time of replacement. Tensioner was replaced at 150,000 miles with the reccome ded maintenance. Replaced it again with chain to get it under the shp's warranty
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
