Timing tensioner failure? - PeachParts Mercedes-Benz Forum

mbgizwiz · #1 02-03-2007, 04:53 PM

Hello all:
I just bought my first Benz (and my first diesel) a couple of weeks ago, a '78 240d. It's great. A friend of mine followed my lead and picked up an '81 240d less than a week later. He got a better deal, he thought, until his car broke down within ten minutes after he bought it. He had it towed to my place, where it still sits.
He said it lost power and just konked out, but I wasn't there. The next day I started to fool around with it, and got it to start. It seemed to make more noise than it I thought that it should. It sounds like a very mettallic clatter, like a very loose lifter. When I started it again later in the day, it made less noise, at first, but then more after it idled for a few minutes. Often, after revving it a time or two, the noise will change in volume and violence anywhere between outright unbearable and almost inperceptible. I began to suspect something loose about the timing chain, so I took the valve cover off this morning.
Nothing looks out of place. But when I put a wrench on the cam and turn it clockwise, it is clear that the tensioner retracts, and the injection pump side of the chain goes slack. That seems to me to say that the tensioner is kaput, as I'm under the impression that it is a hydraulic thing with a spring behind it, and it's behaving as if it is only a spring. My manual hasn't arrived, so I'm just looking for a confirmation of this diagnosis.
Or any other ideas??
Thanks
Paul

Scott98 · #2 02-03-2007, 06:06 PM

Quote:

Originally Posted by mbgizwiz

But when I put a wrench on the cam and turn it clockwise, it is clear that the tensioner retracts, and the injection pump side of the chain goes slack.

Don't ever do this. Use the power steering pulley (though others will tell you not too - arguments both ways) or use a socket on the crank to turn the engine.

The tensioner shouldn't retract. It's sounds like that is your likely culprit.

Scott

mbgizwiz · #3 02-03-2007, 06:41 PM

Thanks, Scott.
Now I'll hunt one up and swap it out.

It looks pretty easy.
Like I said, I'm new to these cars. What's the concern about cranking the engine with the cam nut? Breaking something? Having the chain jump teeth on the sprokets and losing timing? Normally I wouldn't have taken this approach just to turn the engine over, but I was looking for an issue with the tensioner, and it appears that by doing this, I found it.
Paul

Scott98 · #4 02-03-2007, 06:45 PM

The bolt on your camshaft is set to a specific torque. If you are new to MBs you will learn that tolerances and torque specs are very specific and must be followed carefully. Be sure you buy a new tensioner gasket along with the tensioner. You can get them both from this website.

Good luck,

Scott

Ken300D · #5 02-03-2007, 07:27 PM

I don't know why, but the timing chain tensioner on your 240D is different from those on 300D cars. On the 300D, there is a ratchet type mechanism that prevents the tensioner from going backwards to release pressure against the chain. There's nothing of the sort on a 240D tensioner, just a spring.

So, yes, it will retract fairly easily with just a spring providing forward pressure. You may well be looking in the right area and found a weak tensioner or stretched chain. Try picking the top of the chain off the camshaft timing gear with your fingers. If it lifts up easily, that may be a problem. Look at the inside of your valve cover. See any abrasions from the chain hitting it? That's a big problem.

I think what you should be looking for is a very loose, stretched chain that has slipped a tooth on the cam gear. Are you aware of the rough timing marks on the cam gear and tower? Do a search on camshaft timing here.

Ken300D

Scott98 · #6 02-03-2007, 08:15 PM

I've never had the tensioner retract on my 240 and I've been in there a lot lately checking cam timing and resetting cam timing. Though when I did remove the tensioner and reinstall it, it retracted at first until the oil pressure built back up. But all the other times it never retracted.

mbgizwiz · #7 02-03-2007, 09:01 PM

The tensioner in this car retracts pretty easily. Either there is no oil in it or the check ball inside is stuck open with a piece of crud. I believe if it was full of oil, as it appears to be designed to be, at least when the engine is running, it would be rock hard.
Apparently there is a gallery that supplies oil to this thing. Is it fed from the line under the bolt on the water pump? Perhaps it has been assembled incorrectly to the block somehow and isn't getting the supply it needs at all.
It began to rain, and this project will have to wait a week. I'll post the results.
Paul

Mustang_man298 · #8 02-03-2007, 10:45 PM

Quote:

Originally Posted by Ken300D

I don't know why, but the timing chain tensioner on your 240D is different from those on 300D cars. On the 300D, there is a ratchet type mechanism that prevents the tensioner from going backwards to release pressure against the chain. There's nothing of the sort on a 240D tensioner, just a spring.

My understanding of this was that they used the same type tensioner on both the 300 and 240, but the ratchet type was a newer design of the old spring only unit put into the newer (later) engines and used as an improved retrofit replacement when the older type ones fail.

Scott98 · #9 02-04-2007, 09:36 AM

Quote:

Originally Posted by Mustang_man298

My understanding of this was that they used the same type tensioner on both the 300 and 240, but the ratchet type was a newer design of the old spring only unit put into the newer (later) engines and used as an improved retrofit replacement when the older type ones fail.

Come to think of it, my 240 tensioner did not have the spring in it. It must have been replaced at some point.

deerefanatic · #10 02-04-2007, 05:42 PM

uh........ It should have a spring in it anyway...........

Definitely somethings up there..........

Mustang_man298 · #11 02-05-2007, 12:56 AM

Agreed, the spring should still be there, only difference between the two is the added ratcheter that prevents potentially jumping chain teeth from cranking/starting with no oil pressure behind or a loss of pressure during operation to keep the chain positively held. if yours is missing that spring, count your stars you had the ratchet type, as it probably has been what is saving your engine every time you started it up.