timing chain 13Degrees off - Page 2 - PeachParts Mercedes-Benz Forum

JonL · #16 02-14-2009, 10:04 AM

I suppose it's possible that the chain is stretched, but I don't understand why the left cam would show no indication of stretch while the right shows quite a lot of stretch. Unless there is more to the story that we don't know... perhaps this has been apart recently and reassembled incorrectly, or the heads have been cut, or there are offset keys installed, etc.

Gurunutkins · #17 02-14-2009, 11:36 AM

from the pictures I posted the shortest length of chain is between the rhs cam sprocket and the crank sprocket its only about 20 inches of the 6ft of chain the major "loose" part of the chain is from the bottom crank sprocket to the lhs (usa passenger side) cam sprocket and another 2 ft 6 between the lhs cam sprocket and the rhs can sprocket (where the chain dips down into the valley to the idler wheel) so the lhs sprocket has the greatest chance to be out of time. what happens if you turn the engine to get the cams at their respective timing marks? where does the crank read? try the test for chain stretch listed by tomguy at this link

http://www.peachparts.com/Wikka/M117TimingChain

Unless I am reading what you are saying incorrectly your lhs cam is the one reading out?

cheers
Barri

Gurunutkins · #18 02-14-2009, 11:37 AM

OK I just realized I am reading what you are saying wrong and it is your rhs cam that is out of alignment

sorry

RC4U12 · #19 02-14-2009, 12:23 PM

do you kow how that center sprocket is held in place? or does any body? right now i am at work and my engine is at home will check then ...i just cant fathom this as the problem ,,i thought maybe the wrong guide installed to make the chain not rest on that guide,,but your explenation or the wrong guide seems to be the only thing that it could be..but tonight i will take the cam gear off and check for offset keys...Jeff

Quote:

Originally Posted by JonL

Here's a theory... I don't recall how the chain makes the 90 degree turn in the "valley." I guess there's a sprocket in there. The bearing or bushing for that sprocket has failed, and the sprocket is now riding high in the block by some amount. That will make the effective length of the chain longer between the right and left cams making the right cam run retarded compared to the left. It will cause the chain to ride high off the rail on the left cam as shown. It will also make the chain run high enough to wear a groove in the cylinder head as shown in the picture. You're going to have to pull the front cover, my friend. Have fun. You're lucky you didn't let it get worse.

Gurunutkins · #20 02-14-2009, 02:01 PM

the center sprocket is held in place by 2 bushings, one bushing in the block and another in the front cover. the bushing is maybe 1mm thick. however even if the bushing had failed it would not put the rhs cam out of alignment. the cam is pulled clockwise by the crank turning clockwise (the rhs cam is the leading cam not the trailing cam). there is nothing between the rhs cam and the crank its a straight run. if you had jumped a tooth you would be crashing valves into the pistons as its an interference head and 1 tooth is enough. a really stupid question but you are turning the engine clockwise when looked at from the front?

RC4U12 · #21 02-14-2009, 04:30 PM

Yes i am turning clockwise..i have a good knowledge but not a mechanic by trade..i really think its a wrong guide and maybe an offset key.. i really dont think its the center gear..but i am taking all the steps when i get the right parts...my guides are all plastic except the tensionguide..and red or brown in color ..in '74 are they all suppose to be metal backed?...i did take the right vavle cover off my '73 and that one has only the metal tension guide and there white but that engine has 190,000 miles..... i will take off the left cover to see if the chain rests on the guide where mine is not.. Sunday projects.. Jeff

RC4U12 · #22 02-14-2009, 04:36 PM

I am calling the right hand cam the passenger and it seems you are calling the drivers rh? as the crank would pull the drivers and the drivers cam pulls the passenger and this theory works then..in every place i look i think the engine sides are pasenger right and drivers left{us version} Jeff

JonL · #23 02-14-2009, 04:44 PM

Quote:

Originally Posted by Gurunutkins

the center sprocket is held in place by 2 bushings, one bushing in the block and another in the front cover. the bushing is maybe 1mm thick. however even if the bushing had failed it would not put the rhs cam out of alignment. the cam is pulled clockwise by the crank turning clockwise (the rhs cam is the leading cam not the trailing cam). there is nothing between the rhs cam and the crank its a straight run. if you had jumped a tooth you would be crashing valves into the pistons as its an interference head and 1 tooth is enough. a really stupid question but you are turning the engine clockwise when looked at from the front?

Let's get our terminology consistent. In the US, we have left hand drive cars. The driver's side is the left side. Left and right is defined from the driver's point of view. So when you are looking at the engine from the front of the car, the cam on the driver's side is the LEFT cam, or lhs as you put it. the LHS cam is pulled by the crank. The RHS cam is pulled by the LHS cam.

My idea of the sprocket bushing being worn fits the symptoms as described. I'll agree that it would be a very, very unusual failure mode. However, if the timing chain has not been molested, it's hard to see what else it could be. If the chain is stretched, I'd expect to see the LHS cam be retarded maybe 5 degrees if the RHS is retarded 13 degrees. But the LHS is reportedly right on the mark, and the RHS 13 degrees retarded. I think my theory fits, but I think it is more likely that someone attempted to retime the cams with a badly stretched chain and screwed up what was a bad idea in the first place. Like maybe they put an offset key in the LHS, and didn't do anything on the right.

This one's got to be looked at very carefully IMO to prevent a future big time failure.

Gurunutkins · #24 02-14-2009, 04:51 PM

Ok the passenger side cam (us car) is the trailing cam. that is the one that will get out of synch if the chain has stretched. if the chain has stretched then it will slap and wear grooves in the head just like you are seeing. read the article by tomguy in one of my last posts on checking your chain stretch. it always gets confusing when discussing sides. what you seem to be seeing is very typical of a stretched chain. the chain doesn't necessarily rest on the guides, they are just guides and depending on tension the chain may well not touch them all the time. the older cars came originally with metal rails, if they have been changed they were usually exchanged for plastic rails which then need to be changed every 30 or 40K max or 5 years. you can still buy metal rails, febi has started making them again. Try the chain stretch link I posted earlier I am fairly sure thats a lot of your problem. let us know
cheers
barri

RC4U12 · #25 02-14-2009, 05:11 PM

that article is how i got here and is what i did ..Jeff

RC4U12 · #26 02-14-2009, 05:27 PM

so if if the chain was already changed as the guides are not stock and i have another 13 degrees of strectch ..HOW many miles are on this thing?? Jeff

JonL · #27 02-14-2009, 07:41 PM

You are not alone...
Check this thread:
timing marks don't line up

Maybe it IS just a stretched chain, although I can't understand how the left cam will show no stretch and the right cam shows 13 or 14 degrees.

shadetree77 · #28 02-14-2009, 08:25 PM

hi...that' last post with the link is from me....the weird thing on my car is that the chain does not make any noise on startup (i think it would if there's 14 degrees of stretch)....i just aquired this 420 sel very cheap because the passenger side valve train was making a lot of noise (found 2 badly worn lobes, caused the oil tube being blocked)...so i don't really know what's wrong with the chain either...i haven't checked for offset woodruff keys, but i plan on pulling the passenger side camshaft sprocket tonight to see if has one...the car i have has approx. 350,000 miles on it, but has been cared for very nicely...the chain guides look fairly new (not brown or amber looking)..the chain was replaced at some point with a genuine mercedes repair chain (has fishook clip on it)...the whole suspension looks new....i hope we can find a problem to our problem

shadetree77 · #29 02-14-2009, 10:57 PM

pulled the camshaft sprocket....no offset woodruff key...don't know where to go from here

RC4U12 · #30 02-15-2009, 11:04 AM

well this morning i went out again and this is a slot cut by the chain..i did try to move the chain there before and you can move it up and down easily to hit the head.. i now understand that its only a few inches of chain from the left cam to the crank and all the stretch would be noticed as late on the right cam as this is where 85% of the chain strech comes in.. so the 13 degrees of stretch just like i explained.. is now explained to me anyway..new guides and chain have to fix it..
that center gear between cams is not failed or the bushings..a thin rod pushed down to gear gets no movement and seems solid..
I think my engine is going to like this..My valve timing on the right bank will be 15 degrees earlier after the 2 degree offset key is installed with the new chain..Jeff

Quote:

Originally Posted by tyl604

In picture #4 it looks like something has ripped a slot out of the flange.