380SL Timing chain "slap" after new installation - PeachParts Mercedes-Benz Forum

earnest37 · #1 01-31-2012, 04:42 PM

Changed the timing chain in my daughter’s ‘85 380SL. Actually pulled the engine, replace all valves on driver’s side since all were bent. Had both heads renewed including .003" milling, etc. Replaced all six slides/ramps including the one on the chain tensioner. Replaced both the intermediate and guide/return gears. Problem: When engine is running the new timing chain "slaps" against the head on the passenger side. This produces a clicking sound which sounds a lot like a very loose valve lifter. Better description: After the chain rolls over the top of the cam gear and begins its downward travel through the head and toward the guide gear in the center of the engine it somehow gets some slack and snaps upward against the aluminum head. This doesn’t make sense because I’ve replaced all of the slides and they are all correctly positioned. In addition the chain rotates in a clockwise direction when viewed from front of engine. This means the chain is being pulled over the top of the cam gear downward around the return gear. To me this should mean the chain is always tight from the cam gear to the return gear. How the cam gear can "overrun" the engine rotation to create slack is beyond me...but it definitely does.
The only thing I can determine is that the chain tensioner, for some reason, is not keeping the chain tight enough. The tensioner has a fairly strong spring in it to take up any slack and is additionally tensioned by oil pressure from the engine. Oil pressure is good. I guess my question is this: Does the chain tensioner need to somehow be bled of air in order for it to apply sufficient pressure against the chain tensioner ramp? If so, how is this accomplished? Inquiring minds want to know.

Roncallo · #2 01-31-2012, 07:30 PM

I very rarely replace chain tensionrs but maybe you need one. Also are you shure you cut the chain to the right length. The cam can and will over run the engine with slack in the chain due to cam lobe pressure. This is why it is necessary to hold back pressure on the cam when checking timing.

Strife · #3 01-31-2012, 08:13 PM

The chains are different lengths between the M116 and M117. It would not be impossible to be supplied the wrong one (the 117 is longer). The difference is so great that it one case, it wouldn't reach, and in the other case, I think it would be completely obvious long before you started.

I would change the tensioner. Note that some people (including myself) have had problems with aftermarket units. Unless you are on a serious budget I would buy the MB model at the best discount that I could find.

I suspect that you had to compress the tensioner slightly in order to perform this job. If it was old that may have finished it off and it may have lost its ability to retain oil pressure and therefore tension. If you feel that it is "springy" when you start compressing it while installed and then it suddenly gets MUCH more difficult to compress (the oil and one-way valve) this is DEFINITELY a sign that it is old. If you can actually compress it with your hands and get oil out of it this is DEFINITELY a sign that the valve is aged.

That being said, I wonder if you did this change by the book and removed the RH rockers to alleviate the effect that Roncallo is writing about. I would take the covers off, remove the RH rockers, and zero the engine (distributor at ZERO, the cams as close to zero as possible, and the timing mark at zero. You may find that you are a tooth off.

I would NOT run this engine whatsoever, period, until this problem is found and fixed. And this "slapping" may have cracked your new guides already but they are still in one piece-for now. I bit the bullet and bought heavy duty Meyle metal and rubber guides so that if I have a tensioner problem, I will have plenty of warning and no excuse but to take preemptive action.

ADDITIONAL IMPORTANT NOTE: Rereading your post, I noticed that you had the heads milled. This changes the geometry of the valve train and timing. You may even have to get offset woodruff keys for the cam gears to truly get the engine "zeroed".

meltedpanda · #4 02-01-2012, 03:53 PM

wow Strife, you need to write a book, nice explanation and keen to the point

Stretch · #5 02-01-2012, 04:15 PM

As a diesel owner I have to ask the petrol heads - do the tensioners need to be pre-filled with oil like on an OM617?

rowdie · #6 02-01-2012, 06:02 PM

Quote:

Originally Posted by Army

As a diesel owner I have to ask the petrol heads - do the tensioners need to be pre-filled with oil like on an OM617?

I don't know about the later models the early ones do. Submerge in oil and and manually pump it up.

Strife · #7 02-02-2012, 10:37 PM

The Febi tensioners were shipped with "this side up" labels, and were in sealed plastic bags...not too promising for holding oil for a long time! Like, the distributor and shippers are going to maintain that orientation...

I pumped mine up full of oil, and then I discovered that it was impossible to actually install it. I compressed it a bit, and then (after installing the tensioner bolts with LOTS of threads engaged!!!!!) let the bolts do the rest. I do not recommend this because it might put a strain on the one-way valve. Under ordinary circumstances, the tensioner should only go one way, out, as the gears and chain age and the tension gets looser.

There are a lot of variables in play here. The wear of the gears, if extreme enough, can make even a brand new chain looser and require offset keys to correct. I've also read on a mechanical engineering site that old gears can wear a new chain fairly quickly. From what I've seen, the cam gears bear the brunt of the wear. The MB ones are expensive, the aftermarket, not so much.

You don't really notice the wear until you put an old gear up against a new one.