Premature Chain Stretch from Bad Cam Bearings? - PeachParts Mercedes-Benz Forum

Strife · #1 11-26-2008, 01:08 AM

On my 85 380SL, I did the timing chain job at 95K, and had cam bearing failure on the LH cam and bearings at about 106K (not the oil tubes, the engine had a tough life before I bought it, I had to replace the RH cam at 95K, upon purchase, before doing the chain due to a worn lobe). The lower guide broke on the LH side at 119K and I've replaced the LH guides (with METAL guides), the cam gears, the chain, and the tensioner. Originally, I thought the tensioner went bad but I'm guessing that I had 8-10 degrees of stretch and the tensioner was pretty darned close to the limit. You won't believe this, but the e-clips came off (probably due to the broken guide piece bouncing around) and the link and outer plate were being held by a piece of plastic from the guide about 1/4 the size of a grain of rice!

Anyway, after doing this, the new tensioner is much more "pushed in", and the stretch is probably no more than 2-3 degrees. It's hard to say what changing the cam gears was worth out of that improvement - they were scored and definitely sharper than the new ones.

I'm guessing that the bad LH cam bearings (the cam was not spinnable by hand) stretched the chain prematurely.

Does this sound reasonable?

duxthe1 · #2 11-26-2008, 11:15 PM

Were the cam towers discolored? If the cam was dragging enough to stretch the chain I'd expect the cam towers to be noticibly darkened.

ARNOLDLewis · #3 11-26-2008, 11:32 PM

a stretched timing chain (or worn gears) will affect both valve and ignition timing. If the wear is severe and the chain (or a belt) jumps timing, the engine may run rough and/or possibly backfire, or it may not run at all.

Strife · #4 11-27-2008, 12:31 AM

Quote:

Originally Posted by duxthe1

Were the cam towers discolored? If the cam was dragging enough to stretch the chain I'd expect the cam towers to be noticibly darkened.

Discolored, hell - they were mostly black.

Anyway, I replaced everything and got the car on the road tonight - I'm just upset that I can't really put my finger on the cause of the problem and/or that I've permanently solved it. I really wish that I could have changed every gear involved (the crank gear comes to mind) but it's a $65 gear and $1000 to get at it.

I really played with the timing system a lot while doing this trying to come up with scenarios for chain slap. One thing I wonder about, if you have an older engine with compression leakdown, old valve springs, etc., the RH cam might turn a little further right after the engine stops, leading to the piece of chain between the gears to get a little "slack". In this case, even with new everything but an old engine, the chain is occasionally going to go slack in this area. I now have metal guides in the LH head, so this should help a lot.

duxthe1 · #5 11-27-2008, 10:19 PM

Evenly black across all of them? That sounds like an oil supply problem for the entire head. How about the other side cam and towers? If there was cam bearing clearance issues I'd only expect that cam tower to be blackened. The only time I've seen all of them blackened was due to oil starvation and it was on both sides. After all of these problems I'd be looking into replacing the engine. It gets to a point of throwing good money after bad.

A little off topic but.......

The reason these engines have problems with the valvetrain is due to the loading and unloading on the cam. If you were to turn the cam on an assembled head on the bench you would notice that as any valve is lifted the cam gets harder to turn up to the maximum lift. Once past max lift the valve spring actually starts to push the cam forward as the valve closes. That puts a push - pull motion into the cam drive as each successive valve is lifted and closed. As the cam is being "pushed forward" during closing the slack in the chain at the tensioner gets moved between the right and left cams. That is until the next lobe lifts a valve and that slack gets pulled out, sometimes violently enough to break the guide rails. Well we all know things snowball downhill from there.

The reason the six cylinder engines don't have cam drive problems, often going several hundred thousand miles, is because the extra cylinders place lobes opening while others are closing canceling out the push - pull effect.

Strife · #6 11-27-2008, 11:29 PM

There was indeed an oil starvation problem on the LH side, and here's how I think it went:

Both sides oil tube fittings were replaced at 95K. There is no question that they were installed correctly, down all the way. Later, the rearmost bearing became clogged. This heated the bearing up to the point where the oil tube fitting melted and no longer sent even a trickle of oil to the other bearings, and the same thing happened to those. I found bits of material in the bearing oil passageway. Both sides tubes were removed again, new fittings installed after the tubes were cleaned to to a mirror-like shine internally.

Since then (after new LH cam and bearings), the problem hasn't returned. But I didn't change the chain, etc after all that abuse, which I should have done.

I agree with your thinking on the push-pull effect, because you can see this effect when you spin the engine slowly by hand. A bad combination of RPM, load, and acceleration / deceleration could cause a harmonic enough to break the guide. I've installed two metal guides on the LH side so that should be that, I hope! Nonetheless, I'm going to be inspecting things every oil change from here on out for premature stretch.

This is not going to be one of those "250K motors", but I'm hoping I can get another 60 or 70 out of it before I have to do the heads, and at that time I'll assess the whole thing.

duxthe1 · #7 11-28-2008, 01:27 AM

That push pull effect wouldn't be that bad if the tensioner could hold the chain tight under all conditions. Unfortunately it doesn't. The tensioner has a spring but it isn't strong enough to fight against the valve springs pushing the cam forward. It relies on oil pressure to hold the chain tight. The tensioner doesn't ratchet forward like most other mercedes engines. That alone would help tremendously. The reason they don't, I think is that the chain is too long and it's length changes too much between hot and cold. So the best thing you can do is keep the engine full of clean fresh oil. M116 and M117s are more sensitive to this than any other MB engine.