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

duxthe1 · #5 11-27-2008, 11:19 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.