Do timing chain guide rails really do anything (because if so I've work to do)?
 

89 300E
Okay, this might seem a bit niave, but can a flimsy plastic guide rail really do anything to keep the timing chain on the sprockets? I can see how a failed tensioner would allow slack which would allow the chain to hop off the sprocket. But I just can't imagine the chain jumping off the sprocket as long as the tensioner is good. There really is an enormous amount of tension in the chain on the pull-side, and there's only a guide rail at the bottom of the slack side. I recently pulled the head and replaced the chain and tensioner. When I was in there I noticed the guide rail on the driver's side was literally crumbling (the pull side). I removed the bits that were cracked and didn't pay much more attention. I'm wondering if I must go back and replace the rails. In the cases of chains going bad, was it the chain that broke, or the tensioner that failed, or did the chain just hop off the sprockets because of bad rails?

Thanks.
Robert

They are called chain guides because they guide the chain on its trip around the sprockets. If they weren't there, the chain would flop back and forth.

The chain is not pulled smoothly around and around. It experiences an acceleration torque when a cylinder 'fires' and a deceleration until the next cylinder ignites. These torque pulses can set up waves in the chain between the sprockets, in spite of the tension. The guides flatten the waves and guide the chain to the sprocket.

I'll bet MB put them where they were needed... ;)

Best Regards,
Jim

Do a search. Guide rail replacement is considered normal preventive maintenance. Failure of them seems to be rather common on old/high mileage engines, and potentially catostrophic. (And all over an inexpensive, easily replaced part.)

Jeff Pierce

Lots of good advice, You Really Want To Heed!
 

All the above is correct. It is designed to compensate for torque, wear in the gears and natural stretching in the chain! Without it you would get about half the life before catastrophic failure! A valve through the piston really makes these run poorly! :D

In the cases of chains going bad, was it the chain that broke, or the tensioner that failed, or did the chain just hop off the sprockets because of bad rails?
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Many cases of "chain failure" are actually caused by the plastic becoming brittle with age, breaking, and falling into the chain which then jumps the sprocket and/or time and causes massive engine failure. Most MB experts say that the plastic pieces should be replaced at least every 10 years/100K miles, whichever comes first.

I removed the one on my car about 9 years ago because I had to take it out then found it to be too much work to put it back in place. I dont miss it after 300K. never noticed any difference.

Interesting. I think I should go back in there and replace the guides. I did remove the loose pieces of plastic, but then again...

Thanks!
Robert