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

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89 300te 222,222
92 300e 190,000

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.

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Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

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

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Jeff Pierce

Current Vehicles:
'92 Mercedes 190E/2.3 (247K miles/my daily driver)
'93 Volvo 940 Turbo Wagon (263K miles/a family truckster with spunk)
'99 Kawasaki Concours
Gravely 8120
Previous Vehicles:
'85 Jeep CJ-7 w/ Fisher plow (226K miles)'93 Volvo 940 Turbo Wagon
'53 Willys-Overland Pickup
'85 Honda 750F Interceptor
'93 Nissan Quest
'89 Toyota Camry Wagon
'89 Dodge Raider
'81 Honda CB 750F Super Sport
'88 Toyota Celica
'95 Toyota Tacoma
'74 Honda CB 550F

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!

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.

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1986 560 SEL (159K miles)

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

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89 300te 222,222
92 300e 190,000