Need advice

[rwolak]

I have a '78 450 that has approx. 96K miles. Although I haven't noticed any problem, I am considering replacing the timing chain guides. Should I change only the upper guides and tensioner, or should the lower guides also be changed? Are the lower guides ever a problem?

Any input will be appreciated.

TIA

Bob

[Mistress]

I just replaced the chain and all the guides on my 90' 420 SEL at 169,000. Reasoning behind that was it's all apart now so do the whole thing...

[kip Foss]

With those miles I would first check for chain streach. If it is good just chance the upper guides and the tensioner. The mechanic I use is the master mechanic at the local MB dealer. He has a shop in the side and charges half what the dealer does plus you don't have all the EPA rag disposal charges. He says he has seen double row timing chanes easily go well over 250k miles. The important thing is the upper guides and the tensioner. They are not difficult to replace and relatively cheap. The lower guides apparently don't wear as quickly as do the uppers.

[Jim B.]

Originally Posted by Siduri19

I just replaced the chain and all the guides on my 90' 420 SEL at 169,000. Reasoning behind that was it's all apart now so do the whole thing...

Siduri, I wish I was as smart (lucky) as you to have had the benefit of a forum like this. From what I understand you bought a car that was behind on maintenance and you are catching up, doing the most important stuff first,and you will reap then the benefits of a well maintained W126. One free benefit of the W126 is you don't have the self leveling hydraulic suspension in your 420SEL, they just take ordinary Bilstein shock absorbers. So you are ahead of the game there. Plus, more leisurely accelleration for better mpg than the 560 cars. Zebrano wood instead of burlwood. The 420 is a fine car if you get a good one.

Smart owner, the timing chain, tensioner, and guide rails ought to be replaced at around 100,000 miles. That is just good preventative mainenance. At 169k you are overdue. The guide rails are made of plastic and after awhile they just get too brittle and simply tend to crack and crumble. When they let go, often on the left hand side, the timing chain slips off the rails and the pistons collide with the valves, BIG trouble, a new engine can set you back $7-12k.

Jim B.

[cbdo]

The lower guides are constantly in oil, and virtually never cause problems; they shouldn't need attention.
The upper guides absolutely need replacement. The tensioner is routinely done as well at this point. The chain may or may not need replacement in terms of stretch, but usually is done. In my own case, it was 145K and I'd just bought the car; as far as I could determine, it had never been done, so I set up the appointment--just in time as it turned out; it was noisy the morning I started it up to drive to the mechanic, and they called a few hours later to tell me the left guide was broken! I had the chain replaced as well because everything else was apart and I didn't want to have to think about the chain at all for a nice long while.