108 Timing chain rattles

I have the 3.5 V8 in my 108. On hot start up there is a terrible rattle from the timing chain.
This is, I believe the chain tensioner not doing its job for the first few seconds after start up. It doesn't happen on cold starts as its a bit reluctant to fire and the cranking gets the oil pressure up enough to tension the chain.

I've had conflicting advice on this. An MB mechanic says I shouldn't drive the car till I have changed the chain and slide rails. The local garage mechanic says this will do nothing , drive the car and don't worry so much as the chain is quiet after a few seconds.

Neither solution makes me happy . Has anyone any experience with a similiar problem.

[ctaylor738]

My 3.8, which is a close cousin to your 3.5, had the chain jump at start-up. What happens, I think, is the sudden acceleration of the valve train causes the loose chain to "wave" over the right sprocket. The chain jumps and you instantly have eight bent valves and no compression on the right side.

There are lots of posts on how to measure chain stretch. And while you have the valve cover off, you can inspect the slide rails. But I think you are at risk and recommend you replace the tensioner immediately.

[MikeTangas]

I wouldn't listen to the local's advice of "just drive it". I would stick a tensioner in it ASAP.

My chain started to rattle several months ago, so I just parked it. Once the rattle starts you are on borrowed time, start up is when the chain will likely go as Chuck experienced.

I replaced my chain because it appeared to have about 6* stretch (8* is the limit) and tensioner. Found that the old tensioner would not prime up and hold tension. The new tensioner should be primed, or pre-loaded to ensure tension on initial start up.

Now the rattle is gone and I'm enjoying the 4.5 again.

Understand that if the chain breaks, or more likely in your case, jumps a few teeth on a cam sprocket, valves will collide with pistons, disabling the car and requiring an expensive repair.

By all means, park the car until you have the tensioner, and perhaps the chain, replaced.

[Aaron]

Agreed,

Go tell the local garage mechanic to jump in the lake and get your car over to the MB specialist and have him put in a chain, tensioner and four upper guide rails. It's one of those "You can pay me now or you can pay me later" things. Would you rather drop $1,300 or so for the above work or $5,000+ for a rebuilt engine?

[Tomguy]

Aaron: Can that be done without removing the engine? Just curious, I've been thinking about it lately (when I get around to replacing my camshaft on the (right?) side... that is the passenger side, correct?).

[MikeTangas]

Chain, tensioner and rails can be done without pulling the engine. In fact, the chain and tensioner, plus adjusting the valves is only about a 4 hour DIY job (taking ones time, with plenty of breaks). Figure another hour to hour and a half to add doing the rails (if all bolts and pins go smoothly).

Thanks for the advice people.

I also read Mikes posts on his similiar problems so I am now pretty sure on what I need to do. Tensioner, may as well do the chain, may as well do the rails.

I have a few questions though. What is 6*, for that matter what is 8*? Also Mike you talk about something to attach the new chain to the old chain when you are feeding it in, I don't quite get this bit and it sounds tricky can you give me some more detail. One last question should I get a non genuine chain for $95 ( thats about $55 US ) or a genuine chain for $195. I think I know what the answer will be.

[wbain5280]

Check this out. Complete instructions on chain changing:

http://www.pindelski.com/cars/W126/W126_chain_rails/W126_chain_rails.html

Pretty much the same engine.

Regards

[MikeTangas]

6* and 8* mean 6 degrees and 8 degrees, using the asterisk in lieu of the degree symbol (no degree symbol on my keyboard ). As far as attaching the new to the old, just use the new master link and c clips. Try to get a JWIS or a FEBI chain and you should be OK. Thomaspin's picture page will be a great reference as the 3.5, 3.8, 4.5 4.2 and 5.6 are all similar enough with the main difference being chain length (198 links for the 4.5 and 5.6, not sure on the others).

Thanks to all.

I'll let you know how it goes.

[william rogers]

I was quoted 450 to do chain, rails and tensioner on my 500SE at a very good indy shop. I decided to do a cover off job myself and even though it quiet a chore I found one of my bottom rails very worn and was also able to replace oil pump chain at same time.I would look for a good indy shop an have it towed there first hit on the key is all it takes to ruin you day............
William Rogers.........

I have been scared off doing the bottom guides and oil pump chain as it sounds like a major job, timing cover alignment or something. Did you have any major hassles?

[william rogers]

yes It was really a lomg drawn out job and there were many snags and hassels during the whole procedure I wished I had not committed to it.I worked without a manual thinking that my many years of wrenching would get me through BAD MISTAKE but it runs great.........
William Rogers.........