I have 198K miles on my 98 ML and the dealer is telling me I better have the timing chain replaced before it jumps time. They say this is a preventative measure. I haven't seen anybody talking about this kind of issue on the forum yet - Is this a real concern?

It's a double row chain. Should be pretty sturdy. I have never read or heard of one being changed as preventive maintainance.

^ ^ ^ ^ ^ ^ Ditto.

On the older diesels, such as my 79 300SD, there are timing marks and procedures for checking timing chain wear (ie: "stretch"), as well as guidelines for changing the chain as part of preventive maintenance. proper oil changes and use of synthetic oil can extend the chain life from less than 100,000 miles to well over 300,000 miles. YMMV!

I do not have access to an ML manual but suspect that similar procedures exist. the chain tensioner may also be at fault. My 98 ML rattles slightly on cold startups but has done so since I bought it 30,000 miles ago. It just clicked over 150,000 recently. I blame the change in design of the chain tensioner on the start up rattles. The old tensioner design had a stepping piston that took up the slack under oil pressure and then held it at the most recent step achieved. Newer ones are purely hydraulic so when the engine is shut down and allowed to stand, the oil leaks out and the chain can go slack. Note: There is a small spring inside the tensioner that is supposed to hold the timing chain tight when the engine is off, but oil pressure is really important here. THIS IS A GOOD REASON TO KEEP YOUR FOOT OFF THE THROTTLE WHEN STARTING YOUR MERCEDES!

Finally, I bet a tensioner and chain replacement on a V6 would run close to $2000 in the dealer's shop, so this is no simple decision.

I would say keep using synthetic oil and hope for the best.

Jim in Phoenix

I would leave it alone. It's not too labor intensive to replace but I've yet to see someone break one of those. If there's no abnormal noises or running issues let it be. Save your money for something better than a 98 ML.

Checking the chain for stretch isn't a bad idea at 200K miles.. they will stretch & that can cause problems.

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