Hello, I recently had a complementary inspection done to my '94 E320 wagon with 160,000 miles on it and the only suspect items were the two leaking oil cooling lines in the front bottom part of the engine and the belt tensioner. The tensioner is making a slight rattling noise when starting and during operation when something like a compressor kicks in. What happens when a tensioner completely fails and how dangerous is it for the engine? Quoted cost for the tensioner job is ~$400.

If it fails, and/or the chain breaks, you will have catostrophic damage when your pistons meet your valves.

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)

Pat,
I think he is talking about the belt tensioner, not the timing chain.
We just did a comprehensive thread on that same problem, so visit here:

All M104 engine owners, Please Read

the oil cooling lines are easy to fix, but belt tensioner
is a shop item, IMHO!

I plan on having it done in the shop but I still don't know what happens after a complete failure of the tensioner.

your car has only one belt driving all the pulleys in front of the engine. If the tensioner fails you would have no tension in the ONLY belt that drives the pulleys to the alternator, a/c compressor, engine, water pump, fan, etc. You will not go far after the failure, not to mention the damage that a flying rubber belt can do to those components already mentioned, and the radiator (which is right in front of that belt).
Other than that, you should not worry about it.
Good luck!

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J.H.
'86 300E

Maybe I'll drink less coffee while responding!

I should have never taken that speed reading class in the 70's.

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)

The rattling sound may be the worn bushing holding the shock to the shock mount...that is a fairly simple and less expensive fix than replacing the entire unit.

I've not heard of any belt tensioners failing in such as catostrophic manner as mentioned in the previous post. More than likely, the shock will wear out and the entire assembly will no longer be adjustable when a new belt replacement is in order, or a component sharing the belt needs to be replaced.

It is a moderate DIY job to perform, but time consuming, mainly due to the lack of open access to all of the components and fasteners involved. Some areas of the tensioner can't be readily seen, and have to be accessed by feel.

If a water pump replacement was in order, the task would be a bit easier...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I will not worry too much at this time, be sure te shock (rubber bushing) is in good condition. Replace the tensioner when the next time you are working around the belt.

jack

The sound you are describing could very well be the tensioner shock rattling. The mounts are prone to wear out. If its the upper shock mount, you can do a quick temporary fix by adding a rubber washer or o-ring on the mounting bolt. It worked for me.