Wow, never woulda guessed. Thx for the info!

 

How to replace timing chain tensioner
In every OM642 timing chain tensioner should be replaced every 100k ml.

 

Replace timing chain tensioner.
Will take you 1 hr and $150.

 

Great job Alex! Thank you for shooting this video!

 

Timing chain replacement data point:

I replaced my timing chain tensioner as Alex Garage suggested. Very easy. My tensioner came apart the same way his did in the video. I installed the new tensioner and torqued it to spec. The chain “slap” was even worse.

I think this is evidence the the timing chain has stretched and that the chain needs to be replaced.

It seems to me that installing a new tensioner is only applicable if the timing chain hasn’t stretched (or the stretch is “minimal”). If the chain has stretched the new tensioner won’t extended far enough to take up the slack.

Appreciate any feedback and/or correction to my supposition.

 

Alex Garage is completely correct. Replace the tensioner first before changing the chain. I was having difficulty starting and significant chain rattle at 138k miles. Was told to replace the chain. There is no way a Mercedes diesel timing chain wears at 138k. I replaced the tensioner and the truck has never run smoother(since I’ve owned it). This job is 30 minutes. The only change from Alex Garage is that I did it from the hood and did not remove the wheel. It is a PITA, but easier than removing front tire and splash guard. Angle the replacement tensioner pointing upward into the block more than you think is appropriate, that is the trick to line up the threads. Finally, a tensioner that is over extended means the tensioner is bad, NOT the chain!! It is the pressure applied to the chain, not the travel that matters. I can’t stress enough, these chains should last a LONG time. Expect 100k tops from a timing chain tensioner. I wish I did mine years ago as the motor runs so much smoother now.

 

One more note. After changing the tensioner, it took several minutes of idle for the rattle to go away. Give the motor 5 minutes to run for the slack in the chain to work it’s way out. It made some funny noises and then ran smoother than I’ve ever seen.

 

GL3502011, was the rattle issue that you faced only on cold start or was it a consistent rattle hot or cold?

 

the rattle was consistent hot or cold but did build over time. I first noticed it at start up, then as the tensioner continues to wear it will become a constant rattle like something is broken in the engine. It also became difficult to start as the timing is affected by a loose chain. Replace the tensioner and let it run for a few minutes, the rattle will go away

 

got it! Just bought the tensioner. Any tips on changing it. I plan on going the same route you went and remove from the top.

 

yes, it took about 20 minutes, the trick is the angle getting the new one in is a bit different than you would expect. It is easier if you remove stuff. I did find a video which recommended removing the wheel I think which I did not do.

 

This video was helpful but I was able to change it from above

 

I can second this.

The current severity of chain slap on my 2012 GL350 is that there is a second or two ratlle/slap of the chain during cold start only. If I start the vehicle 4-6 hours later there will be no rattle. It seems like this only happens when it sits longer than 8 hours.

I removed my current tensioner, and I found the same issue with the tensioner pulling apart with one hand as Alex and Napa had. It was fully extended.

Replaced it with a new tensioner; first thing that caught my eye is that the new one is much shorter than the current. At the time I am thinking that tensioner is automatically adjusted with oil/oil pressure from the engine since the current one is fully extended and when you push/retract the tensioner I notice that the body of it is holding some oil. I put this new one in and started it up, same result as Napa, chain slap was even worse and consistent.

I waited 10 minutes to see if the rattle would go away as GL3502011 mentioned that it takes some time for the slap to go away. It did not go away after 10 minutes.

Decided to put the old tensioner back and the chain rattle issue was fixed (no longer consistent, but still happens on cold start). When I removed the new tensioner I can see that it did not extend at all and you can see that there was no contact/scar marks of the tensioner to the timing chain.

My opinion is that the cold start rattling is not ideal but still ok to drive for now. This is my thought because the chain is still tensioned properly during 99% of operation (when car is not cold). I am thinking that once the rattling gets to the point where it happens on every start or is consistent or you have extended crank issues then it is time to change the timing chain. I may be totally wrong here so please let me know if I am and please explain why you think so. Thanks!

 

This is a very confusing thread. I have read all the posts three times. They seem to contradict themselves. One says one thing, one says another.

 

These actually do not lock and remain fully extended UNTIL oil pressure is reached . Thus the slack fix when cold starting .

Oliver, what do you mean by this. Are you saying this in relation to your picture of the new tensioner that is extended in your picture?

 

So your not talking about the 642 engine.

 

i am talking about the OM642 engine in my 2012 GL350

not a mechanic but am an engineer so bare with me I am going off of my limited mechanical knowledge.

My understanding is that oil pressure is not dependent on engine temp. Oil pressure is an output of the oil pump in the engine which is driven internally which means that oil pressure is reached virtually instantly once the engine starts cranking.

going back to the tensioner. I gave it the benefit of the doubt and let the engine run for 10 minutes and still tensioner did not extend AT ALL and was not willing to wait any longer as the chain was rattling and hitting the timing cover.

i agrée with you though thread is confusing. Alex has stated he changed the tensioner and it worked. Napa and I also tried it with no luck. If you would like to try it I say go ahead it really is a 10 minute job. No need to remove the plastic wheel well cover.

 

This is not entirely correct. First of all, a hot engine has hot oil. Hot oil is less viscous, so the oil pressure can be lower, especially at lower RPM, than a cold engine.

Second, the hot oil tends to drain back into the sump once the engine is shut down. The more oil that drains out, the longer it takes for pressure to return during cranking or initial running. Add to that the more viscous cold oil and you could be looking at 15 - 30 seconds after a cold start to make full oil pressure everywhere in the engine.

So, I'm guessing either the tensioner is faulty, or the oil flow was blocked.

 

Thanks John for chiming in, I knew there was other external factors regarding pressure but we can both agree that pressure builds very quickly and the 10 minutes run time was definitely more than enough for the tensioner to have extended and remove the slack from the chain. John you had mentioned that the oil flow was blocked or the tensioner was faulty. I did double check if oil flow was blocked; I had blown compressed air into the oil port and noticed that there was oil in the body of the tensioner. Not saying that oil flow was unrestricted but it was definitely not blocked as oil made it to the tensioner. I do want to state that the tensioner I bought was from FCP euro and did not look the same as the tensioner that I pulled out. I put a snapshot of exactly what I purchased for my vehicle and maybe the wrong tensioner or faulty tensioner was purchased. I would appreciate any inputs from Mercedes techs on this forum on this.

If anyone is thinking about doing this, I would say to just pull the trigger and try the tensioner swap, it if doesn't fix the issue, you now have a tensioner on hand for when you decide to do the timing chain swap. This is my current plan; hoping the tensioner can keep the chain taught for atleast a couple more years.

 

the tensioner doesn't hold oil pressure when car sits for a long period of time.

as you crank, pressure does build up but it isn't instant, and these cars start very quick..

I have another car that has the same issue. I drive it only on weekends and end up pulling fuel pump fuse in order to crank for 10-15 sec and it avoids start up rattle.. If I restart it after an hour or so, there is never rattle..

I wonder if the tensioners have a built in one way check valve to retain oil pressure.

If not, the chain is most likely stretched, and this is the cause of tensioner not able to fully extend to required point to avoid rattle.