Hello everyone. We have a very nice, well serviced (every five thousand miles) 2013 C250 (my sons) that has recently developed an issue. Had the timing chain tensioner replaced as a preventive maintenance item while the alternator was being replaced (bearing issue). The car has 125k miles and never had any startup related timing chain noise on cold start until this tensioner was replaced. New Mercedes dealer part. Now when starting the car, there is a less than one second chain rattle when the engine is cold. The timing was checked not too long ago per the WIS (almost perfect alignment, exhaust mark off a "hair" but likely due to normal wear) and the cam sprockets are like new with no discernible wear.

Anyone have any ideas? Did something change with the tensioner part? Always use a 229.5 0w40 or 5w40 (Mobil or Liqui Moly). The car has not been driven much since this was done.

Any thoughts / comments are appreciated.

Thank you.

Is the CEL on? Even if it isn’t check for a pending P0340(62). That code may not set the CEL unless it occurs 3 times in a row.

No CEL's and no pending/stored codes of any kind. I've checked with both of my scanners.

Then it’s got to be the chain and not the cam phasers.
If I understand the post, you had a shop replace the tensioner.
There is specific sequence on releasing the tensioner. They should know it.
This is the sequence as I remember it without a camshaft alignment fixture in place.
With the tensioner out, rotate the crankshaft just far enough clockwise to place the slack on the tensioner side of the chain.
Install the tensioner.
Rotate the engine slowly counter clockwise. If it’s quiet in the shop a click can be heard when the tensioner releases. It should also be felt in the wrench as the tensioner takes up the slack.
Install a new tensioner cover plug.

Thanks for the reply. I believe they followed the correct sequence. It's just such a strange occurrence. Not sure where else to go with trouble shooting. Once the initial 1/2 second or so of noise is over, the car runs totally silently with no noise during idle, acceleration, etc. No codes, and I do have quality scanners.

Any other suggestions? J

Those timing codes can be seen on inexpensive hand held code readers. When I get this kind of “what the heck” result on something I did myself, I go get a hands on opinion from one of the gear heads I know. My only suggestion is to bring it to the attention of the shop that did the tensioner.