2009 W204 OM646 Serpentine Belt Issue

[Aiden]

Good afternoon all,

Today I changed the thermostat on my W204 C220CDI, got everything bolted up, did an engine start and noticed straight away that the serpentine belt had slipped off.

There wasn’t any tension in the belt at all and I can’t figure out how to re-tension the belt.

The only thing I can think of that may have caused this, when I was unbolting the plate next to the thermostat housing (that sits just above the belt), I’d accidentally undone the wrong E10 bolt, below the one I needed for the plate. I noticed oil on my hands, have I potentially ‘depressurised’ the tensioner? The offending bolt was nipped back up again. The belt has been correctly routed but is doesn’t have enough tension.

Thanks in advance for any help and happy new year to you all.

 

[Uncle Benz]

The tensioner is sprung, very unlikely to be a problem with that. Are you sure you have the correct belt routing?

 

[Aiden]

Hi there - thanks for the reply. I will double check the routing, I took it from the Haynes manual. The belt was tight before I did the thermostat.

 

[EmilysDad]

this is the perfect job for taking a photo before you start.

 

[Aiden]

this is the perfect job for taking a photo before you start.

Oh yes, in my defence it was just supposed to be the thermostat being changed, not anything to do with the belt!

 

[Blobcat]

Oh yes, in my defence it was just supposed to be the thermostat being changed, not anything to do with the belt!

There’s plenty of pics in the interweb, just do a search on your engine

 

[Aiden]

There’s plenty of pics in the interweb, just do a search on your engine

I will do, I have used the picture in the Haynes manual - I’m more wondering why it has slipped off in the first place.

 

[Uncle Benz]

Can you show us which bolt you mistakenly loosened?

 

[Aiden]

Can you show us which bolt you mistakenly loosened?

I will have to get a picture in the morning, but I would describe it as an E10 bolt that is just below the [E10] bolt that holds a roughly triangular shaped plate.

 

[Droverunner]

I've changed the belt and all the pulleys on our 646 and at the moment I can't think of a single bolt that would lose tension and cause oil to run out. Be interesting to see your picture.

 

[Aiden]

Good morning all,

Thank you so much for all the assistance yesterday afternoon. The issues are now solved, and I think I know why what happened happened.

The E10 bolt mistakenly loosened - a total red herring. Nothing to do with either the belt or the thermostat. I think it’s something to do with fuelling, as there is a line from the common rail to it.

The tension on the belt itself? I had looked at the wrong drawing, the 651 engine. As soon as I corrected the belt route for the 646, the tension was there (once I’d levered the tensioner down to get the belt over).

I believe when I was getting the thermostat out, I had inadvertently slipped the aux belt off the nearby roller, leading to it completely coming off when I started the engine.

Thank you all again and happy new year.

 

[Droverunner]

Ahh a good result in the end... well done.