Repair-It

This probably doesn't even warrant a thread but whatever. I think my thermostat is doing what these all do: the rubber stuff disintegrates and the thermostat over-flows and then the engine runs too cold. I don't have a CEL but the temp will mysteriously drop from warmed up sometimes, and maybe warmups are slower than they should be. Plus, now that I have the AMG oil temp display, the oil temp also suggests the engine is not getting up to temp.

This should be an easy job.
Thermostat: A2722000515
https://www.mercedesbenzpartsshop.com/o ... 2722000515

Gasket: A2722030180
https://www.mercedesbenzpartsshop.com/o ... 2722030180

Looks like they stock both. Under $100 for everything.

O ring
A0039971889

Not quite as easy as I was envisioning.

The S-class version of the M273 uses a unique cast aluminum manifold that connects to the thermostat. Also the new thermostat came with a gasket so that was a waste. Forgot to order an O-ring for the manifold so I ran to the dealer and paid retail for that ($10).

The only other "trick" to this job is I had to loosen the bolts on the air pump and flex it to the left just slightly so I could get my E10 on the upper thermostat bolt. Maybe if I had some kind of low profile deal or maybe a ratcheting E10 I could have done it without moving the air pump.

Did not see anything visually wrong with the old one.
Vacuum filled and should be good for another 100k.

The heat is definitely working better now. And the oil warms up more. Sweet.

I think my thermostat is defective? On my drive home temps fluctuate kind of like what was happening originally. Oddly it never seems to happen on my drive into work. Can't really explain that. But I may see if I can get another one from the dealer under the warranty.

Probably unlikely, but you may want to try to see if there are any air pockets resulting in the temp swings before you pull/swap the t-stat.

I agree.

Called dealer parts and asked about any warranty. They said I have to have service look at it and determine if there is actually a defective thermostat or something else (or it's "fine"). If they determine there is a defect with the thermostat they will actually replace it. Including labor. This kind of baffles me.

I assume if there is not a defect I get stuck with the diagnostic fee which is probably 2x the cost of the thermostat....

Interesting though. I will check coolant level and also see what kind of diagnostic I can run in DAS (which they would have). I don't fully trust that it will instantly misbehave based on what I am seeing.

Perfect coolant level. That's not it. Can't be air either as it initially was fine for a while then started this fluctuation. It's not going high and coming down. It's coming up to normal and then dipping.

Found the thermostat test. I have never run this before because it has specific starting temperature requirements and I guess it's always over 55C when I've tried before. So I ran it. Looks like it graphs a min and max acceptable temperature and then the red plot is the actual. So I guess you just watch it over the 600 second test and see if the red plot stays in the bounds. It runs the cooling fan on high which makes sense.
Definitely trending bad. And there it is. So it finished like that. It's not automated though. So I said, not ok. Oh, there's something wrong with my thermostat! Amazing, I hadn't already determined that.

There is also this screen that I could monitor while driving. Shows actual vs target coolant temp. I'm sure this would deviate while driving as well. So I think this was the smoking gun. Provided they run the same test the result should be the same. Hopefully they accept that it's a bad thermostat and not go chase wiring with the breakout box...

You can print this post and bring it to talk to the service advisor with...