I recently bought an '85 300sd with 268k.
The owner clipped some kind of high curb & damaged/distorted but didn't break the radiator.
Then he let it sit for 14 years in Mississippi, full of good coolant the whole time.
I put in a new Nessens radiator & it ran hot, the thermostat never opened.
I bought a Mahle/Behr thermostat off eBay & it got hot again. I put it in boiling water on the stove & it didn't open until it hit 220ºF.
I bought another of the same off Amazon & it does the very same thing.
I bought a Mercedes one from this site. Engine and back of the thermostat housing still got 200ºF hot. I put the thermostat in a pan on the stove & it opened up at 185ºF.
I put it back in the car, made extra sure the system was completely full & it got hot again. The back side of the housing gets too hot to touch yet the front of the housing stays cold as does the radiator.
When I pull the thermostat cover off, it's always completely full of coolant.
My first job at 16 was working at an indy Mercedes shop when this car was new. I've been working on them ever since & have owned quite a few, especially diesels.
What the heck am I missing?
Thanks y'all.