Your thermostat mixes hot water exiting the block with cool water from the radiator in order to supply water entering the block at 80*C.....after the water circulates through the engine it exits the block roughly 10*C higher than when it entered, or about 90*C......this is good!!!

If you have an MB Service Manual, see section 20-005 "Coolant Circuit and Engine Cooling"......this will give you a nice write up on how your cooling circuit works and what to expect from it....

And FWIW the Behr OEM Thermostats have a little arrow stamped into the edge ......this little arrow points up.....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG