I'd like to try a new thermostat in the 1984 300SD. It has the tendency to run a hair below 100C - but there are times when I look down at higher speed on the Interstate and see it at 85C or so, then it moves back up.

Radiator exterior looks clean and so did coolant during a recent flush and renewal.

Also considering the possibility of the water pump having problems, but usually you get a leak failure rather than a pump vane breaking off.

When I procure the Behr on FastLane, I see it comes with the sealing ring.

Do I need to buy the Thermostat Housing Gasket? It says its for the housing to head junction.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles