You definitely don't want to run the engine without a thermostat. The temperature should be stamped on the thermostat. Call your local MB dealer and verify that the thermostat you bought is the correct one for your engine. Assuming it's the correct thermostat, is it installed correctly and is your cooling system filled with a 50/50 coolant mix? You may need a new cap for the expansion tank.

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Fred Hoelzle