Check the temperature of different sections of hose/radiator, etc.. and you should be able to at least tell if you are circulating hot water (thermostat is opening) through the radiator. If one hose is hot and the other hose it not then you have an issues in the circulation system. On my 240D I had to get the thermostat from a local parts house because the MB thermostat would not open.
I will say that the warmer/hotter weather we have been having has done wonders for both the 240 and the 300D.

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Jim