MBZ are designed such that the thermostat opens the passage to the radiator AND closes the recirculation passage at the same time. Without the thermostat to block the recirculation passage there is very litte pressure to force coolant through the radiator and they will surely overheat.

Also, often old 'green' antifreeze begins to react with minerals in the water and the aluminum to form insoluble hard deposits in the radiator tubes. these have to be mechanically removed by 'rodbing it out.' Never use anything but MBZ antifreeze or the Orange stuff. Regular green stuff leaves those deposits when th eanit-corrosion additives give up the ghost.