Saw the exact same thing today on a 300e. Customer had a bad radiator cap and a expansion tank leaking. They had air in the system from it leaking and spewing out coolant so much. We replaced thermostate , fill water pump ( below thermosate) full of coolant, disconnect upper radiator hose , fill coolant tank full of coolant until running from radiator . Take upper radiator hose full of coolant and put back on . Make sure resevoir full, put new cap on and run engine until thermostate open. Run heater until heat comes out .This should get all air out of system .Use Mercedes antifreeze , same price as any other good coolant.
