I also have a 240D, mine is an 80 but I believe its the same cooling system wise. There is a bolt in the top of the thermostat housing that when removed while running and filling will allow you to get most of the air out. As soon as your antifreeze mixture begins to come out replace the bolt and check to make sure the radiator is topped off before replacing the radiator cap. After driving for a day or two I'd check it again and top it off one more time. Good luck!