I have a 2002 E39 M5 with 78K miles and have changed the coolant twice (I've owned it just over 3 years). A problem I run into is that I can't get more than 2.1 gallons of old coolant out. Capacity is 12 liters (~3.1 gal). I've tried leveling the car before draining but that made no difference. It's possible that the remaining fluid is in the heater core or the system that allows residual heating with the engine off but I don't know of a way to force that part of the system to drain. Note that I do drain the radiator and remove both engine block plugs. What am I missing?