Are you sure you bleed the system correctly? BMW are notorious for being hard to bleed. The radiator water level need to be higher then the T-housing. You should part uphill...start the car....turn heater on full...open the radiator and the t-housing bleed screw...and keep pouring water in constantly and do not let the water level in the radiator go below the t-housing level. (NOTE: car must be at working temp for this to work properly since the thermo needs to open) You MIGHT still have a little air in there. You know all the air is out when your filling the radiator and the t-housing bleed screw has a constant flow of water. Im sure you might already know this, but it took me trail and error to finally do it correctly. I almost bought thermogasket (very reputable product to seal cracked cylinder heads and bad head gaskets) but then bleed it correctly and tada. No more overheating, no more low water level, and no water anywhere. Try it.