Cooling system

Wow, what a dramatic question/situation....and all I can do is deduce from your story-line.... If the coolant reservoir on the BMW is totally empty, it is likely that air has contaminated the system...so when you filled it back up, you really did not fill and bleed the system. As a result, there was not much coolant in the radiator, and a large air pocket I would imagine as well. When the car heated that pocket (which the water pump cannot push/remove) to a certain temperature...it may have compromised your radiator. Stop everything. Let the car cool completely. Then, start and refill gradually, making sure that all the air escapes and the system is full. Now, as you refill, if everything pours out of the radiator (perhaps at the temperature switch, as there is no "heating element,") Well then there is your problem... The cooling system has to hold pressure and be free of contamination/air...for that BMW to operate properly. Oh, and turn on the heat full blast...as that opens up the path to the heater core, and assists in bleeding air when you refill the system. It also confirms the flow of the coolant itself! That's the best I can do with what you have provided...but if you would like to give me further details...I can help you knock this one OUT!