Selina, it sounds like you have a leak somewhere, The Cooling system of vehicles is
pressurized. So with a leak, air is getting into the system, you are then replacing the coolant
that doesn't get recirculated, and that doesn't help cool the engine down, so it begins to
overheat. You added coolant to the expansion tank which helped turn off the coolant light,
the sensor is in the expansion tank, but the car is still hot and the coolant needs to
recalculate through the system. So you need to find the leak. It could be the expansion
tank itself, as it ages, it can crack, water pump or thermostat assembly. Dried white spots
are an indication of the leak when the car is cooled down as well as light blue liquid. It is
crucial you don't drive the car anymore, since the BMW's had aluminum heads and you can
easily warp them damaging the engine. IF the parts are original and you have over 100K
miles, I suggest you purchase a cooling kit which has the water pump, hoses, thermostat,
expansion tank, etc. and some BMW antifreeze from an online auto parts store like
FCPeuro,if one parts has failed the others are right behind them. There are DIY's online that
you can follow, it isn't all that hard if you can turn a wrench. OF course taking it to a
mechanic is always an option too. Good Luck.