Coolant level

One of two things, it is either leaking out somewhere, which you should be able to see puddles on the ground or while its running open the hood to see if its leaking out on your motor, hopefully its only a radiator hope or something, this is the cheapest fix. 2. It's burning out internally in your engine, if this is the case you should notice white smoke out of your tailpipe, bad news is if this is the case, its going to get pricey fast because that would indicate a bad head gasket or cracked head.

put new rad in bmw 316i water level stays but left hand side hose stays cold tried to bleed it no air in system