The reason why the radiator needs to be replaced is the same reason the rest of the cooling system needs to be replaced: Plastic. The radiator has plastic side tanks, which after 75-100k miles, or 7-10 years, will eventually become perilously brittle from being subjected to years of heat cycles. The cooling systems in BMWs are a known weak spot for this very reason, and due to the aluminum cylinder head, even one mild overheating due to a cooling system failure can warp and/or crack the head, resulting $1200+ worth of damage. For reasons unknown, BMW used low quality plastics in the cooling system, which means that unlike with many other car manufacturers, we have to replace our cooling systems preventively, BEFORE something breaks and the engine overheats. With the radiators, the hose necks will suddenly snap off and you will dump all of your coolant out in an instant. Before you realize what's going on, your temp gauge will be creeping toward the red and it will already be too late. The same idea is true for all the other plastic items in the cooling system - the plastic will look totally fine, until it snaps without warning and you have a catastrophic system failure on your hands. So this is why you must replace the cooling system parts which, although they may look to be in perfect working order, they are a serious liability if they have over 75k miles on them.
The cooling system parts you should replace are the following:
- Expansion tank
- Expansion tank cap
- Bleeder screw (replace with brass unit)
- Thermostat housing (replace with aluminum unit)
- Water pump (be sure to use one with a metal impeller)
- Upper & lower radiator hoses