I know we all know what this is,,,,,if you have owned a BMW for sometime,,,,,you probably have replaced one in your life time. I know we have all seen these expansion tank crack on the side and start leaking coolant everywhere inside the engine compartment,,,,and it’s always at the wrong time when it cracks,,,,but sometimes they don’t crack,,,,sometime you notice you always have to top off coolant like every week,,,but you don’t see any signs of coolant leaking,,,,what might be happening is you see this hole on top of the expansion tank,,,,well the tank can leak out of here,,,but you don’t see it. Here’s the underside of the tank,,,,the leaks is not major,,,,but enough to cause a problem,,,if you pressure test the cooling system,,,,you can hear air hissing out of this hole.