Keep an eye on your cooling system. They can develop leaks in places you can't see. 2 places that are the usual suspects are the external coolant pipe that runs around the back of the block and (mostly relates to G63B's) is the coolant chamber in the base of the inlet manifold. They are known to corrode out and form small pin holes from all of the crap from the block slowly collecting in it. And don't quote me on the diaphragm. I've seen overflows with and without, but if coolant isn't returning to the system and you're getting air you must be losing coolant somewhere.