[MEATMASTER]

Not sure what it all might be, but check these points first. Remember, the expansion tank on the engine should not be filled more than half full of coolant. It is there to collect air and allow the coolant to "expand" inside the tank without overlowing. If you fill it full, most likely it will overpressurize the expansion tank and dump into the overflow tank, and if that fills up, it will flow out of the top of it. As long as your engine is fine (head gaskets not blown and so forth), the only other thing it might be is a non-sealing radiator cap or the more rare event, the cap spring itself not up to the rated 13 lbs or more pressure capacity. If so, simply try a new cap.

MEATMASTER "...cooking sausages on an intake manifold is an art Martha Stewart couldn't even dream of..."