[Blackcat]

This is an old method to check for a blown head gasket. Take off cap(cold engine). Start car. Watch the coolent(flow). When the engine reaches operating temp. The fluid will flow in the radiator. Watch for a while(several min). Normal flow will have no or very little bubbles. Blown head gasket will have bubbles in the flow. Having the cooling system pressure checked should also show a bad head gasket.
This seems to be your problem.