possible radiator core frozen if no anti freeze in it,
pressure cap rubber failure, or a more serious fault, cylinder head cracked or the head gasket has failed .
compression test each cylinder, or use a radiator pressure cap tester on to the expansion bottle. run the engine and WATCH if pressure proceeds higher than the rated pressure of cap very quickly, If it does the head is cracked and the excess pressure is entering the water system .
wirecutterman, December 2009
