To much build up pressure in coolant system!

White smoke out the exhaust? With that info I'm with RRR, it's either the head gasket or a crack. If you have the equipment to do a cylinder leak down test that would be the next step, or a compression check as already mentioned. The leak down test would be a little more accurate in pinpointing the leak rather than just telling you the low cylinder(s), but at that point it's pretty moot as it all has to come apart anyway.