[dsraven]

check the cap to see if it has a gasket on the end, like the spring loaded end,plus another gasket right up under the part you turn. the second one seals the cap against leakage when the spring loaded end releases rad pressure into the overflow. older style rads didn't have that second seal, just the initial spring loaded seal. the second seal or gasket on the newer style should seal against the rad neck at the very outside or top part. when the rad cap blows off the coolant is supposed to go into the overflow tank and the tank should be sized to take the amount. if it is dumping outside then the cap is leqking or the overflow is not big enough or you have another issue. a pressure test would be a given, if you have access,but in the meantime look for the obvious, watwr pump lek hole, intake manifold ends where they meet the heads etc.