I hope he thinks the water pump is not pumping, therefore hot water in the engine is not getting to the radiator. me, i'd put a pressure tester on it and see how fast and how high the pressure rises and watch the temp on the motor and the rad. with a laser, then decide. a new cap is cheap, but usually when the fail, they won't hold pressure. maybe get a 2nd opinion? could be a head gasket or cyl. head problem. good luck, chuck.