i'd say headgaskt to. if you remove the radiator cap and tape a clear plastic bottle with the bottom cut off on top of the opening and pour water in till it comes slightly out of the radiator then start the motor. allow a bit of time till any trapped air comes out. then see if any bubbles come out. there is also a test kit that i believe you can get, that you can test the radiator coolant with. i think it tests for Co2 but i've not had any experiance with this myself.