Q: my overflow tank is full and if i add more water to my radiator where will that water go?

To begin with: you don’t want to put straight water into your cooling system. You need to use a 50% mixture of water and anti freeze. You put it in at the radiator cap. Start with the engine cold and off, and fill the radiator slowly, then, leaving the radiator cap off, start the engine and turn the heater onto MAX. As the engine warms up, the level may go down some so you continue to top it up. Be careful looking into the radiator, if the cooling system is building pressure for any reason, it may belch HOT coolant out into your face! Continue to run the car until the engine is warmed up and there is heat coming out of the vents. Then re-install the cap. The overflow tank is just that, it catches any coolant that blows out of the radiator because of pressure buildup. I like to leave it about 1/2 full in most cases. If the system is working properly, the radiator will blow out small quantities of fluid, and then draw it back in as the engine cools. If you would rather have someone else do this for you, you can contact Your Mechanic. They will send a technician out to your home or office to Top off your cooling system for you.