adding water

put water in your over fill resvior

If the radiator cap is missing the radiator coolant will leak out as the engine warms up and will definitely cause over heating. Replace the cap and make sure the antifreeze/ coolant concentration is OK if flush the cooling system and replace the coolant. The radiator cap on all modern cars creates pressure in the cooling system of about 15psi. The pressure increases the boiling point of the cooling system.