Warning! Do not remove the expansion tank cap on a hot engine, as hot fluid vapors can burn you.
The coolant level in the expansion tank varies with temperature. On a cold engine, the coolant level should be at the mark on the side of the expansion tank. After the engine warms up, the coolant level should rise above the mark.
Wait until the engine has cooled down and slowly unscrew the plug from the expansion tank.
Pour a mixture of water and antifreeze into the expansion tank up to the mark. Screw the plug onto the expansion tank.