The radiator cap should only be removed when the engine and cap are cool to the touch. Coolant expansion tanks are usually translucent and have clearly marked minimum and maximum levels (sometimes cold and hot markings). Having to continually add coolant is an indication of a more serious problem that should be diagnosed and repaired. Be advised, the use of generic "green" coolant may not meet the specifications of your vehicle. There is a wide array of coolants, specified by each manufacturer for their vehicles. Should your vehicle require an addition of coolant, please verify in your owner's manual you are using the correct fluid for your vehicle.