did you just add water? and how many times did you only add water? the coolant .. which is in your radiator and coolant overflow tank should be a mixture of 50/50 water to antifreeze (the green stuff) you should go to a acura dealership to pick up a bottle of pre-mixed coolant. you could be boiling it off. check for leaks and check for white smoke out the tail pipe .. its either leaking, evaporating, or burning off. Get a new radiator cap while you are at it. .