I will second Kai's post. you CAN use tap water and it will cool the car fine, but a gallon of distilled water is less than a buck. I've replaced the radiator on my '67 mustang and let me tell you....it wasn't pretty! flush your system when you get spare time and do it right. and for a few extra bucks, a bottle of Water Wetter [made by Red Line, I think] can't hurt [you only add a few capfulls].