Get some of the radiator flush from Walmart and clean out the system, then add fresh coolant, Prestone works fine. You are only talking about $8 or $9 for a bottle. Also, be sure that you dilute it properly with distilled water, which is usually about $1 for a gallon. In NY, since it gets pretty chilly in the winter and hot in the summer, I run around 65 or 70% coolant to 35 or 30% water. Also in Walmart, you can pick up those little coolant testers for less than a dollar that tell you how much the coolant will protect.