Car radiators contain a 50/50 mixture of engine coolant and water. The coolant mixture circulates through the car's engine, where it draws heat from the engine, allowing the engine to cool. The coolant then travels back through the radiator, where it passes between the tanks via small tubes fitted with cooling fins. As the coolant moves between the tanks, the cooling fins draw the heat out of the coolant and release it into the atmosphere. This process cannot work unless the radiator contains the proper amount of coolant and water.
Open the radiator or coolant overflow tank, depending on the year, make and model of your vehicle. Never open a cooling system unless the car's engine is cold. If your car has a radiator cap, the cap will be located at the top of one of the radiator tanks. If not, the overflow tank is usually translucent and equipped with a cap that has a coolant symbol on the top.
Place the funnel into the radiator or coolant overflow tank, if necessary and then pour in one gallon of coolant. At the time of this writing, there are at least six different types of engine coolant. Make sure you know what type of coolant your car uses before purchasing any. Never mix different types of coolant, regardless of what the coolant may say on the bottle.
Pour one gallon of distilled water into the radiator or coolant overflow tank. Never use tap water, as hard water deposits can plug up passageways in your radiator and the engine's water jacket.
Pour half of the remaining coolant into the coolant bottle you emptied in Step 2. Make sure the two coolant bottles now have half a gallon in each one. Fill the first bottle with half a gallon of distilled water to create a 50/50 mixture.
Pour the 50/50 mixture into the radiator or coolant overflow tank until the tank is full. Start the engine and turn the heat on inside the car. Allow the engine to run until the temperature gauge starts to rise and shut the engine off. Top the radiator or coolant overflow tank with 50/50 mixture as needed and reinstall the cap.
Never open up a cooling system when the engine is warm or hot. Pressurised steam heated above 93.3 degrees C can and will escape, potentially causing serious injury or death.