To keep the water in the cooling system from boiling or freezing, the water is mixed with coolant or antifreeze.
Car Smarts: Most coolants contain about 95 percent ethylene glycol, a chemical that stops water from freezing or boiling even in extreme temperatures. (Ethylene glycol is toxic; there are nontoxic coolants that contain propylene glycol instead.) In addition to the glycol, coolant also contains rust, corrosion, and foaming inhibitors, so coolant does more than just keep the water in the system in a liquid state: It also helps to prevent the formation of rust on the metal surfaces of the engine and the radiator, lubricates the water pump, and keeps the liquid from foaming as it circulates through the system. Since the early 1960s, auto manufacturers have designed the cooling systems of most vehicles for a 50/50 mixture of ethylene glycol and water, which is still generally considered the proper proportion of coolant to water for thecooling systems of most vehicles.
Today's engines require specially formulated coolants that are safe for aluminum components. There are long-life (sometimes called "extended life") coolants with organic acid rust and corrosion inhibitors that promise to last for as long as five years. Automakers use some of these coolants as original fluid in new vehicle radiators made of aluminum (they can't be used in anythingexcept aluminum radiators).
Caution: Due to differences in Japanese, Korean, European, and American compounds, you can shorten coolant life by putting the wrong stuff in your vehicle's cooling system. So make sure that you use the automaker's recommended coolant toensure the longest life and best protection for your vehicle.
Tip: If your cooling system is operating properly, you shouldn't have to keep adding liquid to it.
From Auto Repair for Dummies, copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana. Used by arrangement with John Wiley & Sons, Inc.