Most vehicles have coolant recovery systems. Some are pressurized and serve as the fill tank when you need to add coolant, but there are many vehicles still on the road that require you to add liquid directly to the radiator. The following cautions apply no matter which type of vehicle you have.
Never add cold liquid to a hot engine!
Caution: Adding cold liquid to an engine that's hot can crack the engine block because the hot metal contracts sharply when the cold liquid hits it. If you must add liquid to an engine that's still warm (and I can't imagine why you should) always do so slowly with the engine running. This way, the cold liquid joins the stream of hot water that's circulating through the system rather than falling all at once into the system when you start the engine again. This technique isn't foolproof, however, so I advise patience instead.
Under normal conditions, a 50/50 mix of water and coolant is preferred for most vehicles. If the day is extremely hot or cold, a higher proportion of coolant/antifreeze may be necessary.
Don't overfill the system!
Eco-Logic: If you overfill the system, the extra liquid gets hot, expands, and flows out of the overflow pipe. That may not seem too terrible, but because coolant is toxic, it can harm animals or children, who love its sweet taste.
If you don't have coolant on hand and you just need to add a little liquid to the cooling system, plain old tap water will do. But try to maintain a good coolant level by adding a similar amount of straight coolant the next time you add liquid to the system.
Caution: Several types of coolant are on the market. If your vehicle has an aluminum engine, make sure that the coolant container specifies that it's safe for aluminum engines. If you have extended-life coolant in the system, only addextended-life coolant to it.
Tip: Keep some coolant in your trunk to use in case you're driving in an area where you may not be able to find the brand of coolant for your system.
From Auto Repair for Dummies, copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana. Used by arrangement with John Wiley & Sons, Inc.