Changing the antifreeze in your car's cooling system is important for two reasons - corrosives and scale build up in the cooling system and the additives in the antifreeze wear out, just as they do in motor oil. And keeping the right amount of antifreeze in is important for maximum fooling effectiveness, as well as for providing adequate protection against the coolant freezing when the temperature drops.
Every two years you should drain the coolant (which consists of water and antifreeze) from the engine, flush the system, and refill with clean water and new antifreeze. Here's how to do it:
Park the car on a level area, or if possible on a slight incline. If you park on an incline, point the front of the car down the incline. That's for maximum drainage later.
Put a pan under the radiator for the coolant to drain into. Look at the bottom of the radiator. You'll see a drainage petcock. Loosen it, but not enough so coolant can flow out. Just enough so you can turn it easily with your fingers.(Be sure while you're doing this, of course, that the engine is off, and that you have the ignition keys in your pocket. You don't want someone to fire up the old buggy while you've got your arms and hands in the neighborhood of the fan.)
Remove the radiator cap. The engine should be cool when you remove the cap. Just press down on the cap, and twist it counterclockwise until it comes off.
Start the engine and let it run 10 minutes or so until it reaches normal operating temperature. This causes the thermostat in the engine block to open so the coolant in the engine can drain out. And as with motor oil, when the fluid is hot, the contaminants in there tend to flow out instead of staying behind.
Stop the drain petcock at the bottom of the radiator, and let the coolant drain out. When the coolant stops draining, shut the petcock with your fingers and pour a can of cooling system flush into the radiator. Then fill the radiator with water.
Start the engine and let it run another 10 minutes or so.Leave the radiator cap off now. And keep an eye on the coolant level in the radiator. Add water if the coolant level drops.
Shut the engine off, pocket the keys, open the drain petcock and let the coolant drain out.
Close the petcock and fill the cooling system with water from a garden hose. After the system is filled, open the petcock all the way, and adjust the flow from the garden hose so that it matches the flow from the drain petcock.
After you've got the flow adjusted, start the engine and let it run for awhile. As it runs it circulates the water through the cooling system, flushing out unwanted scale and contaminants.
Keep an eye on the water flowing out the drain petcock. When it becomes clear, stop the engine and let all the water drain out.
Now close the drain petcock with your fingers, then tighten it with pliers. Don't overtighten, just snug it up.
Now pour antifreeze in the radiator. Add the amount indicated on the antifreeze container to give you freezing protection down to the temperature you want. You need to know your car's cooling system capacity to do this (which you can find in a service manual for your car, or by asking the service manager at a local dealership).
After putting in the antifreeze, if there's any room left in the radiator, finish filling with water. Now run the engine for 10 minutes or so, keeping an eye on the coolant level drops. Continue running the engine and adding the water until the coolant level stops dropping and the radiator remains full. Shut off the engine and replace the radiator cap.