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How To Flush Your Coolant

how to change coolant

Winter is fast approaching, and you may think that means you don’t have to worry about your car’s cooling system. People assume that if it is cold outside, the chances of overheating are slim. While that is true, if you still have years old coolant in your radiator, you may be in for a nasty surprise when the temperature drops.

Antifreeze is not just a clever name. As you might have guessed, its main job is to keep the fluid in your car’s cooling system from freezing solid, expanding, and breaking things. Water actually does a better job of cooling hot parts than antifreeze, but it also freezes at 32 degrees, and boils more readily. Besides staying liquid, modern antifreeze is chock full of chemicals to reduce corrosion in aluminum and iron parts, and lubricate the water pump.

You should be flushing and filling your coolant about every other year, or with each 30k mile service. Do this regularly, and it is easy, requiring just a drain and fill, repeated three times, to get the old coolant out of the motor, radiator and heater core. Put it off for too long and the tiny passages in the radiator start to get plugged. When that happens there are flush kits with detergent and an adaptor to clean the whole system out with your garden hose.

Remember, never work on the cooling system of your car when it is hot. The coolant in the radiator is under pressure when hot and can spray you when you open the cap, resulting in a nasty burn.

Also, a word of caution about antifreeze: Antifreeze, even the new lower toxicity formulas, is poisonous if you drink it, but it is also very sweet tasting, and candy colored, so it can be attractive to small children and pets. Never leave an open drain pan of antifreeze around. Always pour it into sealable jugs and dispose of according to you local laws. Never pour it in a storm drain or on the ground.

For this you will need a large flat drain pan that will fit under the radiator, and hold about two gallons. The type you use for oil changes will work, but you don’t want to use one that has already been contaminated by oil.

You will also need a funnel, a bucket, a source of water, and several gallons of new antifreeze. Check the color of the antifreeze currently in your system, and match it to the new fluid at the auto parts store. Or, just purchase one of the “universal” types that are compatible with green, red, or orange.

Make a mental note of whether you have pre-diluted, or full strength antifreeze.

Out With the Old

Open the radiator cap, and the cap on the overflow bottle so air can get into the system. With the car and motor cold, position the drain pan under the radiator drain petcock. If your car doesn’t have one or you can’t get to it, you can remove the lower radiator hose instead. Open the valve, or loosen the hose clamp and pull back the hose, and let the fluid drain into the pan.

Close the drain, or reattach the radiator hose, and fill the radiator with fresh clean water. Just water, not antifreeze. Start the car, and turn the heater controls to hot to circulate fluid through the heater core. While the car idles, continue to top off the water level in the radiator. After a few minutes, when the electric fan has come on, shut down the car and let it cool for a few hours.

Empty, Fill, and Empty Again

Once the radiator is cool to the touch again, repeat the process above. Open the drain valve, or remove the hose, and drain the water and what little coolant has been flushed out. This time the fluid you drain should be closer to clear, with not much of the colored coolant present.

Close the drain, reattach the hose, fill the radiator with clean water and run the car again. Once it is warm, shut it down, let it cool and drain again. After filling and draining it three times you should have almost clear water coming out of the radiator. Button it all up again nice and tight.

This is a great time to replace any heater or radiator hoses that are old, spongy, or looking marginal.

Fill, Then Fill Again

At this point you are ready to fill the car with a fresh antifreeze and water mixture. The back of the bottle ought to give you a dilution table, but 50:50 mixed with water is generally fine. If you have pre-diluted antifreeze, just use it as is.

Check your Haynes Manual for any air bleed screws that should be loosened when refilling the cooling system. These are typically placed at high points of a cooling system where air bubbles may be trapped.

Fill the radiator to the top of the filler neck with your antifreeze water mix. If your car has a pressure cap on the overflow tank or upper hose, fill it from there. When full, close any air bleed screws, and start the engine.

With the engine running the level of coolant ought to immediately go down slightly. Top it up again, and close the radiator cap. Fill the overflow bottle to the “cold” mark and let the car run.

The motor needs to run until the thermostat opens or the electric fan comes on, so the hot coolant in the motor can mix with the fluid in the radiator. You may want to take a ride around the block to get it to warm up faster. Once warm, park the car and let it sit again for a few hours until cool to the touch. Then once again, pop open the radiator cap and fill with diluted antifreeze.

Check the level in the radiator first thing in the morning before driving to work for the next several days.

Flushing the old coolant out of your car like this will save you from failed water pumps, plugged radiators, and possibly even a blown head gasket in the future.