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How to Flush Your Cooling System

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Rust, scale and other contaminants can clog the cooling system and cause engine overheat.

Rust, scale and other contaminants can clog the cooling system and cause engine overheat.

The Purpose of a Flushing Your Cooling System

A cooling system flush will help the system get rid of:

  • dust,
  • rust, and
  • metal shavings.

Contaminants floating around in coolant fluid can be removed by simply draining the system. With a cooling system flush though, you run a special chemical to help dissolve and wash out these contaminants out of the radiator, coolant pockets and other passages inside the engine.

Usually, car owners won't flush the system until they find evidence of rust and scale, or the engine begins to overheat because of restriction issues. And with newer car models, there seems to be not much reason for it. Newer engines have better resistance, and modern antifreeze offers better protection than before. Nevertheless contaminants can still accumulate over time and, eventually plug the radiator, water pump, and water passages in the engine.

A variety of flushing chemicals are available at auto parts stores and online to help you clean the cooling system. If you are dealing with heavy buildup though, you may need to go to a shop and have the system serviced with power flushing equipment. Another option is to simply replace the radiator and clean the rest of the system with a proper flush.

Whether you need to get rid of buildup or just want to service the system to prevent clogging and overheating, the following procedure will help you do a cooling system flush.

You can remove the lower radiator hose to drain the cooling system, if a petcock or drain valve is not provided on your radiator.

You can remove the lower radiator hose to drain the cooling system, if a petcock or drain valve is not provided on your radiator.

1. Flushing the Cooling System

Just like changing the system's coolant, flushing the system is part of the system maintenance. Although some car manufacturers recommend flushing the system when replacing the coolant (every 2 to 5 years), some car owners do this every other coolant change, or at longer intervals.

Cooling systems in newer vehicle models tend to be more resistant to rust and buildup. Still, flushing the system is necessary from time to time to prevent damage to system components and coolant flow restriction.

The following is a simple cooling system flushing procedure you can do in your own garage. However, if you believe your radiator is restricted or plugged, taking it to a shop for a professional flush or replacing it with a new unit may be a better option.

Draining the Coolant

Draining the coolant from the radiator will only remove so much of the coolant in the system. Usually, there'll be about 30 to 50 percent of the old coolant left inside. If you need to remove as much of the old coolant as possible, repeat this procedure two or three times using distilled water, as described below.

When draining the system, you may want to disconnect the lower radiator hose instead, if your radiator doesn't have a drain plug or petcock, or is difficult to access. This is a more practical solution in some cases.

  1. Wait for the engine to cool.
  2. Place a catch pan on the ground, under the radiator drain. This drain plug is located at the bottom, usually on the driver's side, of the radiator. If necessary, consult your vehicle repair manual. If you don't have this manual yet, you can buy a relatively cheap, aftermarket copy online. Haynes manuals come with many step-by-step procedures for many maintenance, troubleshooting, and repair projects you can do at home.

    On some models, you may need to lift and secure the front of the vehicle with jack stands and remove a protective cover from under the engine to access this plug.

  3. Set the temperate control on your dashboard to "Hot" to open the heater coolant control valve to the heater core.
  4. Disconnect the overflow tank or coolant reservoir from the radiator and remove the tank from the vehicle. Empty the plastic tank into the catch pan and reinstall the reservoir.
  5. Open the radiator drain plug or petcock valve, and remove the radiator cap, or overflow tank cap.

    If you've decided to disconnect the lower radiator hose instead:

    • Place the catch pan under the lower radiator hose.
    • Remove the lower hose clamp at the radiator.
    • Twist the hose back and forth as you pull the hose off its radiator fitting. If necessary, slip a thin screwdriver between the hose and the radiator fitting to break the hose loose.
    • Disconnect the hose from the radiator and drain the coolant into the catch pan.
    • Reconnect the hose and secure the clamp to the hose.
  6. After the coolant has finished draining from the radiator, close the drain plug or petcock valve.

Don't Contaminate!

Store used coolant in a closed container in a safe place for later recycling.

Wait for the engine to cool before starting to drain the system.

Wait for the engine to cool before starting to drain the system.

Flushing the System

  1. After draining the radiator, fill the radiator with distilled water through the radiator neck and add the chemical flush. If your car doesn't have a radiator cap, add the water and chemical through the reservoir or expansion tank. Follow the instructions that come with your flushing chemical product and make sure it's safe for your radiator.
  2. Engage the emergency brake.
  3. Set the transmission to Park or Neutral.
  4. Start the engine and let it idle.
  5. Set the heater to Hot MAX.
  6. Add more water through the radiator neck to top off the radiator as the engine idles.
  7. Install the radiator cap.
  8. Let the engine idle or drive the car for about 15 to 20 minutes, so that it reaches operating temperature and allows the chemical to circulate and clean the system. If the instructions for your chemical agent differ from these steps, follow the instructions that come with your product instead.
  9. Shut off the engine and allow it to cool for about 10 to 15 minutes.
  10. Drain the radiator by opening the drain valve or disconnecting the lower radiator hose.

    Depending on how much buildup your system has accumulated, you may want to repeat steps 1-10 once or twice more before finally adding fresh coolant to the system. Usually, you want to see clear water draining from the radiator before refilling the system with fresh coolant.

  11. Refill the system with 50 percent distilled water and 50 percent antifreeze. Use the antifreeze recommended by your car manufacturer. Fill the reservoir with coolant as well to the Cold mark. Then bleed the system (see the next section).
  12. From time to time, check the coolant level in the radiator and reservoir when the engine is cool and add coolant as necessary to bring it up to the correct level.
You can add an anti-rust solution to your fresh antifreeze for better protection.

You can add an anti-rust solution to your fresh antifreeze for better protection.

2. Cooling System Bleeding

Once you add fresh coolant, bleed the system to prevent overheating. You can find the proper cooling system bleeding procedure for your particular model in your vehicle repair manual.

Also, this other post can help you bleed the system as well.

After bleeding the system, you can extra-protect your cooling system by adding a quarter bottle of anti-rust solution to the radiator or expansion tank as well. You can find the product in most auto part stores.

Watch the following video to get some visual clues about how to eliminate air pockets when filling the cooling system.

3. Benefits of a Cooling System Flush

A cooling system flush will help:

  • maintain the efficiency of your car's cooling system
  • prevent clogging
  • prevent overheating
  • increase system components' service life

For better protection, you can flush the system every time you have to replace the coolant at the recommended maintenance schedule suggested by your car manufacturer. Usually every two or five years, depending on the type of antifreeze you use. Or do it every other time, and whenever you need to replace the water pump. Consult your car owner's manual or vehicle repair manual.

When you flush the cooling system at appropriate intervals, you not only help increase system service life, but prevent one of the most common engine problems, cooling system failure.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Dan Ferrell