How To Flush a Car Radiator

Your car’s radiator contains a mix of antifreeze and water called coolant, which is responsible for regulating the temperature of your engine when the outside temperatures are extreme. It also helps to prevent corrosion.

To keep this essential fluid working it at its best, your radiator should be flushed and your coolant changed roughly every two years.

Signs you need a radiator flush

There are a few key signs to look out for that indicate that you may need to flush your coolant:

  • The coolant is due for replacement
  • The coolant antifreeze strength is too week or too strong
  • The coolant is contaminated
  • Your engine is overheating, even when the coolant is topped up

How to flush coolant system

What you'll need

Please note: Always work with a cold engine. Do not attempt to access the cooling system when the engine is hot, as this can cause serious injury.

Step 1: Position your tray

Before you begin flushing, place your tray or container beneath your bonnet. The radiator drip valve is usually located in the lower right corner of the radiator.

Step 2: Drain the coolant

Remove the radiator or coolant reservoir cap under the bonnet, then unscrew the drain valve below. If you don’t have a drain valve, you will need to remove the bottom radiator hose. The coolant should begin to flow out into your container.

Once it’s empty, close the drain valve or refit the hose.

Step 3: Add your radiator flush

Now the radiator is empty, you can add your cleaning flush solution. Depending on the fluid, you may need to top the radiator up with water. Be sure to follow the instructions on the container.

Step 4: Run your engine

Loosely fit the radiator or coolant reservoir cap to enable air to escape, then start your engine and leave running for around 15 minutes to give the cleaner time to work.

Step 5: Drain the cleaner

Once the engine has cooled, repeat step 2 to remove the cleaning fluid.

Step 6: Top up coolant

Now the system is clean, you can top up your radiator or coolant reservoir.

Make sure that any air that has entered the cooling system has been bled out before driving the vehicle on the road.

Tighten the radiator or coolant reservoir cap. Recheck the coolant level after a 5–10-mile drive, topping up only when the engine has cooled.

Coolant is a mix of antifreeze and water – you can either use premixed coolant or buy antifreeze and dilute it with water yourself. Ideally when adding water to a cooling system it should be distilled to prevent scale. Find out more about what antifreeze you need here.

Important: Antifreeze must only be disposed of correctly. It must never be put down the drain, toilet, or in the bin. Instead, take it to your local recycling centre and they’ll instruct you where to put it.

Halfords coolant change

If you’d like us to drain and refill your coolant for you, you can book our Coolant Change service at one of our 600 garages.

Our experts will remove and safely dispose of your old coolant, replacing it with manufacturer approved Comma coolant to provide maximum protection against corrosion, overheating and freezing.