How to replace engine coolant

Your car’s engine coolant (or antifreeze) keeps the engine operating at the optimum temperature. It also prevents your engine from seizing in extreme weather and helps to prevent corrosion and deposits that could damage crucial engine components.

Changing your engine coolant is usually done as part of a major service. However, changing the antifreeze in your car is a task that a confident home mechanic can do with the right tools and approach.

Why change your engine coolant?

The engine coolant in your car works hard to control heat from the engine and protect core engine components from damage during cold weather. The coolant is pressurised and circulates around your engine where it heats up. It flows through the system to the radiator, which cools it down before returning it to the engine block, where it heats up again.

The coolant in your car is a mixture of water and antifreeze (ethylene glycol or propylene glycol). Most cars operate sealed systems, but even in those, coolant levels can drop and require a refill or replacement.

Manufacturers provide recommendations on how often the engine coolant should be replaced. Usually, it's done as part of a major service, approximately every 30,000 miles.

As well as part of regular servicing and maintenance, there are other reasons why you may choose to replace your coolant. You may spot debris in the expansion tank and want to take a closer look. The rubber hoses or radiator can degrade, become damaged and can cause the system to fail resulting in leaking coolant and an overheating engine.

Whatever your reason for replacing the coolant, it’s vital you choose the right type. You can find details in your owner's manual or use our free tool to find the proper coolant for your car.

Essential equipment to replace coolant

To replace your car’s engine coolant, you will need the following:

  • Gloves
  • Vehicle jack
  • Jack stands
  • Torx bits
  • Screwdrivers
  • Pliers
  • Hex keys
  • Draining container
  • Funnel
  • Wheel chocks
  • Engine coolant/Antifreeze

How to replace engine coolant

We’ve broken the process of replacing your coolant into four steps:

  1. Preparing your car
  2. Draining the coolant
  3. Refilling the system
  4. Testing the car

Here’s how to replace the coolant in your car.

Prepare your car

You'll need to secure the car and may need to raise it from the ground to check the hoses and drain the coolant. Here's how to safely secure your car so that you can access the bottom of the radiator:

  • Ensure the car is on level ground.
  • Place the jack under the car at the jack points on the vehicle. (If you don’t know where these are, you can find details in your owner's manual or search online). When in the right place, raise the car using the jack.
  • Once the car is raised, place jack stands underneath the car at strong points (the frame or subframe are ideal).
  • Set the height on the jack stands to enable you to access the coolant drain plug and then lower the car onto the stands using the jack.
  • Test the car is secure on the jack stands by shaking it slightly.
  • Place wheel chocks on the rear wheels.

Don’t ever work under your car if it’s only secured on the jack. Once you’re confident the car is secure, it’s time to drain the coolant.

Drain the coolant

  • When the engine is cold, open the bonnet and locate the expansion tank (or coolant reservoir). While wearing gloves, slowly turn the filler cap anti-clockwise to release any pressure, before removing it entirely and put it somewhere safe.
  • Underneath the vehicle, you'll need to locate the coolant drain. This may be a tap, but it’s now common to remove a hose that is at the lowest point of the engine or radiator. Place a suitable container under the tap or hose to capture the coolant when it drains.
  • Open the tap using pliers and / or remove the hose, directing it toward the container.
  • On closed systems, you’ll need to add air to the system to remove all the coolant. The radiator, thermostat housing or even a top engine hose will have a bleed screw that enables air to enter the system when you open it.
  • When the coolant has finished draining, close the drain tap and / or reattach the radiator hose.
  • Before refilling the coolant, check the hoses are in good condition, secured and not split. If they need replacing, now is the time.
  • Empty the container of coolant before putting it back into position for refilling.

Antifreeze is a hazardous chemical, so you must dispose of it safely. You can search online to find where to safely dispose of your engine coolant, with local councils often allowing you to take it to a recycling centre.

Replace the coolant

It’s now time to replace the coolant. First, you’ll need to find the right coolant for your vehicle. You can choose from a ready-mixed solution or concentrated antifreeze (which you will need to mix with water). The exact process applies for both types, but you shouldn't add water to the system if using a ready-mixed engine coolant. Most concentrated antifreeze must be mixed 50:50 with water.

  • Place a funnel in the top of the expansion cap and slowly pour in a couple of litres of antifreeze. Slowly adding the correct amount of antifreeze helps to prevent airlocks from forming. (If you have a radiator bleed screw, ensure it’s open while refilling the coolant.)
  • Now, add the water.
  • Take your time and regularly stop and squeeze the top and bottom rubber hoses to help move any trapped air to the expansion bottle. It’s very important to prevent air from becoming trapped in the system.
  • Keep going until you see fluid emerging from the bleed screw. When this happens, tighten it.
  • Once the bleed screw is secure, top up the coolant to the max level and fit the cap in place.
  • Remove the container from underneath the car to check for any leaks or puddles of coolant.

Check the system

Now safely remove the jack stands. To do so, raise the car using the jack to free the stands. Remove them and gently lower the car to the floor. Before driving, you should check the cooling system is working correctly and all parts are securely in place.

  • Start the engine and run it at idling speed for between 10 and 15 minutes.
  • Keep the engine running until the electric cooling fan activates. When this happens (you’ll hear it kick in), increase the engine speed gradually from 2000 to 3000 rpm.
  • Now, stop the engine and allow it to cool down completely.
  • You can check in the engine bay and underneath the car for any leaks. Look at the components you've removed or refitted.
  • Next, check the coolant level in the expansion tank and top-up if required. The level may have dropped as any remaining air is expelled and the engine cools down.
  • If you’ve added more coolant, refit the filler cap and you’re ready to go.

You should always check the coolant level after your first trip. Ensure the engine is cool so you get an accurate reading, and top-up if necessary.

Need some help changing the coolant?

Changing the coolant is a relatively simple task if you have the time correct tools and follow our step-by-step guide. If you need some extra help, book into a local Halfords Autocentre. Our expert mechanics are always on hand to help prepare your car for the road ahead.