How an engine’s cooling system works

what coolant to use

Coolant is your engine’s lifeblood. The fluid keeps the motor at the correct operating temperature, helping it to warm up quickly in cold weather and not get too hot when under stress – and when mixed with the correct antifreeze it prevents damage in cold weather and stops corrosion (so always add it, even if you don’t live in a cold climate).

Your vehicle's coolant system consists of an expansion tank, a radiator, a water pump, a thermostat and a series of pipes that connect all of these components to each other and the engine block. The coolant runs through channels within the block to help transfer heat away from the engine and keep it at peak operating temperature throughout the year, under a range of operating conditions.

How long does coolant last?

Coolant is deeply important. It’s the liquid tasked with coursing through your engine to keep it cool and corrosion-free. It also works its way through the interior heater matrix to keep the cabin warm.

Take a look at your car’s handbook for coolant change intervals. Some older models – including classics – need a change every two years. They require what's known as Inorganic Acid Technology coolant, which is kinder to the seals and gaskets used on older engines. It's typically blue in colour but is becoming harder to find in shops. Don't be tempted to swap it for longer-lasting 'Organic' coolant (see below), because it's likely to damage your classic's seals.

Modern vehicles can go for 10-15 years or even more with the fluid they come out of the factory with – you may see 'lifetime coolant' referred to, which is often (but not always) pink in colour. This coolant is also known as Organic Acid Technology. Once you change this 'factory fitted' coolant for aftermarket coolant, we'd recommend renewing it every five years, unless the instructions on the bottle say otherwise.

Whichever coolant your car needs, we'd recommend checking its strength each autumn, and certainly before cold weather arrives, to make sure it's still at sufficient strength to protect your vehicle from a deep freeze.

However, if you're changing a part that's connected to the cooling system, such as the water pump, you'll need to replenish some or all of the coolant well before these intervals.

Can’t I just use water?

Your engine needs coolant. You should never just put water in your cooling system. Water on its own will course through your engine and do nothing to prevent corrosion. The coolant additive you put into your engine is chemically balanced to react in certain ways under certain conditions.

In winter, the additives in coolant prevent it from freezing. Use plain old water and it'll freeze in your engine block. Water expands when it freezes, which will crack the block. And that’s game over.

Those additives also raise the coolant's boiling point, to prevent the engine from overheating, particularly when it's working harder if you're towing or in hot summer weather.

When to change your coolant

“If you can’t remember the last time the coolant was changed, change it now”

If you're planning on changing the coolant as part of a car's servicing procedure, always consult your vehicle’s handbook for recommended intervals. If you can’t find evidence that the coolant was changed when it should have been – and especially if it’s looking cloudy and discoloured, change it now. Part of the cooling system service involves flushing.

Always wait until the coolant is cold before draining it. Don’t allow antifreeze to come in contact with your skin or painted surfaces of the vehicle and rinse off spills immediately with plenty of water. Never leave antifreeze lying around in an open container or in puddles on the floor; children and pets are attracted by its sweet smell and may drink it.

Check with local authorities about disposing of used antifreeze. Many communities have collection centres which will dispose of antifreeze safely. Never dump used antifreeze on the ground or pour it into drains.

All cars are slightly different, so if it is time to change your coolant, find your car for specific instructions.

How to change your coolant

Here's an example of how it's done

A very brief summary of the task:

  1. Make sure the engine is cold and raise the front of the vehicle on axle stands. You may need to remove the underbody shield
  2. Place a container under the radiator and undo the drain valve
  3. Remove any other drain plugs as instructed by Haynes. Flush the system as many times as necessary
  4. Refill the system with the correct antifreeze, following Haynes’ instructions for bleeding/topping-up procedures

Why you should change your coolant

Draining, flushing and refilling the coolant will help prevent formation of rust and corrosion, which can impair the performance of the cooling system and cause engine damage.

Over time the antifreeze chemicals in the coolant lose their effectiveness, especially if you top up the coolant with water regularly. In harsh winter weather this can lead to the coolant freezing and expanding, which can damage components.

Tools you will need

Only basic tools are required for this job, although you may need to raise the car to remove the undershield.

  • Pan or bucket to collect water
  • Trolley jack (if necessary). Not your car’s emergency jack
  • Axle stands
  • Ratchet and socket set
  • Flat-bladed/Phillips/torx screwdriver
  • Rags
  • Pliers

Parts you may need

  • Coolant
  • New hoses and clamps

How much does new coolant cost?

Coolant £2-£5 per litre
Garage fee savings £100-£150