What is antifreeze, and why does your car need coolant all year?

Antifreeze is more than something for cold weather. It's part of the coolant that keeps your engine at the right temperature all year round.

What is it for?

Your engine makes a lot of heat. A mix of antifreeze and water circulates through the engine and the radiator to carry that heat away. The antifreeze does three jobs:

• Stops the coolant freezing when the car is parked in the cold. Frozen coolant expands and can crack the engine or radiator.
• Raises the boiling point, so the coolant doesn't boil over when the engine is working hard.
• Protects against corrosion. It contains additives that stop rust and scale building up inside the engine and radiator.

Why it's mixed with water

Antifreeze is normally mixed about 50:50 with de-ionised or distilled water. Pure antifreeze doesn't carry heat as well as a mixture does, and it's less effective at protecting the engine. Don't use ordinary tap water, because the minerals in it can leave deposits in the system.

Most coolant is sold ready-mixed or as a concentrate. Check the label, and top up with the same type that's in the car.

Colours and types

Coolants come in different chemical types, often sold as green, blue, orange, pink or yellow. Colour is only a rough guide, because manufacturers use different dyes. The safe rule is to use the type your car's handbook specifies. Mixing incompatible types can cause the additives to react and clog the system, which can lead to a costly repair.

Checking the level

1. Park on level ground and let the engine cool completely. Never open the cap on a hot engine, because the pressure can release scalding steam.
2. Find the translucent expansion tank and look at the level against the MIN and MAX marks.
3. If it's low, top it up with the right coolant mix to the MAX line.
4. If you have to top up more than occasionally, there may be a leak. Get it checked.

How often should it be changed?

It depends on the car and the type of coolant. Some need changing every couple of years or so, and many modern long-life coolants last considerably longer. Your handbook or service schedule will say. If the coolant looks brown, cloudy or has bits floating in it, it needs changing straight away.

Safety

• Antifreeze is poisonous. It can taste sweet, so keep it away from children and pets, and clean up spills straight away.
• Don't pour old coolant down a drain. Garages and household waste sites can dispose of it properly.
• A warning light for engine temperature, or a gauge reading high, means stop somewhere safe and switch off. Driving on can wreck an engine.

At your service

A good service includes a coolant level and condition check, and often a test of how much freezing protection it still has. It's one of the cheapest ways to avoid a big repair bill.