Engine coolant: how to check it, when to change it, and what it costs

What coolant does

Only about 30% of a petrol car's fuel energy actually moves the car; much of the rest becomes heat, which leaves through the exhaust and the cooling system. The cooling system keeps the engine at its working temperature, around 90 °C on most cars, in summer traffic and on frosty mornings.

Coolant is a mix of water, antifreeze (glycol) and protective additives. It has four jobs:

• Carrying heat away. It flows through channels in the engine, picks up heat and gives it off in the radiator.
• Stopping it from freezing. Plain water would freeze in winter, expand and crack the engine. A correct mix protects to about −25 °C or lower.
• Preventing rust and deposits inside the engine, the radiator and the pump.
• Lubricating the water pump's seal and the thermostat.

How it flows:

• The water pump pushes coolant through the engine.
• The thermostat stays closed while the engine is cold, so it warms up quickly, and opens at working temperature to send coolant to the radiator.
• The radiator at the front of the car cools it, helped by the fan at low speed.
• The heater core uses the hot coolant to heat the cabin.
• The expansion tank (the reservoir you can see) takes up the coolant as it expands when hot, and shows the level.

Why coolant wears out

Coolant doesn't get used up like fuel, but it ages:

• The protective additives wear out. Over the years, the corrosion inhibitors are used up, even if the level and the frost protection still look fine. Then rust and deposits start to build up inside the engine and radiator.
• It gets diluted. Topping up with plain water over the years weakens the frost protection.
• It gets contaminated with rust, scale or, if something is wrong, oil.

That's why manufacturers set a change interval in years and kilometres, and why "the level is fine" isn't enough.

When to change it

Your manufacturer's interval comes first. It varies with the coolant type: older coolants every 2 to 4 years, many modern long-life coolants (such as VW's G13) about 5 years or 100,000 km. Many owners have it changed with the timing belt, when the water pump is replaced anyway.

Your driving style doesn't change this. Your maintenance level does.

How to check your coolant

The level

1. Only with the engine cold: ideally before your first drive of the day. Never open the cap on a hot engine. The system is under pressure, and hot coolant can spray out and scald you.
2. Find the expansion tank: a white or see-through plastic tank in the engine bay, often with a warning symbol on the cap.
3. The level should be between MIN and MAX. Look from the side; you usually don't need to open it.
4. If it's low, top up with the coolant your manufacturer specifies (premixed, or concentrate mixed with distilled water as the label says). In an emergency, plain water gets you to a workshop, but have the mix corrected later.

If you have to top up again soon, there's a leak or a fault. Have it checked.

The frost protection

A cheap antifreeze tester from a petrol station or car shop (a few euros) draws up a little coolant and shows down to what temperature it's protected. In central Europe it should protect to at least −25 °C. Many workshops check it for free before winter.

The colour

Colour doesn't tell you the type: manufacturers choose it freely, and the same colour can mean different chemistries. Look for:

• Brown or rusty coolant: the protection is used up, change it;
• An oily film or a milky, mayonnaise-like layer in the tank or under the oil filler cap: oil and coolant are mixing, possibly a head gasket problem. Get it checked straight away.

Which coolant does your car need?

There are several chemistries (with names like G11, G12, G12++, G13 at VW, or OAT, HOAT and silicate-free elsewhere), and they don't all mix.

• Use exactly the specification in your owner's manual or printed on the expansion tank. It's not about the colour.
• Don't mix different types. Some combinations weaken the corrosion protection; mixing silicate and silicate-free coolants can form a gel that clogs the system and damages the pump. G11 and G12 must never be mixed.
• If you're unsure what's in there, top up only with distilled water to get by, and have the coolant changed for the right type.
• Use distilled water for mixing concentrate, not tap water: hard water leaves scale.

Warning signs

• The temperature warning light (a red thermometer in waves) or the temperature gauge in the red.
• The coolant warning light for a low level.
• The level keeps dropping.
• Coloured drops or puddles under the front of the car: green, pink, blue or yellow.
• A sweet smell in the car or from the engine bay.
• White smoke from the exhaust that smells sweet, after the engine is warm.
• The heater blows cold when the engine is warm: often too little coolant or air in the system.
• Brown, rusty or oily coolant.

What happens if you don't change it

1. 1The protection wears out. Nothing changes that you can see or feel.
2. 2Rust and deposits build up in the engine, radiator and heater, and the system cools less well.
3. 3Parts fail early: the water pump starts leaking, the thermostat sticks, the radiator clogs, and the heater gets weak.
4. 4Frost damage. With diluted coolant, a hard frost can freeze it and crack the radiator or even the engine block.
5. 5Overheating. Replacing a blown head gasket often costs around €1,000 or more, and a seized engine can mean the end of the car.

Why not just keep topping it up?

• Topping up keeps the level, not the protection. The additives in the old coolant are still used up.
• Plain water dilutes it. Every top-up with water lowers the frost and corrosion protection.
• A dropping level is a symptom. Coolant doesn't evaporate in a sealed system: if it keeps going down, something is leaking.

How coolant is changed

1. 1The engine is left to cool down completely.
2. 2The old coolant is drained from the radiator drain plug or a lower hose into a container.
3. 3The system is flushed with clean water if the old coolant was dirty or a different type.
4. 4New coolant is filled in the correct mix, often with the heater set to hot so the heater circuit fills too.
5. 5The system is bled of air. The engine runs with the cap off or the bleed screws open until the thermostat opens and no bubbles come out. Air pockets can cause overheating.
6. 6The level is checked again when the engine has cooled, and topped up.
7. 7The old coolant goes for recycling. It's toxic and tastes sweet to animals, so it must never go down the drain or onto the ground.

At a workshop this takes about 45 minutes to an hour.

How much a coolant change costs

The coolant for a typical car costs about €28, the same everywhere, and the job takes about 45 minutes, so the labour rate makes most of the difference. Figures are estimates for an independent workshop, VAT included.

Questions people ask