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Why coolant matters more than most drivers think

Coolant — still widely called antifreeze — does two jobs at once. In winter it stops the water in your engine freezing and cracking the block, and in summer it lifts the boiling point so the engine can run at around 90°C without the coolant turning to steam. On top of that, it carries corrosion inhibitors that protect the radiator, water pump, heater matrix and head gasket from rust and scale. Those inhibitors wear out over time, which is why most manufacturers specify a change interval, often anywhere between two and ten years depending on the type used.

The good news is that checking the level takes five minutes and a quick look under the bonnet. Catching a low level early is far cheaper than replacing a head gasket after an overheating episode.

Rule one: never open the system when it is hot

The cooling system is pressurised, and when the engine is hot the coolant inside can sit well above 100°C. Crack the cap and it will erupt as scalding liquid and steam, which is a genuine hospital trip. Leave the car at least three hours — ideally overnight — before you touch anything. If you have just driven it, wait. As a rough test, squeeze the thick top hose: if it feels firm, there is still pressure in there.

Park on level ground too. A slope will give you a false reading, and you may think you need to add coolant when you do not.

Finding the reservoir and reading the level

Almost every modern car has a translucent plastic expansion tank, usually near the front of the engine bay with a cap marked with a warning symbol or the word "coolant". Look for MAX and MIN markings, or two horizontal lines. When the engine is cold, the level should sit between them, and closer to MAX is perfectly fine. Coolant expands as it warms, so a level a little above MAX on a cold engine is not a problem — the system simply pushes the excess into the overflow.

Older vehicles sometimes have no separate tank, only a metal radiator cap. On these, check the level at the filler neck with the engine cold; the coolant should be visible just below the neck. Have a torch handy, because a stained plastic tank can be hard to read.

Take a proper look while you are there. Fresh coolant is usually a clear pink, red, orange, blue or green. If it looks like brown sludge, or there is an oily, mayonnaise-like film on the cap or in the tank, do not simply top it up. That can point to a failing head gasket or an oil cooler problem and needs proper diagnosis.

Choosing the right coolant — the mistake that costs money

Coolants are not interchangeable. Most modern cars use OAT (organic acid technology) coolant, typically pink, red, orange or purple. Older vehicles often use IAT coolant, usually blue or green, which contains silicates, and some use a hybrid formulation. Mixing incompatible types can cause the additives to drop out of suspension and form a gel that clogs the heater matrix and radiator.

Check your handbook, and if you can see what colour and type is already in the system, match it. If you genuinely cannot identify it and the level is only slightly low, adding a small amount of deionised water is the safer option until you can get the system flushed. Do not guess with a random bottle from the garage shelf.

• Pre-mixed coolant: pour it in neat. Easiest and safest option for most drivers.
• Concentrate: must be diluted. A 50/50 mix with deionised water protects down to roughly -37°C, which covers every UK winter comfortably.
• Water: use deionised or distilled water, not tap water. Hard water leaves limescale deposits inside the radiator and heater.

Topping up, step by step

• Confirm the engine is completely cold and the car is on level ground.
• Lay a rag over the cap and turn it slowly half a turn. Let any remaining pressure hiss out before removing it fully.
• Pour in the correct pre-mixed coolant or your 50/50 mix, slowly, up to the MAX line. Spilled coolant is slippery and toxic to pets, so rinse any splashes away.
• Refit the cap firmly. Start the engine and switch the heater to hot, which opens the matrix and helps push trapped air through the system. Let it idle until the cooling fan cycles on and off once or twice.
• Switch off, let it cool completely, then recheck the level. A trapped airlock can drop the level noticeably, so a second top-up is often needed.

If the level keeps dropping

Losing a centimetre over several months is normal — coolant slowly evaporates and minor seepage happens. Topping up every week is not normal. Common culprits include a perished hose or loose clip, a weeping water pump seal, a failing radiator, a leaking heater matrix, or a filler cap that no longer holds pressure.

Look for crusty white or coloured deposits around joints and hoses, a sweet syrupy smell when the engine is warm, or sweet-smelling steam from the exhaust. A slow leak can become a fast one on the motorway, and an overheated engine can warp the cylinder head — an expensive repair that a pressure test at a garage would have caught for a fraction of the cost.

Make it part of your routine

Check the coolant monthly and before any long trip or the first cold snap of the year. Ask your garage to test the antifreeze strength at service time, since a weak mix protects less than you think. If the coolant is murky, has never been changed, or you have been topping up with water, book a full flush and refill. Finally, keep a litre of pre-mixed coolant in the boot — it has rescued a great many drivers at the roadside.