Engine Coolant (Antifreeze)

Coolant is usually a mix of water and a glycol-based antifreeze with corrosion inhibitors. The mix lowers the freezing point, raises the boiling point and protects the metals and seals of the cooling system. Colours vary by type and maker and are not a reliable guide to compatibility.

The system is pressurised, which raises the boiling point further. That is also why you must never open the cap on a hot engine. Use the coolant type the manufacturer specifies; mixing types can reduce protection. Coolant types differ by their corrosion inhibitors, for example OAT, HOAT or silicate-based fluids. Colour alone does not identify the type reliably; the vehicle specification in the owner’s manual is what matters.

The water pump circulates coolant through passages in the engine, where it picks up heat. The thermostat sends it through the radiator once the engine is warm, and the heater core in the cabin uses the same coolant to heat the car. The system is pressurised by the expansion tank cap, which raises the boiling point of the coolant well above 100 °C. When the engine cools, coolant is drawn back from the expansion tank.

Low coolant from a leak can overheat the engine within minutes. A sweet smell, coloured drips or a falling level are signs of a leak; white smoke and loss of coolant can point to a head gasket fault. Coolant is poisonous and tastes sweet, so keep it away from children and pets. Check the level only when the engine is cold, and top up with the correct mixture rather than plain water when possible.