The temperature gauge creeps above its usual position on a hot day in traffic. Or a sweet smell appears after a long drive, and there is a small green or orange puddle under the car. The cooling system is one of the most important — and most overlooked — systems in a car. When it fails, an engine can overheat quickly, causing expensive damage.
Key Takeaways
- The cooling system circulates coolant through the engine and radiator to keep the engine at its ideal operating temperature.
- The pressure cap raises coolant’s boiling point; never open a hot cooling system.
- Coolant types use different corrosion inhibitors; mixing incompatible types can reduce protection.
- Rising temperature gauges, low coolant, sweet smells and white exhaust smoke are signs to investigate.
What the cooling system does
Engines produce a lot of heat. The cooling system keeps the engine within a narrow temperature range, warm enough for efficiency and emissions control but cool enough to prevent damage.
The main components
Coolant (antifreeze). A mixture of glycol (usually ethylene glycol) and water, with additives that prevent corrosion. It lowers the freezing point and raises the boiling point.
Water pump. Circulates coolant through the engine and radiator. It is driven by a belt or, in some engines, electrically.
Thermostat. A valve that stays closed while the engine warms up, keeping coolant within the engine. When coolant reaches a set temperature, it opens and lets coolant flow to the radiator.
Radiator. A heat exchanger at the front of the car. Hot coolant passes through thin tubes while air flows over fins, removing heat.
Cooling fans. Pull air through the radiator when the car is moving slowly or stopped.
Pressure cap. Seals the system and keeps it under pressure. Pressurised coolant has a higher boiling point.
Expansion tank (overflow reservoir). Holds extra coolant as it expands when hot and returns it as it cools.
Heater core. A small radiator inside the cabin that uses engine heat to warm the interior.
Hoses. Carry coolant between components.
Why pressure matters
Under pressure, coolant can operate well above its normal boiling point without boiling. If the pressure cap fails, coolant can boil at lower temperatures, causing overheating.
Never open the radiator cap or pressurised expansion tank on a hot engine. Hot coolant can spray out and cause severe burns. Wait until the engine is cool.
Coolant types
Coolants differ mainly in their corrosion inhibitor technology:
- Inorganic additive technology (IAT): traditional coolant, often green, with silicates and phosphates. Common in older vehicles; shorter service life.
- Organic acid technology (OAT): longer-life coolant using organic acids. Used by many manufacturers.
- Hybrid organic acid technology (HOAT): combines organic acids with some silicates or phosphates.
- Phosphate-free or silicate-free variants specified by certain manufacturers.
Color is not a reliable guide to type. Different brands use different colors, and some colors overlap between types.
Mixing incompatible coolants can reduce corrosion protection and, in some cases, cause deposits. Use the coolant specified in your owner’s manual. If you must top up in an emergency, water can be used temporarily, but the system should be serviced with the correct coolant mix afterwards.
Common cooling system problems
Low coolant. Can result from leaks in hoses, radiator, water pump, heater core or head gasket.
Thermostat stuck closed. Causes rapid overheating.
Thermostat stuck open. Engine takes a long time to warm up, poor heater performance and reduced fuel economy.
Failed fans. Overheating in traffic but normal temperature at speed.
Clogged radiator. Reduced cooling capacity, especially under load.
Water pump failure. Leaks from the weep hole, noise, or overheating.
Head gasket failure. Can cause coolant loss without visible leaks, white exhaust smoke, bubbles in the coolant or oil contamination.
Warning signs
- Temperature gauge higher than normal
- Coolant warning light
- Sweet smell, especially after driving
- Puddles under the car
- White smoke from the exhaust
- Poor cabin heat
- Steam from the engine bay
If the gauge reaches the red zone, pull over safely and turn off the engine. Turning the heater to maximum can draw some heat away from the engine temporarily while you find a safe place to stop.
Maintenance
- Check coolant level in the expansion tank when the engine is cold.
- Inspect hoses for cracks, swelling or soft spots.
- Follow the coolant replacement interval in your manual. Long-life coolants may last many years, but intervals vary.
- Have the system pressure-tested if you suspect a leak.
Frequently Asked Questions
Can I use only water in my cooling system? Only temporarily in an emergency. Water lacks antifreeze protection and corrosion inhibitors.
What mixture should I use? Many coolants are sold premixed 50/50. Concentrated coolant should be mixed according to instructions.
Why is my coolant level dropping slowly? Small leaks, a weak pressure cap or internal engine problems are possible causes. Have it checked.
Is coolant toxic? Yes. Ethylene glycol is toxic to people and animals. Clean spills and store coolant safely.
The bottom line
The cooling system protects your engine from heat damage. Use the correct coolant, never open a hot system, and investigate rising temperatures, leaks or sweet smells early.