How the Cooling System Works

When fuel burns inside the engine, roughly one-third of the energy becomes mechanical power. Another third goes out the exhaust. And the final third becomes heat that stays in the engine. That heat would melt the engine block, warp the heads, and destroy the pistons if it were not removed continuously. The cooling system is the heat removal machine. It circulates a liquid — coolant — through passages in the engine block and cylinder head, absorbs the heat, and carries it to the radiator where it is dumped into the outside air. Think of it like a conveyor belt for heat. The coolant picks up heat inside the engine, carries it to the radiator, drops it off, and goes back for more.

Cold coolant from the bottom of the radiator flows into the engine through the lower radiator hose. The water pump pushes it through passages in the block around the cylinders, then up into the cylinder head around the combustion chambers — the hottest areas. The hot coolant exits the head through the upper radiator hose and enters the top of the radiator. As the hot coolant flows down through the radiator tubes, air passing through the fins pulls the heat out. The coolant reaches the bottom of the radiator cool again and the cycle repeats. This loop runs continuously as long as the engine is running.

The cooling system operates under pressure — typically 13 to 18 PSI depending on the vehicle. The radiator cap or the reservoir cap contains a calibrated spring that holds system pressure. Why pressure? Because pressurizing a liquid raises its boiling point. Water at sea level boils at 212 degrees Fahrenheit. Under 15 PSI of pressure, the boiling point rises to about 265 degrees. Since the engine operates at 195 to 230 degrees, that extra margin is critical. A weak cap that cannot hold pressure lowers the boiling point and allows coolant to boil over at normal operating temperatures. Always test the cap with a cooling system pressure tester — it must hold its rated pressure. A five-dollar cap prevents a five-hundred-dollar overheat.

As coolant heats up, it expands. The excess flows out of the radiator through the cap and into the overflow reservoir — also called a degas bottle or expansion tank. When the engine cools down, coolant contracts and vacuum pulls coolant back from the reservoir into the radiator. If the reservoir is empty, the system pulls air instead — and air in the cooling system causes hot spots, overheating, and heater core problems. Always keep the reservoir filled to the proper level line. On many modern vehicles, the pressure cap is on the reservoir itself rather than the radiator, and the reservoir is part of the pressurized system.