Ontluchtingsklep koelsysteem

Vents trapped air out of the cooling system to the expansion tank. Modern engines route coolant through a branching network — head and block, radiator, heater matrix, turbocharger bearing housing, exhaust gas recirculation cooler, often an oil or gearbox heat exchanger — with high points where air collects and cannot escape on its own, because coolant flows past a pocket rather than through it.

Trapped air is not a harmless bubble. It insulates, so a pocket resting against a cylinder head is a local hot spot on an engine whose gauge reads normal — the sensor is sitting in liquid somewhere else. It lies to that sensor when it collects around it, giving temperature readings that swing implausibly fast. And it compresses, so a pump generating pressure against a pocket moves less coolant than its speed suggests.

The presentation that misleads people is a system needing repeated topping up with no leak anywhere. Each time the engine works air out of a pocket the level drops, and a pressure test that holds while the level keeps falling is that story rather than a leak nobody can find.

Because the valve sits at a high point in hot coolant, surrounded by seals and hose joints, coolant getting into its connector is a leading cause of its electrical faults — a genuine fault with undamaged wiring, where fitting a new valve into a contaminated plug simply repeats it.