Many people find the radiator pressure cap on their vehicle but don’t know the purpose of it. Many think that cap was for more than just show. In fact it plays an important role in a pressurized cooling system.
When you set the vehicle into motion the temperature of the engine increases. We know that when we increase the pressure on a liquid, we raise its boiling point. The coolant boiling temperature is 106°C when it’s under 1 Pressure per Square Inch(psi). We know that the more pressure we can hold in a system, the higher the boiling point. The Radiator pressure cap maintains the system pressure between 15psi to 18psi which is optimum. Maintaining this Pressure would increase the boiling point of coolant upto 125°C. If we don’t maintains this Pressure the coolant will boil off leaving the engine in the worse condition.
When the coolant pressure (& temperature) inside the cooling system increases above its optimum level the pressure cap releases some of the coolant into reservor tank. Remember, heating a liquid makes it expand. When the engine shuts off the coolant begans to cool lowering the pressure in the system creating vaccum. So the Radiator cap allows coolant in the reservor tank to return to the system. This transferring of coolant is done by make use of spring loaded valve mechanism.