The cooling system of an engine is designed as a sealed system with the engine coolant held under pressure. When under pressure, water and engine coolant boil at a higher temperature. As your coolant and water increase in temperature and approach the boiling point, the state of the liquid begins to change to a gas, seen as steam. A gas is much less efficient in cooling your engine than a liquid. So incre
asing the pressure increases the boiling point, allowing the coolant and water to do a much be
tter job at removing heat. Keep in mind that internally coolant running through the channels within the engine will come on contact with metal that is well above 210
°
F so preventing the coolant from boiling is essential. By increasing the pressure, you are increasing the boiling point and the coolant remains in a state of liquid rather than converting to a gas. Heated coolant also expands so a closed loop system will require coolant to be held in rather than boiled out. A closed loop system will also need an overflow tank which is reclaimed upon cooling of the system.
Role of the Radiator Cap
Functioning radiator cap will maintain its rated pressure. Different vehicles require different pressure ratings so check your owners manual for the correct cap for your vehicle. A periodic check of your radiator cap is important especially when troubleshooting a cooling problem. Allow the engine to cool to the touch and remove the radiator cap. Check the radiator cap gasket. The gasket is located on the underside of the radiator cap. The gasket seals the cooling system keeping pressurized coolant in the closed loop system. Inspect this gasket. If it is hard, cracked or split, or if the rubber is brittle or missing then it should be replaced. A bad cap can be the reason for lost fluid. A bad cap that is not tightly sealing in cooling fluid can allow the fluid to boil off, especially when the engine is shut off. The reason for this is un-circulating fluid will not be cooled in a hot engine and can increase in temperature as it sits, high enough to boil. The effect of this can cause fluid to be pushed out of the radiator via the radiator cap. If the cap is properly sealed it will be pushed into the overflow tank. If the seal is damaged or missing it can spill out of the system. If the gasket is weak or damaged, it will not hold the pressure necessary to maintain a higher boiling temperature. Always make sure you have a good cap, an overflow tank and never fill your overflow tank beyond the 'cool' high mark.
Also keep in mind that your cooling system may be leaking pressure somewhere else other than the radiator cap.
