Coolant circulated by the water pump, flows through the engine blocks where it picks up engine heat and flows into radiator passing through combustion chamber and thermostat. As the coolant inside the radiator heats up to its boiling point, the liquid expands and creates additional pressure inside the system. When the pressure is above its psi rating, the spring loaded valve of the radiator pressure cap opens and allows excess coolant to flow into the overflow tank. Once the pressure is released the radiator cap closes. Coolant flows through thin tubes of radiator, gets cooled by the air stream and returns back to the radiator and then to the water pump. This is how an engine cooling system circulates the coolant and keeps engine from overheating.
Purpose Of A Radiator Cap
The purpose of the pressure cap is to seal and protect coolant from the outside air and maintain coolant pressure inside radiator. It has a two way spring loaded disc valve which opens and closes based on the coolant pressure inside the radiator. At the maximum pressure point which is normally between 15 to 16 psi, the pressure of the coolant forces the valve to open allowing excess coolant to flow into the overflow tank. As the excess liquid is let out, the pressure goes down and the cap valve comes back to its closed position. Thus pressure cap helps in maintaining optimal psi of the coolant when the engine is running.
When the engine is turned off, the coolant will be at its normal temperature. Since there is no hot liquid flowing through the radiator, the system pressure is down. This drop in pressure creates vacuum in the system and pulls coolant in the reverse direction. Thus the excess coolant from the overflow tank is returned back to the water pump via secondary valve of the radiator cap.
Signs Of Radiator Cap Malfunctioning
As mentioned above, pressure cap keeps the radiator system pressurized. If cap malfunctions, there will be no control of pressure in the system. There is a risk of bursts and leaks to the radiator hose and possible damage to the engine due to overheating. In case if the cap is stuck, it will not release fluid and the pressure keeps building up causing radiator leaks and damage to hose. On contrary, if the cap is stuck open and not closing, the fluid will not be properly pressurized. Hence, it is important to check the seals and spring of the cap to make sure it is working properly. It would be even better if you perform pressure test on the cap to check the amount of pressure it is building. If you detect any issue, better replace it with a new one.
Using Lower Or Higher PSI Cap
It is unusual for the pressure cap to fail but if you are replacing it, buy a correct one that is appropriate to your system. Vehicle owners often think using a lower psi cap would minimize the stress or a higher psi cap would increase the pressure holding capacity of their systems. Generally, the ideal psi for radiator caps is between 14 and 16. Using higher or lower value pressure caps causes radiator failures and gives unnecessary problems to other parts of cooling system.
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