Radiator Cap – How Important Is Your Radiator Cap

Your Radiator Cap is more important than you think !

While radiator caps look simple and do not cost much, they are critical for a properly Operating cooling system.

A faulty radiator cap can result in overheating, loss of coolant or major engine damage. It is important to check it periodically and replaced if necessary.

radiator cap-Pressurized cooling systems help prevent overheating and coolant loss. This raises the coolant’s boiling point about 3-degrees F for each additional psi above atmospheric pressure.

Radiator Cap
Radiator Cap

As the engine warms up and the coolant expands it becomes pressurised.. The radiator cap pressure relief valve allows pressure to build up to a specified level, then lets excess pressure to escape.

In an “open” systems found in older vehicles, excess pressure escapes to the atmosphere through an overflow tube. As air enters through the overflow tube, coolant will be lost.

Therefore, “closed” or reservoir cooling systems are used in later model vehicles.

Here, as the coolant expands it goes through the overflow tube into a reservoir. The radiator cap now serves as a vacuum relief and siphon valve. As the engine is cooling the coolant also contracts.This allows it to be siphoned back into the radiator.

Radiator Cap / High Pressure

Radiator caps also serve as pressure relief valves. They will prevent excessive pressure. Unchecked high pressure could cause damage to the radiator, heater core, hoses or water pump seal. The pressure cap also prevents radiator hoses and tanks from collapsing. If air was unable to enter it could cause excessive vacuum..

Always check the radiator cap during routine maintenance. Check only when the engine is cool. Then with the engine off, place a rag over the cap and remove it. Turn it counterclockwise about 1/4-turn until it reaches the safety stop. Allow all pressure to vent before removing the cap by pressing down and turning it counter-clockwise.

Radiator Cap Testing

Pressure test the cap noting the maximum pressure when the cap valve opens. Caps come in a variety of pressure ranges: 4-pound caps (3-5 lbs.), 7- pound caps (6-8 lbs.), 13- or 14-pound caps (12-16 lbs.) and 15- or 16-pound caps (14-18 lbs.). If the Cap fails to hold the rated pressure for one minute Replace it.

Using a cap with the wrong pressure rating can cause over pressurization or too low a coolant boiling point if pressure rating is too low. Since radiator caps are so important and are relatively inexpensive, why not just replace with a new one.

Also inspect the overflow tube connecting the filler neck to the overflow reservoir for looseness, cracking or obstructions. Check the radiator filler neck sealing surfaces for nicks, dents or corrosion that could impair sealing. Inspect gaskets for looseness, cracking, hardening or other damage that allow pressure leakage and coolant to escape.


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