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The radiator cap acts as more than just a "lid" for your radiator; it
keeps your engine cool by sealing and pressurizing the coolant inside
it.
What makes the radiator cap special is that it is designed to hold
the coolant in your radiator under a predetermined amount of
pressure. If the coolant was not kept under pressure, it would start
to boil, and soon you would have boiled all of your coolant away.
However, the radiator (or pressure) cap prevents this from happening
by exerting enough pressure to keep the coolant from boiling.
Normally, water (coolant) boils at 212 degrees F, but if the pressure
is increased, the boiling temperature is also increased. Since the
boiling point goes up when the pressure goes up, the coolant can be
safely heated to a temperature above 212 degrees F without boiling.
What makes this important is that the higher the temperature of the
coolant is, the greater the temperature gap between it and the air
temperature is. This is the principle that causes the cooling system
to work; the hotter the coolant is, the faster the heat in it moves
to the radiator and the air passing by. So, a cooling system under
pressure takes heat away from the engine faster, which makes it more
efficient.
If your cooling system is under too much pressure, it can "blow its
top"! To prevent this, the radiator cap has a pressure relief valve.
The valve has a preset rating that allows it to take just up to a
certain amount of pressure. When you turn the cap on the filler neck
of the radiator, you seal the upper and lower sealing surfaces of the
filler neck. The pressure relief valve spring is compressed against
the lower seal when you lock the cap.
The radiator filler neck has an overflow tube right between the two
sealing surfaces. If the pressure in the cooling system exceeds the
preset rating of your cap, its pressure relief valve allows the lower
seal to be lifted from its seat. Then the excess pressure (coolant,
air) can squish through the overflow tube to the ground or the
coolant reservoir.
Once enough pressure has been released (the caps preset rating), the
pressure relief valve is again closed by the spring.
The pressure cap can be tested with a cooling system pressure tester,
using an adapter, to make certain that it is living up to its
pressure rating. It should be replaced if it fails the test.
Note: Most radiator pressure caps are not meant to be removed.
Coolant should always be added through the expansion (overflow) tank.
NEVER REMOVE THE RADIATOR CAP FROM A HOT ENGINE. REMOVING THE
PRESSURE CAN CAUSE STEAM TO SHOOT OUT AND SERIOUSLY BURN YOU.
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