Removing the cap on a car's hot radiator will put you in the
hospital.
Never remove the radiator cap on a hot engine. Extreme danger is
involved. Before explaining why, a little history is needed.
Many years ago engine water pumps ran full time, pumping the
water through the engine block but not through the radiator.
Thermostats in the top of the radiator contained a spring-loaded
valve, the spring being made of two dissimilar metals
(bi-metallic, just like the ones in your home) and when the
temperature got to 180o, the metals would expand, open up the
valve, and permit the water pump to start pumping the water
though the radiator where the onrushing air due to the vehicle's
motion would cool the water and keep its temperate at 180o. That
means the coolant water in the radiator was not boiling, and it
was safe to take off the cap. The radiator, if vented to the
atmosphere, was prevented from building up any internal pressure
because no boiling took place.
Imagine this experiment Take a glass of boiling water and set it
on the sink top when the house is at 70o temperature. Time how
long it takes to cool down.
Now put the same glass of boiling water in the freezer and it
will cool down much faster. Why?
Because the temperature difference was greater.
In thermodynamic language, the rate of cooling is directly
proportional to the difference in the two temperatures.
Noting this phenomenon in Nature, auto makers concluded that a
much hotter radiator would get rid of heat faster and therefore
could be made smaller and cost less to make. A smaller radiator
would also reduce the car's front end profile, making it more
streamlined and esthetically pleasing.
To do so, it had to seal the radiator.........with a better
cap.....and make a better radiator to hold a little pressure.
Modern engines and radiators Currently car radiators operate at
about 210o to 220o, right about boiling water temperature. But
remember that if one boils water in a container where the
pressure is greater than atmospheric, the boiling point will be
higher than the familiar 212o.
Therefore by making the radiator able to withstand higher
pressures, the coolant can go up to 220o and transfer heat
faster and be smaller than Grampa's 1945 Buick with its180o
thermostat. An added bonus is that the hot water radiators, in
the cabin of the car, used to heat the passengers, can also be
smaller and cost less because they will be processing hotter
water than Grampa's 1945 Buick.
The radiator in a modern engine must be tightly sealed. If the
cap on a hot modern radiator is removed, then all the water in
the radiator, which is, for example, at 220o, above the 212o
boiling point at sea level, would immediately turn to steam. But
when water turns to steam it expands about 1,700 times, which
means it will likely remove the hood of the car in one terrific
explosion and scald the skin off the face or hands of the person
who just unscrewed the cap.
If a hot water boiler in a basement of a house has a failed
thermostat, and the water goes over 212o before the tank
ruptures, the resulting expansion of 30 or 40 gallons of water
into steam equal to 1,700 times the volume of the water will
turn the house into a pile of twigs. And it has happened more
than once.
So, never, ever, remove the cap on a hot radiator in a modern
car. And always keep the makeup reservoir, usually mounted under
the hood against one of the fenders, at least half full.