Lesson 5/Learning Event 2
If the radiator overflow tube is plugged, too much pressure may build up in the cooling system and cause
damage to the radiator or hoses. Check the tube for being plugged by blowing through it with the radiator cap
removed. If the tube is stopped up, clear it out by running a flexible wire or an old speedometer cable through
The radiator pressure cap should seal and regulate the pressure in the cooling system. If the cap fails to seal,
pressure will not build up in the system. Without the proper pressure, coolant will be lost through excessive
evaporation and boiling. If the cap seals but maintains a pressure that is too high, the radiator and hoses may
Always use extreme care when removing the radiator cap from a warm or running engine. In overheated
systems the sudden release of pressure when the cap is loosened will cause steam and hot water to boil out.
This can burn you very badly. To remove the radiator cap when the engine is hot, place a cloth over the cap
for protection. Then turn the cap counterclockwise until a click or notch is felt (about one-quarter turn) to
release the trapped pressure. When all the pressure has escaped, press down on the cap and turn it
counterclockwise until it can be removed.
The best way to find out if the radiator cap is operating properly is to test it with a pressure tester. The
pressure tester consists of a hand pump, pressure gage, and adapters. To use the tester, select an adapter that
fits the radiator cap to be tested. Install the adapter and cap on the tester and operate the hand pump. Watch
the pressure gage and stop pumping when a higher pressure reading cannot be obtained on the gage.
Watch the gage for a while to see if the pressure bleeds off. If the pressure does bleed off, the radiator cap is
leaking and should be replaced. The highest reading obtained on the pressure tester gage is the amount of
pressure that the cap will maintain in the cooling system. If this reading does not agree with the radiator cap
specifications, replace the cap. The proper amount of pressure varies among the vehicle models, so you must
always check the vehicle technical manual for specifications.