Radiator Fill Cap
Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.
The cooling system is sealed and pressurized by a radiator pressure cap. There are two advantages to sealing and pressurizing the cooling system.
| • | Increased pressure raises the boiling point of the coolant. This increases the efficiency of the cooling system. |
| • | Sealing the cooling system reduces coolant losses from evaporation and permits the use of the recovery reservoir. |
As the pressure goes up, the boiling point goes up. Therefore, the coolant can be safely run at a temperature higher than 212 degrees F (100 degrees C) without boiling. The higher the coolant temperature, the greater the difference between it and the outside air temperature. This difference in temperature is what causes the cooling system to work. The hotter the coolant, the faster the heat moves from the radiator to the cooler passing air. This means that the pressurized, sealed cooling system can take heat away from the engine faster. Therefore, the cooling system works more efficiently when the coolant is under higher pressure.
However, the cooling system can be pressurized too much. If the pressure in the system gets too high, it can damage the radiator and blow off the hoses. To prevent this the radiator cap has a pressure relief valve. When the pressure gets too high, it raises the valve so that the excess pressure can escape into the recovery reservoir.
The radiator pressure cap also has a vacuum vent valve. This valve protects the system from developing a vacuum that could collapse the radiator. When the engine is shut off and cools, the coolant volume is reduced. Cold coolant takes up less space than hot coolant. As the temperature of the coolant drops, a vacuum develops in the cooling system. To prevent excessive vacuum from developing, the vacuum valve opens to allow outside air or coolant from the recovery reservoir to flow into the cooling system. This relieves the vacuum that could otherwise cause outside air pressure to collapse the radiator.
