RADIATOR PRESSURE CAP.- The radiator pressure cap (fig. 1-45) is used on nearly all modern

Figure 1-45.-Radiator pressure cap. -

engines. The pressure cap closes off the overflow pipe and prevents loss of coolant during normal operation. It also allows a certain amount of pressure to develop within the cooling system. The pressure raises the boiling point of the coolant approximately 3 degrees for each pound and permits the engine to operate at higher temperatures without loss of coolant from boiling.

The pressure cap contains two spring-loaded valves. The larger valve is called the pressure valve and the smaller one is called the vacuum valve. A shoulder in the radiator filler neck provides a seat for the bottom of the cap assembly and a gasket on this seat prevents leakage between the cap and the filler neck.

The pressure valve acts as a safety valve to relieve extra pressure within the system. The cooling system may be designed to operate at various pressures between 4 and 17 psi, depending on the manufacturer's specifications. The pressure valve in the cap is preset by the manufacturer. When replacing a pressure cap, make sure you use a cap with the proper pressure setting that is usually marked on the top surface of the cap.

The vacuum valve opens only when the pressure within the cooling system drops below the outside air pressure as the engine cools down. This automatic action of the vacuum valve prevents collapse of the hoses and the radiator.

CAUTION

ALWAYS REMOVE THE RADIATOR CAP SLOWLY AND CAREFULLY. Removing the cap from a hot, pressurized radiator can cause serious burns from escaping steam and coolant.

COOLANT AND ANTIFREEZE.- Since water is easily obtained, is cheap, and has the ability to transfer heat readily, it has served as a basic coolant for many years. Some properties of water, such as the boiling point, freezing point, and natural corrosive action on metals, limit its usefulness as a coolant. This is counteracted by the use of an antifreeze. Manufactured under many different trade names, the most commonly used type of antifreeze is ethylene glycol. Ethylene glycol is a chemical compound composed of a mixture of ethylene and glycerine derivatives. Maximum freezing protection is achieved by mixing 60% ethylene glycol with 40% water. This mixture protects the cooling system to a temperature as low as minus 62F. Ethylene glycol has a very high boiling point, does not evaporate easily, is noncorrosive, and is practically nonflammable.

WARNING

Because it has a sweet taste, animals and children sometimes ingest spilled coolant. The lead content that antifreeze absorbs while in use makes it a hazardous waste and it cannot be disposed of by being dumped on the ground. It must be containerized and turned in for disposal.

Water Pump

The water pump is the heart of the cooling system. Most engines use a centrifugal water pump (fig. 1-46) that provides a large volume capacity and is nonpositive in displacement. This type of pump has an impeller with blades that force the coolant outward as the impeller rotates. The shaft on which the impeller is mounted is usually driven by a fan belt and revolves in a bushing or in ball bearings inside the housing. For different cooling systems, pumps vary considerably in construction of seals, bearings, mounting, and drive.

Figure 1-46.-Water pump.