engines. The pressure cap closes off the overflow pipeand prevents loss of coolant during normal operation. Italso allows a certain amount of pressure to developwithin the cooling system. The pressure raises theboiling point of the coolant approximately 3 degrees foreach pound and permits the engine to operate at highertemperatures without loss of coolant from boiling.The pressure cap contains two spring-loadedvalves. The larger valve is called the pressure valve andthe smaller one is called the vacuum valve. A shoulderin the radiator filler neck provides a seat for the bottomof the cap assembly and a gasket on this seat preventsleakage between the cap and the filler neck.The pressure valve acts as a safety valve to relieveextra pressure within the system. The cooling systemmay be designed to operate at various pressures between4 and 17 psi, depending on the manufacturer’sspecifications. The pressure valve in the cap is preset bythe manufacturer. When replacing a pressure cap, makesure you use a cap with the proper pressure setting thatis usually marked on the top surface of the cap.The vacuum valve opens only when the pressurewithin the cooling system drops below the outside airpressure as the engine cools down. This automaticaction of the vacuum valve prevents collapse of thehoses and the radiator.WARNINGBecause it has a sweet taste, animals andchildren sometimes ingest spilled coolant. Thelead content that antifreeze absorbs while in usemakes it a hazardous waste and it cannot bedisposed of by being dumped on the ground. Itmust be containerized and turned in fordisposal.Water PumpThe 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 nonpositivein displacement. This type of pump has an impeller withblades that force the coolant outward as the impellerrotates. The shaft on which the impeller is mounted isusually driven by a fan belt and revolves in a bushing orin ball bearings inside the housing. For different coolingsystems, pumps vary considerably in construction ofseals, bearings, mounting, and drive.CAUTIONALWAYS REMOVE THE RADIATORCAP SLOWLY AND CAREFULLY. Remov-ing the cap from a hot, pressurized radiator cancause serious burns from escaping steam andcoolant.COOLANT AND ANTIFREEZE.— Since wateris easily obtained, is cheap, and has the ability to transferheat readily, it has served as a basic coolant for manyyears. Some properties of water, such as the boilingpoint, freezing point, and natural corrosive action onmetals, limit its usefulness as a coolant. This is counter-acted by the use of an antifreeze. Manufactured undermany different trade names, the most commonly usedtype of antifreeze is ethylene glycol. Ethylene glycol isa chemical compound composed of a mixture ofethylene and glycerine derivatives. Maximum freezingprotection is achieved by mixing 60% ethylene glycolwith 40% water. This mixture protects the coolingsystem to a temperature as low as minus 62°F. Ethyleneglycol has a very high boiling point, does not evaporateeasily, is noncorrosive, and is practically nonflammable.Figure 1-46.-Water pump.1-34
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