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. 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. CAUTION ALWAYS  REMOVE  THE  RADIATOR CAP SLOWLY AND CAREFULLY. Remov- ing 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 counter- acted 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 62°F. Ethylene glycol has a very high boiling point, does not evaporate easily, is noncorrosive, and is practically nonflammable. Figure 1-46.-Water pump. 1-34