Overheating in the tow car

Press, 27 March 1979, Page 15

Overheating in the tow car

Overheating problems of various sorts are very common in cars towing caravans. The main reason, of course, is that the ordinary car was never designed to tow in the first place. Overseas. some car manufacturers do produce an optional towing kit, which is fitted when the car is being built. This includes a larger than normal radiator, a fan with more blades and a thermostat which opens at a lower temperature. Most cars with their cooling systems in perfect condition will tow a caravan without overheating but few cars are in perfect condition. Cooling a petrol engine is quite a science in itself. The peak temperature in the cylinders, when rhe petrol-air mixture is burning, may reach 32815 degrees F. This temperature exceeds the melting point of iron, so the engine must be cooled to prevent it from melting. However, the engine depends on heat to operate, and up to a point, the hotter it is, the more efficient it is. When considering overheating problems, it must be remembered that the engine in a car is air cooled. The water in the cooling system is a cooling medium and is used to absorb heat from the engine and later surrender the heat to the radiator. The radiator is then cooled by the air passing over its metal fins. Solving overheating problems can be one of the most frustrating jobs that a motor engineer faces, because the fault could be caused by so many things other than the cooling system itself; such as faulty carburation, incorrect ignition or valve

timing, restricted exhaust, dragging brakes, faulty automatic gearbox or torque convertor, leaking head gasket, cracked cylinder head, and, of course, an over-loaded car. A car is overheating when the cooling water starts to boil. At sea level, water normally boils at 212 degrees F. If the water is pressurised, for each one pound per square inch of pressure, the boiling point will be raised by 3 degrees F. Many modern cars now operate their cooling systems at 15 p.s.i., giving a boiling point of 257 degrees F, provided of course that the pressure is maintained. To pressurise a cooling system a spring loaded water-tight valve is fitted to the radiator cap. As an engine heats up the water from cold, the water expands in the sealed system and this creates the pressure required. At a predetermined pressure, the valve on the radiator will open so that excessive pressure is avoided. If the radiator cap is removed from a hot engine, the pressure will be lost and cannot be restored until the engine goes cold again. For this reason cooling systems should only be checked when cold. If the radiator cap is removed when the water temperature is above 212 degrees F„ and most cars reach 240 degrees F at times, all the water will instantly turn to steam, resulting in severe burns to the person removing the cap. A rough check to ascertain whether a radiator is in fact above 212 degrees F is to let some water drip onto the radiator tank. If the temperature is above 212 degrees F, his water will rapidly boil awav.