In previous articles, we provided a basic overview of how the vehicle cooling system works and provided an in-depth look into components such as the radiator and radiator fan. In this week’s edition, we continue be focussing on three somewhat smaller components, but no less critical! 


2.3 Pressure Cap and Reserve Tank 

As the coolant gets hot, it expands. Since the cooling system is sealed, this expansion causes an increase in pressure in the cooling system, which is normal and part of the design. When coolant is under pressure, the temperature where the liquid begins to boil is considerably higher. This pressure, coupled with the higher boiling point of ethylene glycol (your Anti-Freeze), allows the coolant to safely reach temperatures more than 120 degrees Celsius. 


The radiator pressure cap is a simple device that will maintain pressure in the cooling system up to a certain point. If the pressure builds up higher than the set pressure point, there is a spring loaded valve, calibrated to the Kilopascal (kPa), to release the pressure. 


When the cooling system pressure reaches the point where the cap needs to release this excess pressure, a small amount of coolant is bled off. It could happen during stop-and-go traffic on an extremely hot day, or if the cooling system is malfunctioning. If it does release pressure under these conditions, there is a system in place to capture the released coolant and store it in a plastic tank that is usually not pressurized. 


Since there is now less coolant in the system, as the engine cools down, a partial vacuum is formed. The radiator cap on these closed systems has a secondary valve to allow the vacuum in the cooling system to draw the coolant back into the radiator from the reserve tank (like pulling the plunger back on a hypodermic needle). 


There are usually markings on the side of the plastic tank marked Full-Cold and Full-Hot. When the engine is at normal operating temperature, the coolant in the translucent reserve tank should be up to the Full-Hot line. After the engine has been sitting for several hours and is cool to the touch, the coolant should be at the Full-Cold line. 


Consult your manufacturers handbook as to when your radiator cap should be replaced. To err on the side of caution, many drivers should keep a spare radiator cap in their vehicle just in case the one in use stops functioning. Click here to find a replacement radiator cap for your vehicle 


2.4 Water Pump 

A water pump is a simple device that will keep the coolant moving if the engine is running. It is usually mounted on the front of the engine and turns whenever the engine is running. The water pump is driven by the engine through one of the following mechanisms: 

  • A fan belt that will also be responsible for driving an additional component like an alternator or power steering pump. 
  • A serpentine belt, which also drives the alternator, power steering pump, and AC compressor among other things. 
  • The timing belt, which is also responsible for driving one or more camshafts. 
  • An electric motor (uncommon in production vehicles). 


The water pump is made up of a housing, usually made of cast iron or cast aluminium and an impeller mounted on a spinning shaft with a pulley attached to the shaft on the outside of the pump body. 


A seal keeps fluid from leaking out of the pump housing past the spinning shaft. The impeller uses centrifugal force to draw the coolant in from the lower radiator hose and send it under pressure into the engine block. There is a gasket to seal the water pump to the engine block and prevent the flowing coolant from leaking out where the pump is attached to the block. 


Consult your manufacturer’s handbook as to when your water pump needs preventative replacing. Timing belt driven pumps are often replaced the most frequently, at the same time as the timing belt. This is usually around the 100 000km mark, although it can fail prematurely. Leaks, noises and over-heating could all point to a failing water pump. If your water pump needs replacing, look no further! Click here to see our range 


2.5 Thermostat 

The thermostat is simply a valve that measures the temperature of the coolant and, if it is hot enough, opens to allow the coolant to flow through the radiator. If the coolant is not hot enough, the flow to the radiator is blocked and fluid is directed to a bypass system that allows the coolant to return to the engine. 


This bypass system allows the coolant to keep moving through the engine to balance the temperature and avoid hot spots. Because flow to the radiator is blocked, the engine will reach operating temperature sooner and, on a cold day, will allow the heater to begin supplying hot air to the interior more quickly.


Since the ’70s, thermostats have been calibrated to keep the temperature of the coolant above 91 to 104 degrees Celsius. Prior to that, 82 degree thermostats were the norm. It was found that if the engine is allowed to run at these hotter temperatures, emissions are reduced, moisture condensation inside the engine is quickly burned off, which extends engine life, and combustion is more complete, which improves fuel economy.


The heart of a thermostat is a sealed copper cup that contains wax and a metal pellet. As the thermostat heats up, the hot wax expands, pushing a piston against spring pressure to open the valve and allow coolant to circulate.


The thermostat is usually located in the front, top part of the engine in a water outlet housing that also serves as the connection point for the upper radiator hose. Its housing attaches to the engine, usually with two bolts and a gasket to seal it against leaks. This gasket is usually made of a heavy paper or uses a rubber O-ring.


In some applications, there is no gasket or rubber seal. Instead, a thin bead of special silicone sealer is squeezed from a tube to form a seal.


There is no hard and fast rule when to replace a thermostat. It is sometimes replaced as a precaution when repairing the cooling system, or with a major engine repair. An abnormal engine operating temperature may be a result of a failing thermostat.


A thermostat stuck opened will cause a continuous flow of coolant, resulting in a lower operating temperature. Since the oil operates below temperature, the condition accelerates parts' wear, reducing engine efficiency and increasing emissions over time.


On the other hand, a thermostat stuck in the closed position will prevent coolant flow and cause the temperature to steadily rise. If you fail to notice and keep your engine running, in a matter of minutes your engine self-destructs. Literally.


Either way, your engine will suffer damage. The difference is just in the amount of time it takes. Still, a failing thermostat is not the only cause for an abnormal engine operating temperature.


Other reasons include low coolant level, a bad water pump, a worn out or loose water pump belt, cooling system leaks, a clogged radiator, a failed radiator fan and a collapsed radiator hose. Whatever the cause, it's a good idea to start looking into the problem before it's too late.


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