At sometime or another we all have experienced or are likely to experience the problems of an overheating vehicle, it is almost inevitable. But why do our vehicles get so hot that they feel the need to eject their coolant through various orifices while cloaking the front of the car in a dank cloud and leaving us stranded by the roadside? Is there anything we can do to prevent these problems occurring? Are there any warning signs to look out for that might indicate when a vehicle is going to overheat or the cooling system is going to fail? Well, so many questions, and over the next few articles we are hoping to shed some light on the answers to these questions. However, before we start we really must have some kind of understanding of the cooling system. Both, what is the cooling system? And what makes up the cooling system? Once we understand the basics we can then move on to working out why we are likely to have problems.
So what is the cooling system? The cooling system on your vehicle, apart from running the heating system primarily serves one purpose, which is to remove excess heat from the engine. As the engine burns its fuel, a little less than a third of the released energy is converted into mechanical energy to run your vehicle. The remaining energy is converted to heat. Now whilst some of that heat is expelled through the exhaust system, the remaining heat heats the engine itself. Consequently without some kind of cooling system, the engine would fail due to this excessive heat within a few minutes of startup. There are basically two types of cooling system. Firstly, the liquid based cooling system. Secondly we have the air-cooling system. Because the vast majority of vehicles are liquid cooled and more complex, that is what we are going to concentrate on here.

The heating system of most vehicles is supplied by the cooling system. This is done via a secondary and smaller cooling system that allows hot coolant from the engine to pass through the heater core, fig 3, in the passenger compartment. The coolant then flows back to the water pump. However, instead of a thermostat controlling the flow of coolant around this system, a separate valve called the heater control valve, fig 4, facilitates the flow of coolant along this path.

.................................Fig 8

Electric fan systems normally consist of one or two fans mounted on the radiator. They are usually mounted on the engine side. There will also be some electronics, in the form of a sensor and switch, usually running through the vehicles computer, which will turn the fan on and off as required.
A lot of vehicles with forward facing engines use a mechanical fan system; this consists of the fan, sometimes a fan clutch, fig 11, a fan belt, and radiator shroud, fig 12. On such a system the fan belt turns the fan clutch, which in turn turns the fan, which pulls air through the radiator, all of which is constrained by the radiator shroud. The fan shroud is more or less a tunnel between the fan and the radiator, it’s primary function is to make sure that all the air pulled by the fan comes through the radiator, rather than being sucked from the surrounding area. Without a fan shroud, idle and low speed overheating is more likely to occur.

So why is a fan shroud required for a mechanical system but is not a prerequisite for an electrical system. Well as we have already said an electric fan is mounted directly to the radiator itself. Therefore it is very close to the radiator, and will actually move with the radiator. The chances of it pulling air from anywhere other than through the radiator are very slim. Contrast this with the mechanical fan, which is mounted, in some cases, several inches away from the radiator. The mechanical fan is mounted to and moves with the engine, and not with the radiator. So installing a mechanical fan close enough to the radiator to eliminate the need for the fan shroud would be almost impossible, without the fan causing damage to the radiator when the engines torque causes the engine to move.
Obviously as the engine runs the coolant gets hot, and as it gets hot it expands. Since the cooling system is sealed, this expansion causes an increase in pressure within the system. This pressure increase is perfectly normal and is part of the design of the cooling system. When coolant is under pressure, the temperature where the liquid begins to boil is considerably higher. This pressure along with the higher boiling point of ethylene glycol (antifreeze) allows the coolant to safely reach temperatures in excess of 250 degrees Fahrenheit. The radiator cap simply helps to maintain pressure in the cooling system up to a certain point. If the pressure builds up higher than the predetermined pressure point, there is a spring-loaded valve, which is calibrated to the correct Pounds per Square Inch to release the pressure, and a small amount of coolant is bled off. When this happens there is a system in place to capture the released coolant. It is stored in a small translucent plastic tank in the engine bay. This tank is known as the expansion tank and is not usually pressurized. As the engine cools, a partial vacuum is generated, as there is less coolant in the system than at start up. The radiator cap has a secondary valve, which is designed to allow the vacuum in the cooling system to draw the coolant back into the radiator. 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 expansion tank should be visible at around the Full-Hot line. After the engine has been sitting for several hours and is cold to the touch, the coolant should be around the Full-Cold line.

The water pump is made up of a housing, usually made of cast iron or cast aluminum and an impeller mounted on a rotating shaft, a drive pulley is usually 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 and prevent the flowing coolant from leaking out where the pump is attached to the block. An electrical water pump is basically the same as the mechanical pump but instead of being belt driven, as the name implies, it is electrically powered, fig 14.

This bypass setup is a system by which the coolant is able to bypass the radiator and go directly back to the engine. Depending on your vehicle this bypass passage may be as simple as a rubber hose, or a fixed steel tube. Other engines may well have a passage cast into the water pump or front housing. So when the thermostat is closed, the coolant is directed to this bypass and then sent back to the water pump. This then directs the coolant back into the engine without being cooled by the radiator.
When an engine block is manufactured, special sand is molded to the shape of the coolant passages in the engine block. This sand sculpture is positioned inside a mold and molten iron or aluminum is poured to form the engine block. Once the casting has cooled, the sand is loosened and removed through holes in the engine block casting leaving the passages that the coolant will flow through. These holes then need to be plugged or the coolant will pour out. This is where the core plugs come in. They are simply steel or brass discs or cups as seen in fig 17, that press fit in the holes in the side of the engine block. Steel core plugs do not last as long as brass plugs, they have a tendency to corrode. Replacing core plugs is a job that is either reasonably easy or extremely difficult depending on its location. Usual location for core plugs is the sides of the engine, usually 3 or 4 per side. Core plugs can also be found on the rear of the engine block and in the cylinder heads.
Gaskets are not the first thing that spring to mind as component parts of a cooling system. However, all internal combustion engines require that the cylinder heads and manifolds be securely sealed to the engine block. This is in order to stop oil, coolant and combustion gases from leaking away. In order to seal these components together and prevent such leaks we use gaskets. It’s safe to say that the head gasket, fig 18, has the hardest job of all the gaskets, as it must seal in all the things we mentioned earlier, oil, coolant and combustion gases. A typical head gasket is usually made of a soft composite material that is stamped with ridges that surround all possible leak points. When the head is placed on the block, the head gasket is sandwiched between them. As the head bolts are tightened the head gasket is crushed and forms a tight seal between the two.

There are a number of rubber hoses that make up the plumbing of the cooling system. These hoses connect the various component parts together allowing the cooling system to function. The main hoses in the system are the upper and lower radiator hoses. These two hoses are generally 1.25 to 2 inches in diameter and come in a variety of shapes depending on the vehicle, fig 19. They allow the flow of coolant between the engine and the radiator. Two smaller bore hoses, called heater hoses, are used to supply hot coolant from the engine to the heater core. Normally these hoses are between 0.625 and 0.75 inches internal diameter. A fifth hose, called the bypass hose, may be used on some engine applications as we have already discussed. These hoses are designed to withstand the pressure and heat inside the cooling system. Because of this, they are subject to wear and tear and eventually may require replacing as part of routine maintenance. Finally there is usually a small rubber hose that runs from the radiator neck to the expansion tank. This allows coolant that is released by the pressure cap to be sent to the reserve tank, as we have already discussed. This hose is about a quarter inch in diameter and is not normally part of the pressurized system.

Fig 19