Cooling system and service
Basic cooling system
Heat and temperature
Radiator pressure cap and reservoir
Water pump overhaul
Trouble diagnosis guide
144 part two engines and engine systems
The cooling system absorbs about one-third of the heat
produced by the engine. It not only removes heat, but
is responsible for keeping the engine at its most
efficient operating temperature. Only about a quarter
of the heat generated in the engine is actually used, the
rest of the heat has to be disposed of to prevent
damage to the engine.
There are two general types of cooling systems: air
cooling, in which the air is blown directly over the
engine, and liquid cooling, in which coolant is
circulated through the engine and radiator. Most
automotive engines are liquid-cooled and this type of
system is covered here.
Basic cooling system
A basic liquid-cooling system is shown in Figure 10.1.
The main parts are:
2. water pump
4. radiator hoses
additives) through the engine and the radiator. The
water pump takes coolant from the bottom of the
radiator and pumps it through the engine to the top of
Heat originates in the combustion chambers of the
engine and is transferred (by conduction) through the
cylinder walls and cylinder head to the water-jackets,
where it is transferred to the coolant. As it circulates,
coolant carries the heat from the engine to the top of
Hot coolant passing down through the radiator
transfers heat to the radiator. Heat is then dissipated by
the air which passes over the fins and through the
radiator core. The airflow is assisted by the fan. When
the coolant reaches the lower radiator tank, it is cool
enough to re-enter the engine to remove more heat.
Some heat is also dissipated by radiation. Heat
radiates from the outside of the engine and from the
exhaust into the atmosphere, to other parts of the
engine, and to the body.
The thermostat is located at the outlet from the
cylinder head. This is a heat-operated valve which
prevents coolant from flowing through the radiator
until the engine warms up.
Need for a cooling system
The basic function of the cooling system is to
transfer heat from inside the engine to the outside air.
It does this by circulating the coolant (water with
Combustion of the air–fuel mixture in the cylinders of
the engine produces a considerable amount of heat and
high temperatures. Heat is absorbed by the cylinder
The parts of a liquid cooling system
chapter ten cooling system and service
walls, the cylinder head and the pistons. They, in turn,
must be protected by the cooling system so that they
do not become overheated.
Cooling also prevents the oil on the engine parts
from breaking down and losing its lubrication
properties. While the engine must be cooled, it still
needs to operate at a high temperature. Removing too
much heat would lower the engine’s thermal
efficiency, and useful energy would be lost.
Heat and temperature
With engines, we are concerned with both heating and
cooling. Heat is a form of energy, and it is the heat
from the burning fuel in the combustion chamber that
provides the energy that causes the engine to function.
Heat and temperature are not the same. Heat is
energy, while temperature is the degree of hotness, or
coldness. Something is referred to as being hot when it
is above normal atmospheric temperature, and cold
when it is below atmospheric temperature.
To understand the difference between heat and
temperature, consider two pieces of steel, a large piece
and a small piece. Both can be at the same
temperature, but the large piece will contain more heat.
Effects of heat
When heat is applied or removed from any substance,
it can be affected in the following ways:
1. Change of temperature. Heat applied causes the
temperature to rise, and heat removed causes the
temperature to fall.
All substances expand when heated and contract
when cooled. Gas expands easily to many times its
size, but liquids and solids expand only a small
amount. Their molecules are fixed and are not free to
move like those of a gas.
Behaviour of water
The behaviour of water is different to all other liquids.
It contracts when cooled until it reaches 4°C, and from
this temperature until it freezes to become ice, it
expands. When cooled below 0°C, ice contracts like
any other solid.
Because of this, antifreeze chemicals are added to
the coolant in the cooling system to prevent it from
freezing. Without this protection, the water or
coolant could freeze, and expansion could damage
■ When servicing vehicles that operate in freezing
conditions, special precautions must be taken to
make sure that the coolant contains the correct
proportion of antifreeze.
The thermometer is an instrument for measuring
temperature (Figure 10.2). It consists of a glass tube,
with a bulb at its base filled with mercury. When
heated, the mercury expands and rises up a narrow
bore in the tube. A scale on the thermometer is
graduated in degrees. On the Celsius scale, water boils
at 100°C and freezes at 0°C.
2. Change of colour. Heat applied to metals,
particularly steel, causes a change in colour. If a
bright steel surface is heated, it will gradually
change colour, and, depending on the temperature,
different colours will be obtained. An engine part
that has been overheated can usually be identified
because it will be discoloured.
3. Change of state. Heat can change a solid to a liquid,
and a liquid to a gas. For example, ice can change
to water and water to steam. Metal heated during
welding will change from a solid to a liquid.
4. Change of volume. Heat applied causes expansion,
and heat removed causes contraction. This is
because the molecules of the substance that
is heated will move further apart and so increase the
volume, while molecules of the substance that is
cooled will move closer together and so decrease
Mercury thermometer with Celsius scale –
water boils at 100°C and freezes at 0°C
146 part two engines and engine systems
Heat can be transferred in three ways:
1. by conduction
2. by convection
3. by radiation.
All of these are used to remove heat from the engine.
Heat always moves from a hotter place to a colder one.
Figure 10.3 shows how heat from the coolant in a
radiator is transferred to the cooler air.
Heat applied to the edge of the container
produces convection currents in the water – a
spot of dye in a transparent container allows this effect to
With radiation, heat is transferred across space as rays.
These are transformed into heat when they strike a
colder object, so that the temperature of the object is
Dark-coloured materials radiate heat better than
light-coloured ones. For this reason, cooling fins on
cylinders and radiators are usually painted mat black
so that the heat will be more effectively radiated into
the surrounding air. Dark substances are also good
absorbers of heat by radiation.
Transfer of heat in a radiator
■ There are good conductors and bad conductors of
heat. Metals are good conductors, but asbestos,
wood, paper and most non-metal materials are bad
conductors and so can be classed as heat
There are a number of variations to liquid-cooling
systems. While they all function in a similar manner,
the location of the components varies with the type of
Figure 10.5 shows the arrangement for a transverse
engine. The engine has been sectioned to show the
water-jackets around the cylinders. The radiator is at
the front of the vehicle and the water pump is at the
front of the engine. Arrows show that the coolant flows
from the bottom of the radiator to the water pump, then
through the cylinder block and cylinder head to the top
of the radiator.
This is heat transfer by the movement of the molecules
of the substance. It relates to gases and liquids, but not
to solids. When a liquid or a gas in a container is
heated, it expands and its density is reduced.
The heated particles become lighter and so they
float upwards, allowing the colder, denser particles to
sink towards the bottom of the container. This sets up
convection currents – the principle is illustrated in
Figure 10.6 is a simplified arrangement of a cooling
system viewed from above the engine. Arrows show
the coolant flow. The thermostat, which controls the
flow through the radiator, is at the rear of this engine,
although in many engines, it is at the front.
When the engine is cold, the thermostat is closed.
This blocks off the flow to the top of the radiator, and
the pump circulates the coolant within the engine only.
When operating temperature is reached, the thermostat
In the engine, heat is conducted from the combustion
chamber through the metal parts of the engine to the
coolant in the cooling system.
chapter ten cooling system and service
1 radiator, 2 radiator cap, 3 hose to reservoir, 4 upper radiator hose, 5 lower radiator hose, 6 intake pipe,
7 thermostat, 8 water pump, 9 drive belt, 10 cylinder head, 11 heater inlet hose, 12 heater outlet hose HOLDEN LTD
Diagram of the coolant flow for a carburettor engine
opens and coolant circulates through the radiator as
well as the engine.
This engine has a heated intake manifold. The heat
is provided by coolant flowing through passages in the
manifold. This type of manifold is only used with
carburettors and throttle-body fuel injection. Heating
the manifold improves vaporisation of the air–fuel
mixture before it enters the engine.
148 part two engines and engine systems
To provide heating inside the vehicle, small hoses
carry coolant to and from the heater core which is
located under the dash of the vehicle. The flow of
coolant is controlled by the water valve.
The water-jacket is the name for the spaces around the
cylinders and within the cylinder head that carry the
coolant. These spaces are cast into the cylinder head
and block during manufacture. Because the valve seats
and guides need cooling, the head is designed to allow
the coolant to reach these areas. There are coolant
passages between the cylinder block and the cylinder
head which direct the flow of coolant.
bottom of the radiator, and coolant from the radiator is
drawn into the pump to replace the coolant that is
pumped through the engine.
The impeller shaft is supported in one or more
bearings. A seal between the impeller and the housing
prevents coolant from leaking out around the bearing.
The pump shaft is fitted with a flange for a pulley that
is driven by a belt from the crankshaft pulley.
Most water pumps are driven by an external belt,
but Figure 10.8 illustrates a water pump that is driven
by the timing belt. The pump is mounted to the front of
the engine and fitted with a notched pulley, which is
engaged with the teeth of the timing belt.
Water pumps are usually mounted at the front end of
the cylinder block, between the block and the radiator.
The pump consists of a housing with a coolant inlet
and an impeller (Figure 10.7).
Coolant enters the pump at the front of the impeller.
When the impeller rotates, coolant between the blades
is thrown outwards by centrifugal force and is forced
through the pump body and into the cylinder block.
The pump inlet is connected by a hose to the
Parts of a water pump that is driven by the
timing belt MAZDA
Two types of belts are used for fans and water pumps:
V-belts and ribbed belts.
1. V-belts. With this type of drive belt, friction
between the sides of the belt and the sides of the
pulley grooves enables drive to be transmitted from
one pulley to another. V-belts have a wedging
action in the pulley grooves that helps to prevent
belt slip (see Figure 10.24).
2. Ribbed belts. These are a combination of a flat belt
and a V-belt (Figure 10.9). The pulleys used with
the belts have a number of small grooves, and the
belt has a number of small ribs to match.
A water pump located on the front of the
The purpose of the fan is to produce a large flow of air
through the radiator core. The fan has a number of
blades. They can be made of steel or of a plastic
chapter ten cooling system and service
water pump is operated continuously by the drive belt
whenever the engine is running. The driving member
of the coupling (3) is fixed to the hub and so it also
The fan is attached to the driven member of the
coupling (4), so that it is operated through the fluid in
the coupling, but at a variable speed.
Section through a V-ribbed belt and pulley
Belt-driven engine fans are usually mounted on the
end of the water pump shaft and driven by the same
belt that drives the water pump and the alternator.
Fans are mounted close to the radiator and many
fans are partly enclosed in a shroud. This increases the
fan’s efficiency by ensuring that all the air that is
moved by the fan must first pass through the radiator
Some fans are fitted with a variable-speed coupling.
This is a type of fluid coupling filled with silicone,
which allows the fan to operate only when it is needed.
This is done to conserve energy.
A sectional view of a water pump that has a fluid
coupling for the fan is shown in Figure 10.10. The
The diagrams in Figure 10.11(a) to (c) show the
operation of a viscous fan coupling. It consists
basically of two chambers: the working chamber with
the drive plate (or driving member) and the storage
chamber. The chambers are connected by a valve and
contain silicone oil.
With the valve open, the oil circulates between the
two chambers. With the valve closed, the oil is held in
the storage chamber. The valve is controlled by a
bimetal spring on the front of the coupling. The spring
is sensitive to temperature changes.
The coupling operates as follows:
1. Engine cold. The silicone oil has been pumped into
the storage chamber by the coupling rotating
(Figure 10.11(a)). The valve is closed by the
bimetal spring to prevent oil from entering the
working chamber. The fan will operate, but at a
very low speed.
2. Engine warm. Heat in the engine compartment
distorts the bimetal spring (Figure 10.11(b)). This
pulls against the shift pin to open the spring valve
and allow oil to flow into the working chamber. As
a result, a hydraulic coupling is formed between the
drive plate and the fan housing and this increases
the fan speed.
3. Engine hot. At higher temperatures, the bimetal
spring distorts more and opens the spring valve
further (Figure 10.11(c)). This allows more oil to
enter the working chamber and further increase the
fan speed. A point is reached where the speed of the
drive plate and the speed of the fan are the same.
Water pump with a fluid coupling for the fan
1 bimetal spring, 2 housing, 3 driving member,
4 driven member, 5 pulley, 6 water pump bearing, 7 impeller,
8 coolant inlet, 9 seal, 10 pulley mounting, 11 bearing,
12 fluid coupling TOYOTA
Electric fans can be installed in front of the radiator or
behind the radiator. More than one electric fan is used
when the vehicle is fitted with air conditioning, or
when additional cooling is required.
The components of an electric fan assembly are
shown in Figure 10.12. These include an electric
motor, with a plastic fan fitted to its shaft, and a plastic
shroud. The motor is mounted on the shroud, which is
150 part two engines and engine systems
Electric fan and radiator assembly
located behind the radiator. Two of these fan
assemblies can be located side-by-side.
Electric fans are used extensively on vehicles with
transverse engines. This enables the fan (and the
radiator) to be fitted to the front of the vehicle, something that would be difficult to achieve with a belt
Fan operation and control
Figure 10.13 shows the basic arrangement of an
electric fan and its controls. There are two parts to the
electrical circuit: the fan operating circuit and the fan
1. Operating circuit. The circuit includes the fan
motor, the fan relay and the fan switch. The circuit
can be traced from the top fuse (6) and connector
(7) to the fan motor and then to earth through the
fan relay (4). When the relay points are closed, the
fan will operate.
Operation of a variable-speed (viscous) fan
2. Switch circuit. The switch circuit is from the
bottom fuse (6) through the windings of the relay to
chapter ten cooling system and service
Electric fan and associated components
1 shroud, 2 fan, 3 electric motor, 4 fan relay,
5 fan switch, 6 fuses, 7 connector FORD
the fan switch and then to earth. The fan switch is a
thermo switch that has its end fitted into the cooling
system. The switch is normally closed, but opens
when the coolant temperature reaches 100°C.
■ This is for a single-speed fan. Dual-speed fans are
also used. These are arranged to operate at a
slower speed when less air flow is needed.
Fan switch operation
When the coolant temperature is less than 100°C, the
fan switch is closed. Current flows from the fuse,
through the relay windings and the fan switch to earth.
This energises the relay windings and holds the relay
points open so that the fan does not operate.
When the coolant reaches 100°C, the fan switch
opens, the relay windings are de-energised and the
relay points close. This completes the fan circuit to
earth and the fan operates.
The fan will continue to operate until the coolant
temperature drops and the fan switch again closes.
When this occurs, the relay windings are energised and
the relay points open to stop the fan.
The fan will cut in and out as the temperature of the
coolant varies at the fan switch.
■ The fan relay is an electromagnetic switch. When
magnetised, the relay points open and when
demagnetised, the relay points close.
The thermostat is usually located in a small housing
attached to the cylinder head (Figure 10.14). The
housing also includes the coolant outlet from the
Thermostat and housing
1 air bleed, 2 thermo switch, 3 cover, 4 upper
radiator hose connection, 5 heater hose, 6 thermostat,
7 thermostat housing, 8 gasket HOLDEN LTD
cylinder head to the radiator, and the thermo switch for
the electric fan.
The function of the thermostat is to close off the
coolant outlet when the engine is cold. This restricts
circulation to within the engine until operating
temperature is reached. The thermostat then opens to
allow coolant to flow through the radiator.
The thermostat consists of a temperature-sensitive
device, which controls the opening and closing of a
valve in the coolant passage.
Most thermostats are of the wax-pellet type
(Figure 10.15). In the closed (cold) position, the valve
is held on its seat by the spring, so that coolant cannot
pass through the thermostat. This blocks off the
passage to the radiator, except for a small bleed hole.
As the temperature of the coolant increases, the
wax in the pellet expands and applies pressure to
the rubber diaphragm. This tries to force the pin out,
but the pin is fixed and cannot move, so the pellet
container moves downwards. This moves the valve off
152 part two engines and engine systems
Construction of a wax-pellet thermostat
its seat, opening the valve and allowing coolant to flow
to the radiator.
When the engine temperature drops, the wax in the
pellet contracts and allows the spring to close the
valve, blocking the flow of coolant to the radiator.
■ Thermostats are designed to open at specific
temperatures. For example, a thermostat designated as an 80°C unit will start to open between
78°C and 82°C and will be fully open at 95°C.
The cooling system has a temperature gauge and
sometimes a warning light. Any unusual rise in
temperature is a warning to the driver. The engine
should be stopped and checked before serious damage
A thermo sensor in the radiator or cooling system is
used to operate the gauge or warning light on the
Principle of a car interior heater
The volume of air flow can be controlled by
altering the motor speed. The direction of air flow is
controlled by opening and closing shutters in the
ducting through which the air flows.
The radiator consists of two tanks and a core. The core
is made up of a number of tubes which carry the
coolant between the tanks.
Air from the fan, and also from vehicle movement,
passes between the tubes and removes heat from the
coolant in the tubes. The tubes have fins which
increase the surface area over which the air flows and
this improves heat transfer.
There are two designs of core: cores with centre
fins and cores with horizontal fins. Each has a slightly
different construction. Passenger cars and light
commercial vehicles usually have cores of the tube and
centre-fin type, as shown in Figure 10.17. In this
design, the fins are in the form of corrugated strips
between the tubes. There can be a single row of tubes,
or two or more rows of tubes.
Interior car heater
Vehicle interior heaters have a small radiator that
transfers heat from the cooling system to the passenger
compartment of the vehicle.
Hot coolant from the engine is circulated inside the
radiator and a small electric fan blows air through the
radiator (Figure 10.16). The air absorbs heat so that
warm air enters the passenger compartment of the
The coolant to the heater can be turned off, when it
is not in use, by a shut-off valve which is operated by
the heater controls.
Radiator core with a double row of tubes
chapter ten cooling system and service
Automatic transmission cooler
Radiators for vehicles fitted with automatic
transmissions have a heat exchanger in the lower
radiator tank. This is used to remove heat from the
automatic transmission fluid, which can become very
Pipes from the automatic transmission carry fluid to
and from the heat exchanger. The automatic
transmission fluid is at a higher temperature than the
coolant and so heat is transferred from the automatic
transmission fluid to the coolant as the fluid passes
through the heat exchanger.
The radiator in Figure 10.30 has an automatic
coolant. This enables the engine to operate at higher
temperatures without the coolant boiling. This is
illustrated in Figure 10.19.
The higher coolant temperature also improves the
heat transfer from the radiator to the air. This is
because there is a greater difference between the
temperature of the coolant and the temperature of the
cooling air passing through the radiator.
■ Increasing the pressure of water in a container
naturally increases its boiling point. Because
coolant consists mostly of water, its boiling point is
increased in a sealed cooling system.
Many radiators have a top tank and a bottom tank and
a vertical core, but crossflow radiators have a tank at
each side. In effect, they are radiators that have been
turned on their side to reduce their overall height. The
coolant flows horizontally, or across the radiator from
one side to the other. In other radiators, the coolant
flows vertically from top to bottom.
A crossflow radiator is illustrated in Figure 10.18.
The design enables wider but lower radiators to be
Water in an open container boils at 100°C –
when enclosed in a container at a pressure of
50 kPa, the boiling point is raised to 112°C
Radiator cap operation
The operation of a radiator cap is shown in Figure
10.20. It has two valves: a pressure valve and a
vacuum valve. The coolant expands and contracts as it
is heated and cooled, so coolant is passed to and from
the radiator and reservoir.
1. Pressure valve. This valve is held closed by a
calibrated spring, which determines the pressure in
the system. When the system pressure is reached,
the pressure valve is lifted from its seat. This
releases coolant from the radiator to the reservoir
and prevents excessive pressure from building up in
Cross-flow radiator – coolant flows through
the core from one side to the other FORD
Radiator pressure cap
A pressure cap is used on the radiator to hold pressure
in the system and so raise the boiling point of the
2. Vacuum valve. When the engine cools and the
pressure drops, the vacuum valve opens to allow
coolant back into the radiator. In this way, the
system always remains full and is pressurised when
the engine is running.
Pressure and boiling point
Radiator caps are made in various pressure ratings as
shown in Table 10.1. The boiling point of the coolant
154 part two engines and engine systems
Operation of the valves in a radiator cap
CAP PRESSURE RATING
APPROXIMATE BOILING POINT
in the radiator is raised as shown in the table. This can
be compared with water in an open container which, at
sea level, normally boils at 100°C.
Sealed systems not only maintain the system full of
coolant to provide more efficient cooling, but they
exclude air from the system and help to reduce the
effects of oxidation and corrosion.
■ The radiator cap should not be removed to check
the coolant level. This should be done by checking
the level at the reservoir.
Sealed coolant systems have a reservoir connected to
the vent under the pressure cap in the top radiator tank
(Figure 10.21). This is usually made of plastic so that
the level of coolant can be seen. High and low levels
are marked on the outside of the reservoir.
When the vehicle is operating, the coolant is heated
and it expands. When the pressure lifts the valve in the
pressure cap some coolant will flow from the radiator
into the reservoir, raising the level in the reservoir.
When the vehicle is stopped, the temperature of the
coolant in the system drops and coolant is drawn from
the reservoir back into the radiator. With this arrangement, the cooling system is maintained completely full
at all times.
Coolant is a mixture of water and chemicals
(Figure 10.22). Distilled or deionised water is used
because it is free of harmful chemicals. The coolant
mixture does two things:
1. protects against corrosion
2. act as an antifreeze.
The chemical additives, or inhibitors, protect the cooling
system from corrosion and keep it clean. Additives are
needed in the coolant because of the different types of
metals that are used in the cooling system. Without protection, different metals can react with each other.
Aluminium cylinder heads and other aluminium
parts, such as coolant outlets, would soon corrode if
water were used instead of a proper coolant mixture.
chapter ten cooling system and service
3. Checking for gas leakage.
4. Checking the radiator and cap.
5. Checking the condition of hoses.
6. Checking the condition and tension of the fan belt
and water pump belt.
7. Checking fan operation.
8. Testing the thermostat operation.
9. Draining and flushing the system and refilling with
Coolant is the correct mixture of distilled or
deionised water and chemical inhibitor
The chemicals in the coolant also act as an antifreeze.
Antifreeze solutions are required to prevent the water
freezing when temperatures drop below 0°C. When
water freezes in the engine, the resulting expanding
force could damage the cylinder block or the radiator.
The most commonly used antifreeze material is
ethylene glycol. There are alcohol-based additives, but
these make only temporary antifreeze solutions
because they evaporate at temperatures below the
boiling point of water and are gradually lost. Periodic
additions are needed to maintain an antifreeze solution
of adequate strength.
The ethylene glycol antifreeze materials are of the
so-called ‘permanent’ type. They remain liquid at
the boiling point of water and do not evaporate.
Antifreeze materials are mixed with water in
various proportions. The lower the temperature, the
higher the percentage of antifreeze material needed to
prevent the mixture from freezing.
For most products, the coolant mixture is usually
30% additive and 70% water, but for very cold
conditions, a mixture of 50% additive and 50% water
■ Adding the corrosion inhibitors and antifreeze
chemicals to water also increases its boiling point,
although this is not the main reason for using a
Maintenance checks and servicing of the cooling
system could include:
1. Checking the coolant level.
2. Pressure testing the system for coolant leaks.
Checking the coolant level
In most systems, the coolant level can be seen through
the plastic reservoir. The maximum and minimum levels
are marked and the coolant should be above the
minimum level when the vehicle is cold. Normally, the
radiator cap should not be removed to check the coolant.
Caution: Take care when removing a radiator cap
when the engine is hot. The radiator cap should not
normally be removed unless the engine is cool and
there is no pressure in the system. If the cap must be
removed, then proceed carefully in the following way.
Cover the cap with a piece of cloth to prevent burns
to the hands and coolant spraying onto the arms and
face. Turn the cap only to the first stop and wait to
allow pressure in the system to be released through the
overflow tube. Then turn the cap further to remove it.
■ The radiator cap should not normally be removed
unless the engine is cool and there is no pressure in
Pressure testing the cooling system
A pressure tester (Figure 10.23) consists of a small
hand pump with a pressure gauge that fits on to the
radiator, and an adaptor for the radiator cap. It can be
used to test the cooling system for both external and
It is used as follows:
1. Fit the pressure tester instead of the radiator cap
and apply a pressure which is slightly above the
normal operating pressure (Figure 10.23(a)). If the
pressure holds steady, the system is not leaking.
2. If the pressure drops, check for external leaks at
hose connections, expansion plugs, the water pump
and the radiator.
3. If no external leaks are evident, then a faulty
cylinder-head gasket should be suspected, or the
more serious problem of a cracked cylinder head.
156 part two engines and engine systems
Checking for gas leakage
A defective cylinder-head gasket can allow combustion gases to leak into the cooling system. Apart from
forcing coolant from the system, the gas can form
acids which can cause corrosion.
A general test for gas leakage can be made by
running the engine with the radiator cap removed. The
coolant in the radiator tank is checked for bubbles or
any rise in level that may not be due to normal
circulation by the water pump. If either of these
occurs, then it is likely that exhaust gas is being forced
into the cooling system. A faulty cylinder head gasket
is the probable cause.
Checking the radiator
(a) checking the cooling system (b) checking
the radiator cap REPCO
4. The radiator cap can be pressure tested using the
adaptor (Figure 10.23(b)). The cap is tested to see
that it holds pressure and that the relief valve opens
at the rated pressure. A cap should be discarded if it
does not test correctly, if it is damaged or corroded,
or if the seals have swollen.
Checking for internal leaks
Where an internal leak is suspected, the engine oil can
be checked to see if it contains coolant. If coolant has
found its way into the oil pan, the dipstick could show
overfull and the oil will be emulsified.
Run the engine until operating temperature is
reached. Sharply accelerate the engine several times
and check for abnormal discharge of water through the
exhaust tailpipe, which could indicate a faulty cylinder
head gasket or cylinder head.
■ Small quantities of water that come from the
exhaust pipe are the result of condensation
occurring in the exhaust system and should not be
confused with a coolant leak.
Leaks in the radiator can become noticeable as
corrosion or scale on the tubes and fins below the leak.
Corrosion can also appear around a leak at the joint of
the core and the tank.
A radiator can be tested by removing it from the
vehicle and immersing it in water. The openings to the
tanks are sealed off and air pressure is applied at about
70 kPa. Air bubbles will identify any leaks.
The radiator core can be cleaned externally by
carefully blowing through the core from the rear to the
front to dislodge dust, insects or any other matter
which could restrict airflow.
Checking the hoses and hose connections
The appearance of the hoses and the connections
usually indicates their condition. If a hose is soft and
spongy when squeezed, it has probably deteriorated
internally and should be replaced.
If a hose is very hard and no longer flexible as a
result of overheating, it should also be replaced. Hose
clamps can be examined for tightness, and connections
checked for leaks.
Checking drive belts
The fan belt should be checked to make sure that it is
in good condition. Various belt conditions are shown
in Figure 10.24. These are for a V-belt, but ribbed belts
can suffer from the same conditions.
A belt that has become worn of frayed, or has
deteriorated, should be discarded. A defective belt
will not only cause overheating, but may also lead to
a discharged battery as a result of alternator pulley
chapter ten cooling system and service
Basic fan belt adjustment – the tightness of
the belt is checked by its deflection
Fan belt tension gauge
Possible conditions of V-belts
Belt adjustment (V-belts)
The belt must be correctly adjusted. If it is too tight, excessive loading will be placed on the water pump and alternator bearings. If the belt is too loose, slipping will occur.
The basic adjustment is shown in Figure 10.25. The
belt should be able to be deflected about 10 mm as
shown, without applying undue force. When a new belt
is first installed, it should be adjusted tighter than a
worn belt, to allow for stretching.
A special belt tension gauge (Figure 10.26) can be
used to adjust belts. The gauge is placed on the belt
and force is applied to deflect the belt. The force can
be read directly from a scale on the gauge. Belt tension
gauges are normally used for air-conditioning drive
belts that require higher tension than fan belts.
Fan blades should be checked to see if they are
cracked, bent or damaged. Any damage will cause
vibration and water pump failure.
Clutch drive fans should also be checked if they
work at engine operating temperatures. With the
engine switched off they should be difficult to turn by
hand when hot.
Electric fans should be checked for correct rotation
in relation to the air flow and that the temperature
sensor cuts in at the correct temperature.
Thermostat operation check
Operation of the thermostat can be checked by
suspending it by a wire in a pan of water and heating it
as shown in Figure 10.27. A thermometer is used to
158 part two engines and engine systems
Flushing equipment that uses compressed air and
water pressure is also used. Air pressure is used to
create surges of water and this helps to dislodge scale
and corrosion. The radiator and the water-jackets are
usually flushed separately.
Figure 10.28 shows the arrangement for a radiator
being reverse-flushed. Air and water pressure are being
applied to the bottom of the radiator, and a hose has
been connected to the top of the radiator to carry the
The water-jackets in the engine can be flushed in a
similar way. During flushing and cleaning, the interior
heat control should be turned to the hot position so that
water will circulate through the heater.
Testing a thermostat in hot water
show the temperature at which the thermostat starts to
open, as well as the temperature at which it is fully open.
These temperatures are usually stamped on the
flange of the thermostat.
Water pump check
The most likely water-pump faults are leaks and noise.
Leaks are usually more noticeable cold, and the first
indication may be loss of coolant after the vehicle has
been standing overnight.
Noise from the bearing is a dull rumbling noise.
From the seal, it is a more high-pitched noise.
Draining and flushing the cooling system
A cooling system showing signs of rust or corrosion
can be cleaned with special chemical radiator cleaners
and then flushed with water using a normal water hose.
■ Changing the coolant at regular intervals, using
deionised water and a suitable antifreeze/inhibitor,
will maintain the internal parts of the cooling
system in good condition.
Repairs may require the removal of the radiator, the fan,
the thermostat, the water pump, and the pipes and hoses.
In most cases, the coolant will have to be drained.
Removing and replacing the radiator
Figure 10.29 shows a typical radiator assembly with an
electric fan. The radiator is mounted in rubber insulators
at the top and bottom. It is not bolted firmly to the body,
but is allowed a small movement in the insulators. This
allows it to become a form of vibration damper, which
helps to absorb some of the engine vibrations.
The general procedure for removing a radiator is as
1. Drain the coolant.
2. Remove the upper and lower radiator hoses.
3. Remove the automatic transmission cooler hoses
4. Disconnect the overflow tube from the reservoir.
5. Disconnect the electrical connection from the
6. Remove the electric fan and shroud from the rear of
the radiator, and also the one from the front, if fitted.
7. Unbolt the upper insulator brackets from the body
Method of reverse-flushing a radiator
8. Lift the radiator out of the lower insulators.
chapter ten cooling system and service
Radiator assembly components
To replace the radiator, use the reverse procedure to
removing, with attention to the correct fitting of hoses and
tightness of hose clamps. After filling with coolant, run
the engine. Then recheck to make sure that the system is
actually full, and that there is no air trapped in the system.
Radiators with metal cores and metal tanks have the
tanks soldered to the core. These can be separated by
melting the solder. With the tanks removed, the tubes
of the core can be cleaned, or a new core fitted. When
the tanks are refitted, they are soldered to the core.
Radiators are also made with aluminium cores and
plastic tanks. Figure 10.30 shows a crossflow radiator
of this type. It has an oil cooler for the automatic
transmission fluid in one of the side tanks.
The plastic tanks are flange mounted to the
aluminium core and secured by clinch tabs that are
formed as part of the core. When the radiator is
assembled, the tabs are bent over the flanges of the tanks.
Radiator assembly with an aluminium core, plastic side tanks and an automatic transmission cooler
160 part two engines and engine systems
To make the joints coolant-tight, high-temperature
rubber seals are used between the core and the side tanks.
The tanks can be removed by levering the clinch tabs
away from the tanks. When being assembled, the tabs
are bent over the edges of the tanks to secure the core to
the tanks and compress the seal. Pliers, or a special
clamping tool, are used to bend the tabs into place.
Replacing the thermostat
In most vehicles, the thermostat is located under the
water outlet as shown in Figure 10.14. Its flange fits
into a recess in the housing.
After removal, check the thermostat valve, which
should be closed because the thermostat is cold. Test in
heated water as described previously.
■ A thermostat that does not open will cause overheating, while a thermostat that does not close will
prolong engine warm-up.
Water pump overhaul
Figure 10.31 shows the dismantled parts of a water
pump. Some water pumps are not repairable and must
be replaced with a new component if found to be
faulty. Other water pumps can be dismantled and the
bearings and seal renewed.
Water pumps are dismantled by means of a press and
suitable pressing tools or pullers, which are used to
remove the pulley hub and impeller from the shaft, and the
bearing and shaft from the pump housing (Figure 10.32).
Pressing a bearing into a water pump
The general sequence of dismantling is shown by the
numbers (1) to (5) on the illustration. The sequence is:
1. Cover from the rear.
2. Pulley hub from the front.
3. Impeller from the shaft at the rear.
4. Shaft and bearing from the pump housing.
5. Seal from the housing.
■ Use care when removing bearings, otherwise a
good bearing can be damaged and will have to be
renewed. Careless assembly could also damage the
bearings or seal and spoil a repair job.
Water pump bearings
Figure 10.33 shows a different pump that has been dismantled. It has a rear cover and a pressed steel impeller.
In most water pumps, the bearing and shaft are an integral
assembly. This is packed with grease during manufacture
Dismantled water pump with parts identified
chapter ten cooling system and service
Rust, scale and corrosion that form in the coolant
passages will restrict coolant flow and particles carried
into the radiator tank will clog the tubes.
Overheating can sometimes be caused by a fault in
the ignition or fuel systems. Any loss of power due to
a retarded spark or a poor fuel mixture could cause a
problem, but poor engine performance should be more
noticeable than overheating.
Dismantled water pump – the dismantling
sequence shown is
1 cover, 2 pulley hub, 3 impeller, 4 shaft and bearing, 5 seal,
6 pump housing FORD
and requires no lubrication prior to being installed. In
other pumps, two ballraces with a separate shaft are used.
These require packing with grease before installing.
Both these types of bearings are shown in
Coolant loss is evident by the need to constantly top up
the coolant in the reservoir. Bad coolant leaks are easy
to find, but leaks which occur only under operating
pressure are more difficult to locate and the system
would have to be pressure-tested.
External leaks could come from the radiator, water
pump, hose connections, or core plugs in the cylinder
block or head.
Internal leaks could be due to a faulty cylinder-head
gasket, a cylinder head with a warped surface, or
enlarged water passages in the cylinder head and water
pump due to corrosion. Other less well known causes
of metal loss are cavitation or electrolysis.
1. Cavitation. This is the implosion of small vapour
bubbles against a metal surface causing erosion of
the metal particles. Cavitation is caused by changes
in pressure of the coolant and usually occurs around
the water pump impellor. Correct pressure in the
cooling system is the method of solving the problem.
2. Electrolysis. This is the loss of metal particles,
especially soft metals such as aluminium and solder,
by electrical action in the cooling system. The
electrical source can be from inside or outside the
cooling system. External sources can be cooling fans
or lights with an electrical fault seeking an earth path
through the coolant and radiator core. It can be
measured using an analogue multimeter and is
referred to as stray current. (Note that a digital meter
is not suitable for this test because of its operation
characteristics.) The positive probe is suspended in
the coolant and the negative attached to battery earth.
The reading should not be more than 0.05 volts with
the engine running and all electrical items switched
‘on’. If excessive voltage is detected, each electrical
item can be switched off to find the fault.
Overheating is noticed by a high temperature-gauge
reading. The main causes are loss of coolant and
accumulation of rust and scale in the system. Coolant
that is discoloured or rusty indicates lack of maintenance.
Internal electrical sources are usually caused by
poor coolant condition. Check the coolant condition
using a coolant hydrometer. Discoloured coolant
should be replaced and the cooling system flushed.
Water pump shafts and bearings
Engine overheating, loss of coolant and slow warmup are the most common problems with cooling systems.
162 part two engines and engine systems
Internal leaks can often be detected by running the
engine at fast idle and looking for the formation of
bubbles in the radiator. (See previous section
‘Checking for gas leakage’.)
Oil in the radiator can indicate a leaky cylinderhead gasket, or it could be from a faulty automatic
transmission oil cooler in the radiator. Where an
engine-oil cooler is fitted, a leak in the cooler could
allow engine oil to mix with the coolant.
Water formation, emulsified oil, or an overfull
reading on the engine-oil dipstick could be the result of
an internal leak.
Air cooling, liquid cooling, radiator, thermostat,
coolant, additives, heat, temperature, heat transfer,
thermal efficiency, energy, conduction, convection,
radiation, molecule, heat conductor, heat insulator,
density, state, gas, liquid, antifreeze, thermometer,
Celsius, centrifugal, water-jackets, water pump,
impeller, shroud, variable-speed, coupling, relay
(electrical), thermo switch, crossflow, boiling point,
pressurised, distilled, deionised, inhibitor, ethylene
glycol, heat exchanger, thermo sensor, cavitation,
electrolysis, stray current.
The likely cause of a slow warm-up is that the
thermostat is faulty and not closing¸ or that it has been
The silicone clutch on a variable-speed fan could be
faulty, or an electric fan may not be cutting out when
the engine is cold.
Trouble diagnosis guide
1. Engine overheats:
Loss of coolant
Belt tension incorrect
Radiator fins obstructed
Cooling system passages blocked by rust or scale
Water pump inoperative
Incorrect or faulty pressure cap
Electric fan inoperative or incorrect rotation fan
shrouds are in place
Name the basic parts of a liquid-cooling system.
What is the difference between heat and
What are the four possible effects when heat is
applied to a substance?
What is the unusual effect that occurs with water
when its temperature is reduced?
Heat can be transferred in three different ways.
What are these?
What is meant by radiation?
Briefly, how does a variable-speed fan operate?
Where are electric fans located?
How are electric fans controlled?
How is heat removed from the engine?
Why is it necessary to remove heat from the
2. Loss of coolant:
Loose hose connections
Water pump leaking
Cylinder-head gasket defective
Incorrect tightening of cylinder-head bolts
Cylinder-block core plugs leaking
Cracked cylinder head or block
Warped cylinder-head or cylinder-block surface
Radiator cap defective
How does a water pump operate?
What is a water-jacket?
Where is a thermostat located?
How can a thermostat be checked?
Why are pressure caps used on radiators?
How is a pressure tester used to check a cooling
What is meant by reverse flushing a cooling
3. Engine slow to reach operating temperature:
Thermostat inoperative or incorrect heat range
Electric fan operating continuously
Temperature gauge defective (not indicating true
Why are additives used with water in the cooling system?
What are the possible causes of engine overheating?