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CHAPTER 1

ENGINE COOLING SYSTEM BASIC OPERATION

Automotive engine cooling systems have several functions. They remove excess heat from the engine. They help a cold engine quickly reach operating temperature and then maintain constant engine operating temperature. They also provide heat for the passenger compartment for street vehicles.

Understanding the basic operation of automotive cooling systems begins with a discussion on thermodynamics. Thermodynamics primarily deals with the conversion of one form of energy to another (heat that creates motion) along with the systems used to affect these conversions. The various concepts and laws that describe the movement of heat are called the laws of thermodynamics. These laws further explain the concepts of temperature and heat transfer, which are also important for understanding how a cooling system functions.

Thermodynamics

The main thermodynamic concept is energy, which is the ability to do work. Thermodynamics also deals with heat and temperature and their relation to energy and work. For example, energy can move from one object to another due to the difference in the objects’ temperatures. When heat acts or moves the object through a distance, work is done.

We are discussing thermodynamics because heat is generated in an internal combustion engine, and it needs to be removed by the engine cooling system. By understanding thermodynamics, anyone can design a cooling system that removes enough heat for efficient engine operation, which results in no overheating.

Thermodynamics is simply heat that is used to generate power and cause motion. It is an increase in internal energy of a closed system that is equal to the total of the energy added to the system. If the energy entering the system is supplied as heat and if energy leaves the system as work, the heat is accounted for as positive and the work as negative.

Systsms and Surroundings

In thermodynamics, there are systems and surroundings. The system is composed of particles in balance and the quantity of matter or stuff under consideration. Everything else is called the surroundings.

There are open and closed systems. In a closed system, there is no interchange of matter between systems and surroundings. In an open system, there is an interchange, which is called a process. Any process or series of process where the system returns to its original condition is called a cycle. An internal combustion engine cooling system can be closed, open, or a combination of the two. ■

Thermodynamics developed in 1824 out of a desire to increase the efficiency of early steam engines when French physicist Nicolas Léonard Sadi Carnot wanted to help France with the Napoleonic Wars. In 1894, the definition of thermodynamics was stated as: “Thermo-dynamics is the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.”

The early application of thermodynamics was later applied to mechanical heat engines. It was then applied to chemical reactions. Other forms of thermodynamics have evolved in the decades since.

Thermodynamic Laws

The laws of thermodynamics explain the movement of heat. There are four laws of thermodynamics: the Zeroth law, first law, second law, and third law. We will examine the three that apply to engine cooling system design.

Zeroth Law of Thermodynamics

The Zeroth law states: If two systems are in thermal balance with a third system, they are in thermal balance with each other.

This law helps define the concept of temperature. Temperature is the coldness or hotness of something solid, liquid, or gas. It is the measure of the speed of the molecule vibration of that thing. An increase in temperature indicates that the speed of the molecules has increased. A drop in temperature means a decrease in molecular speed. When heat is removed from an item, it becomes cold with a lower temperature. Heat always flows from the warmer object to the colder object.

Temperature is not a measure of the total quantity of heat; instead, it is a measure of the degree of heat that something possesses. Temperature tells you whether something has gained or lost heat. A thermometer is used to measure temperature, and in today’s auto repair business, an infrared digital thermometer is used.

The First Law of Thermodynamics

The first law of thermodynamics states that heat is a form of energy. It is subject to the principles of conservation, which states energy can neither be created nor destroyed, only changed. This means that a perpetual motion machine of the first kind (a machine that produces work with no energy input) is not possible.

The Zeroth law of thermodynamics deals with the concept of temperature, or the speed of an item’s molecules. It allows the existence of an empirical parameter (the temperature), which means that systems in thermal equilibrium with each other have the same temperature. A particular physical body, for example a mass of gas, can match temperatures of other bodies, but that does not mean the temperature is a quantity that can be measured on a scale of real numbers.

An infrared digital handheld noncontact thermometer is used to measure temperature. These are available online or in retail stores for about $45. It is pointed at whatever needs to be measured and the temperature reading appears on the digital screen. (Photo Courtesy James Halderman)

The Second Law of Thermodynamics

The second law of thermodynamics says that the entropy of a closed system must either increase or stay the same. Entropy has a very precise definition in science that is not commonly known by most people. It can be roughly related to the level of disorder or the loss of information or the amount of useless energy; that is, energy that cannot be used to perform work. For example, a tool drawer system that is disorganized has a higher entropy than an ordered tool system with tools neatly arranged. Similarly, a state in which information decreases or the amount of useless energy increases can be said to be a state in which entropy is increasing.

The first law of thermodynamics states that when energy passes (as work, as heat, or with matter) into or out from a system, the system’s internal energy changes in accord with the law of conservation of energy.

The second law also says that heat transfer always flows from the warmer object to the colder object. When a block of ice melts, it is absorbing heat flowing into it. Engine coolant is cold, so heat from hot engine combustion flows to the colder coolant. Heat cannot spontaneously flow from a colder location to a hotter location.

The second law is an observation of the fact that differences in temperature, pressure, and chemical potential tend to even out over time in a physical system that is isolated from the outside world. The conversion of heat to work is limited by the temperature at which the conversion occurs. No cycle can be more efficient than a reversible cycle operating at temperature limits.

The second law of thermodynamics states the sum of the entropies (disorder levels) of the interacting thermodynamic systems increases. This is to say that a perpetual motion machine of the second kind (a machine that spontaneously converts thermal energy into mechanical work) is impossible.

Thermal Expansion

Another aspect of thermodynamics involves thermal expansion. Thermal expansion is the dimensional changes exhibited by solids, liquids, and gases during changes in temperature at constant pressure. As solids, liquids, and gases heat up, they expand, taking up more room. These changes affect the amount of pressure in a vessel.

The solid engine block must constantly and efficiently deal with different types of liquids and gases. Engine coolant has to withstand drastic temperature changes, and liquid expands and contracts based on these differences. Throughout the expanding and contracting events, the coolant must withstand the pressures in the engine while maintaining integrity.

Let’s examine the laws of thermal expansion: Boyle’s law and Gay-Lussac’s law.

Thermal expansion is the expansion of matter (solid, liquid, or gas). Shown here are two beakers of engine coolant being heated by Bunsen burners. The liquid coolant in the beaker on the right expanded when heated to the boiling point of water (212°F).

Boyle’s Law

Boyle’s law states that the pressure exerted by a mass of a perfect gas is inversely proportional to its volume if it remains at a constant temperature and the volume does not change. In other words, at a constant temperature, the amount of gas depends on the volume in the vessel holding it (high pressure/low volume). If volume increases, the pressure decreases, and vice versa.

Boyle’s law applies to compression in a spark ignition engine as well. When the piston rises on compression, cylinder volume is reduced and the pressure increases. This exerts pressure on the cylinder walls when the same number of molecules now have less space to move.

Gay-Lussac’s Law

Gay-Lussac’s law, or the pressure law, was discovered by Joseph Louis Gay-Lussac in 1809. This law states for a given mass and constant volume of an ideal gas, the pressure exerted on the sides of its container is directly proportional to its absolute temperature. So, if a gas’s temperature increases, the particles move faster and create more pressure.

Pressure

Pressure must also be included in a discussion of thermodynamics. Atmospheric pressure is the force that is exerted per unit area by an atmospheric column (the entire body of air above an area). It is measured by a barometer and expressed in several different systems of units: inches (or millimeters) of mercury (Hg), pounds per square inch (psi), dynes per square centimeter, millibars (mb), standard atmospheres, or kilopascals (kPa). For our purposes, we will use pounds per square inch (psi).

This diagram shows atmospheric pressure and vacuum. The red center line is at atmospheric pressure of 14.7 psi (sea level) with the base line of 0 inches of Mercury (Hg). When the pressure decreases, the vacuum increases.

Atmospheric pressure at sea level is 14.7 psi and decreases as elevation increases. At 5,000 feet (1,524 m) above sea level, a 1 square inch column of air from the earth’s surface to the outer edge of the atmosphere is 5,000 feet (1,524 m) shorter than the same column at sea level. Therefore, the weight of this column of air is less at 5,000 feet (1,524 m) elevation than at sea level. As altitude continues to increase, atmospheric pressure continues to decrease.

Vacuum

Vacuum is the absence of pressure or just low pressure. When a space contains a vacuum, it contains a smaller quantity compared with the amount of air the same space is capable of containing (as dictated by atmospheric pressure). Vacuum could be measured in psi, but inches of mercury (Hg) is more commonly used. A complete vacuum or absence of pressure is equal to 29.995 Hg (101.06 kPa).

Atmospheric pressure and vacuum are used in all automotive systems. For example, atmospheric pressure is available outside the engine air intake. When a piston moves downward with an intake valve open, a vacuum is created in the cylinder above the piston. The air moves rapidly from the high pressure outside the air intake to the lower pressure in the cylinder.

Heat Transfer

Now that we’ve discussed the relationship between temperature, pressure, and volume, it is time to examine how heat affects energy. Heat is thermal energy and cannot be destroyed, only transferred. It always moves from a hotter object to a colder object, as we saw in the second law of thermodynamics.

Heat is transferred in three ways: conduction, convection, and radiation.

The textbook definition of conduction in the sense of heat transfer is transmission through or by means of a conductor. Conduction is heat transfer from molecule to molecule through solids in contact. A rod going into a fire transfers heat from the fire to the other end of the rod, and if you are holding it, ouch.

Conduction

Conduction is the transfer of heat from molecule to molecule through solids and fluids in intimate contact at rest. It is heat transfer from one solid to another. No displacement of the heated body takes place during conduction. The heat travels through a rod via conduction from molecule to molecule until the end you are holding is near the temperature of the end in the fire. The action of a solid to conduct heat is called conductivity.

Convection

Convection is heat transfer by the molecular motion in the heated substance itself. It only takes place in liquids and gases. There are two forms of convection heat transfer. Natural convection is when the fluid motion is caused by different densities in a gravitational field. Forced convection is the method of heat transfer between a fluid and a solid surface in relative motion, when the motion is caused by forces other than gravity.

Most of the heat in a combustion engine flows between the coolant (working fluid) and the engine parts, and it is transferred via forced convection. Heat transfers by circulation though fluids, such as coolant (air in some engines), in motion between the fluid and a solid surface in motion, such as a piston. It is heat from combustion that is transferred to the cylinder wall, transfers to the coolant, and is carried away at the radiator. This form of heat transfer includes conduction as well as fluid motion.

Radiation

Radiation is the transfer of heat through space. This takes place in a vacuum (absence of pressure or very low pressure) or through solids and fluids that are transparent to wavelengths in the visible and infrared range. A small fraction of the heat transferred to the engine cylinder walls from the hot combustion gases transfer via radiation. The radiator in an engine cooling system is the main heat exchanger.

The textbook definition of convection is the action or process of conveying movement in a gas or liquid in which the warmer parts move up and the cooler parts move down. This graphic shows the heat transfer by molecular movement in the heated substance itself, or heat transfer by circulation through the engine coolant.

Cooling System Operation

In a basic cooling system, coolant is drawn from the radiator by the water pump, and this pump pushes the coolant into both sides of the cylinder block. Coolant flows around the cylinders and up into the cylinder heads, where it circulates around the exhaust passages and fire deck areas of the cylinder head. The coolant then flows out of the front into the thermostat housing. This housing stops the main flow when the engine temperature is below the opening temperature of the thermostat. It then directs a small portion back to the pump through a bypass hose or passage. When the thermostat is open, the coolant flows back into the top of the radiator.

Radiation is the process of emitting radiant energy in the form of waves or particles. It is the combined processes of emission, transmission, and absorption of radiant energy. The heat source has no physical contact with the object receiving the heat and the heat is transferred to this object.

Engine coolant flow starts at the water pump with cooled coolant pumped into the cylinder block. It then goes through the heads, which is the hottest part of the engine. The coolant carries the heat of combustion through the intake manifold and thermostat to the radiator, where the heat is removed.

Reverse-Flow Cooling System

From 1992 until 1996, General Motors used a reverse-flow cooling system in the Chevrolet Corvette LT1 Engine. In this system, coolant flows to cool the heads before the cylinder block. There are some import car manufacturers that use this type of system, but all of the domestic original equipment manufacturers (OEMs) use the traditional flow system with the cylinder heads receiving the coolant last before returning to the radiator for cooling.

In the traditional system, the water pump draws coolant from the radiator and the coolant circulates through the cylinder heads first via the water pump and then into the cooling jackets in the engine block. Coolant is then directed back to the radiator where it’s cooled. In the reverse-flow cooling system, the water pump draws coolant from the radiator and the coolant passes through the thermostat on the inlet side of the pump. Vapor is vented off (if there is any) through the air bleed pipe, and the coolant travels down through the bores and into the block. Once the coolant leaves the engine block, it returns to the water pump, where the coolant travels through a cast internal passage to the upper radiator hose back to the radiator for cooling.

The system directs some coolant through hoses to the heater core. A sealed recovery or expansion reservoir connects to the radiator surge tank in order to retrieve the coolant displaced by expansion. As the coolant cools and contracts, vacuum draws the coolant back into the radiator surge tank. The radiator surge tank provides a coolant fill point and a central cooling system air bleed location. A sensor can be used in the radiator surge tank to show the coolant level. When the coolant in the system falls below the recommended level, a warning lamp in the instrument panel turns on.

In a 1992–1996 Chevrolet Corvette LT1 engine reverse-flow cooling system, coolant flows to cool the heads before the cylinder block.

Reverse-Flow System Components

The reverse-flow cooling system consists of these components:

• Air bleed venting circuit

• Coolant recovery reservoir or expansion tank

• Cooling fans

• Cylinder heads that are unique for LT1 block

• Hoses

• Radiator

• Surge tank

• Thermostat

• Water pump

• Water pump gear driveshaft

Cooling System Components

Coolant circulates through an engine and absorbs excess heat through the convection heat transfer process. A combustion engine cooling system can be closed or open. In a closed system, the engine coolant has no contact with outside air. In an open system, the coolant has contact with the outside atmosphere.

The coolant circulating in a typical V-8 engine will absorb heat through the convection process. The flow in this illustration is from the hot side to the cold side of the system. Hot coolant returning with the heat of combustion in the coolant goes into the top or inlet side of the radiator when the heat is transferred to the air moving through the radiator cooling fins over the cooling tubes. It is being sucked out of the radiator at the outlet or bottom by the water pump, where it returns to the engine to start the cooling process over again.

Open systems are things of the past with the exception of marine applications. All current systems are a combination of the two, where the coolant circulating through the engine has no contact with the outside air, but at the radiator pressure cap there is contact through the vacuum or air valve, which allows the entrance of atmospheric pressure.

When designing a new engine cooling system for a street rod or track vehicle, the selection of cooling system components is very important for engine life and performance. Consider the following components carefully.

Water Pump

Early engines relied on thermosiphon cooling. In this system, hot coolant left the top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine. Circulation was powered by convection alone with no moving parts.

The water pump uses centrifugal force to circulate the coolant. It consists of a fan-shaped impeller set in a round chamber called a volute with curved inlet and outlet passages. (Photo Courtesy Jim Halderman)

Modern liquid-cooled engines usually have a circulation pump, which in automotive lingo is called a water pump. The water pump is located between the engine block and the cooling fan. It is driven by the crankshaft through belts and pulleys or directly using a gear. It also has a fan-shaped impeller set in a round chamber (called a volute) with curved inlet and outlet passages. In addition to the housing, the pump also has a coolant inlet and outlet.

The water pump uses centrifugal force to circulate the coolant. To do this, the impeller rotates, forcing the coolant through the engine. The inlet is attached to the bottom of the radiator, where cooled coolant is sucked out of the radiator via the lower radiator hose and flow out the outlet into the engine via the upper radiator hose.

The typical discharge rate is 70 gallons per minute (gpm) with a cooling system capacity of 6 gallons with 3 gallons in the engine. An engine on an engine dynamometer will show the actual amount of coolant flowing through the cooling system.

This high-flow water pump runs in excess of 6,000 rpm and can deliver up to 120 gpm of engine coolant. (Photo Courtesy Meziere Performance)

Water Distributing Tube

Some engines of the past, such as the Ford flathead V-8, used a water distributing tube inserted in the water jacket near the exhaust valve. The front end of the tube received water directly from the water pump. Holes in this tube had discharge jets that provided extra cooling for the exhaust valve seats and stems.

Radiator

Radiators may be manufactured from a range of electrochemically incompatible metals (aluminum, cast iron, copper, brass, solder). Most internal combustion engines are liquid cooled. These engines use a mixture of water and chemicals, such as antifreeze and rust inhibitors called engine coolant, run through a heat exchanger called a radiator, which is cooled by airflow.

Radiators can be downflow or crossflow type, such as the Dual Cooler tank (shown). They can also be designed with a single, dual, or triple pass of coolant through the system. The radiator system may also use a separate expansion tank outside the radiator, which is partially filled with coolant and is connected to the radiator pressure cap. (Photo Courtesy Derale Performance)

The radiator pressure cap uses a pressure valve (left) controlled by a spring that holds pressure in the system until it reaches a specified pressure and then opens. It also contains a vacuum valve (right). As temperatures drop and the coolant contracts, a vacuum is created in the engine’s cooling system. On older open cooling systems without tanks, the vacuum valve opens to prevent the end tanks from being crushed or imploded. On a closed system with an overflow/recovery reservoir or expansion tank, the vacuum valve opens and allows coolant to flow from the overflow tank back into the radiator. As pressure inside the system drops, outside air pressure helps the coolant flow in.

The radiator holds a large column of coolant in close contact with the flowing air. This allows the radiator to transfer heat via convection from the coolant to the outside air. Radiators can be downflow or crossflow type, and they can have a single, dual, or triple pass of coolant through the radiator design.

The radiator system may also use a separate expansion tank outside the radiator, which is partially filled with coolant and is connected to the radiator pressure cap. Coolant expands as it heats up and sends part of the coolant into the expansion tank.

Radiator Pressure Cap

A radiator cap allows an engine’s coolant (water and antifreeze) to expand and contract without allowing air to enter the cooling system. An upper seal protects the system at all times. After the engine warms and system pressure reaches the cap’s rated pressure, a pressure spring compresses and pressurized coolant flows into a reservoir or a coolant overflow tank. This allows for expansion of the heated fluid.

The boiling point of water is always 212°F under standard atmospheric pressure (14.7 psi). If the coolant in a closed system is under pressure greater than 14.7 psi, the boiling point of the coolant will be higher. This occurs because the coolant molecules are compressed by the pressure and will have to vibrate more for the temperature to increase. For every pound of spring pressure, the boiling point is increased by 3 degrees. This means that a cap rated at 15 pounds will increase the boiling point in a system by 45°F with a boiling point of 257°F (212 + 45).

The radiator pressure cap also has a vacuum valve that allows the coolant to flow back into the radiator as the engine cools. As temperatures drop and the coolant contracts, a vacuum is created in the engine’s cooling system. The vacuum valve opens and allows coolant to flow from the pressure cap and either onto the ground (early cooling systems) or back to the radiator (later systems).

As the cap wears, the spring weakens and excessive coolant is allowed to flow into the reservoir tank. This overflow will result in a loss of coolant at the seal between the cap and the radiator as well as the overflow tank. If the system passes a coolant system pressure test with no leaks in the engine or passenger compartments, suspect a faulty radiator cap.

High-performance radiator pressure caps range from 19 to 32 psi. If needed, engines with higher operating temperatures can be designed with OEM parts.

Overflow, Recovery, Reservoir Tanks

When coolant expands from engine temperatures rising, it typically creates steam, which needs to be vented in order to protect the cooling system. Vehicles from the 1950s, 1960s, and 1970s had a hose attached to the radiator’s filler neck below the cap that vented to the atmosphere, dripping on the ground just below the radiator. The steam was able to escape; however, there was another issue. As steam expands so does the pressure, and often the vented cap also expelled coolant through the hose and onto the ground. This system was good for engine cooling but not good for the environment.

This vintage reservoir tank was found on a C3 (third-generation Corvette built from 1968 to 1982), which was the old Stingray version.

In the late 1970s, environmental control regulations required adding a reservoir to that vent tube. This allowed the steam to be expelled and captured the coolant that came with the steam. Capturing that expelled coolant meant that it could then be recovered and reintroduced into the radiator, hence the name recovery or reservoir tank. It is also called an overflow tank. For this book, we will refer to them going forward as reservoir tanks.

Returning coolant to the radiator is possible because the reduced steam pressure allows the atmospheric pressure to push coolant from the reservoir tank back into the radiator through the vented radiator cap. This adds more coolant to your system and helps to keep the engine cooler.

Polymer or plastic tanks have full cold/hot marks that indicate where the coolant should be while cold (non-running) and hot (running). (Photo Courtesy Jim Halderman)

Reservoir tanks use full cold/hot marks to indicate the ideal coolant level while an engine is cold (non-running) and hot (running). These tanks also have a vent. Expelled coolant will enter the tank from the bottom, and when the level rises it will expel through the vent tube. In contrast to the expansion tank, the reservoir tank featured a vented cap and was not required to be above the cylinder heads.

Catch Tanks

A catch tank also collects expelled coolant, but it will be drained from that tank at a later point. You should not confuse the reservoir tank with a catch tank. While both the reservoir tank and the catch tank hold excess coolant, the reservoir tank will automatically put the coolant back in the system, where the catch tank will hold the coolant until it can be emptied.

The reservoir tank will either be plumbed at the bottom of the tank or have a hose internally that runs to the bottom so coolant can be drawn back into the system. A catch tank generally uses plumbing into the top of the tank and does not have a hose that goes to the bottom.

Expansion Tanks

Expansion tanks (also called surge tanks) provide expansion of your cooling system, giving you up to a half gallon more coolant. They are designed so there is space in the tank for the coolant to expand, hence the name. If an expansion tank is overfilled, it will discharge coolant when the system is at operating temperature and can also be used as a system fill point.

An expansion tank with a high-pressure cap provides an area for the expansion of the cooling system, as the name implies. They typically add about a half gallon of additional coolant to the system. (Photo Courtesy Jim Halderman)

Do not remove a pressure cap while the engine is hot. (Photo Courtesy Jim Halderman)

Coolant expands as it heats up and sends part of the coolant into the expansion tank. (Photo Courtesy Jim Halderman)

This late-model turbocharged Chevrolet Cruze uses an expansion tank with a 20 psi pressure cap sealing the cooling system. This system also has a bleed line coming out the top of the expansion tank that returns to the top of the cylinder head.

When an expansion tank is used, the radiator doesn’t require a pressure-relieving cap because the expansion tank cap does all of the things that a traditional radiator cap performed. The expansion tank needs to be located above the cylinder heads. That way the additional coolant stored in the expansion tank can flow back into the main cooling system when the engine cools down.

Expansion or surge tanks are usually sealed systems and not affected by atmospheric pressure. When the engine reaches operating temperature, the pressure valve in the cap closes and seals the system. As temperatures and pressures increase, the expansion tank allows the cooling system to expand without bleeding any off to the atmosphere.

With this type of system, the radiator’s cap will be a non-vented; the expansion tank will have the vented cap in case the pressure exceeds the expansion tank’s capacity. The expansion tank also has a connection to the cooling system through a bypass or a heater hose. Instead of just containing expanded coolant, the expansion tank is part of the cooling system and coolant circulates through the tank as it does through the radiator. When the system balances out, you should not have to add additional coolant.

The expansion tank for a Champion Cooling Systems’ aftermarket expansion tank uses a vented cap, an upper hose that goes to the radiator filler neck below the cap, and a lower hose that connects to the heater core. (Photo Courtesy Champion Cooling Systems)

Recovery Expansion Tank System

The coolant recovery tank system can also be an expansion tank with the radiator pressure cap on the radiator, but this tank is above the cylinder heads and connected to the radiator fill neck with the overflow tube at the bottom of the tank. This way, coolant can flow back and forth between the radiator and the reservoir.

This cooling system with a coolant recovery reservoir is still a closed system. When the pressure within the cooling system gets too high, the pressure valve in the pressure cap will open. This allows the expanded coolant to flow through the overflow tube and into the recovery reservoir. As the engine cools down, the temperature of the coolant drops and a vacuum is created in the cooling system. This opens the vacuum valve in the pressure cap, allowing some of the coolant in the reservoir to be siphoned back into the radiator.

This 2016 Chevrolet Traverse coolant recovery tank system uses a plastic coolant recovery reservoir and overflow tube that is partially filled with coolant. It is connected to the radiator fill neck with the overflow tube at the bottom of the tank.

Under normal operating conditions, no coolant is lost in this system. Although the coolant level in the recovery reservoir goes up and down, the radiator and cooling system are kept full. An advantage to using a coolant recovery reservoir is the elimination of almost all air bubbles from the cooling system. Coolant without bubbles absorbs heat much better than coolant with bubbles.

Thermostat

The cooling system thermostat is located in the coolant passage between the cylinder head or intake manifold and the radiator inlet at the top. Its purpose is to close off this passage when the engine is cold so that coolant circulation is restricted. This allows the engine to reach normal operating temperature more rapidly.

The cooling system thermostat closes off a coolant passage between the cylinder head or intake manifold and the radiator inlet to control engine operating temperature. It is the temperature control valve for the cooling system. (Photo Courtesy Jim Halderman)

Cooling system thermostats use a wax pellet that expands when heated and contracts when cooled down. (Photo Courtesy Jim Halderman)

The thermostat is designed to open at a specific temperature, such as 180°F or 190°F. When the engine is cold, the coolant uses a bypass hose or passage (blocking thermostat) to circulate the coolant through the cylinder heads and engine block.

Most cooling system thermostats use a wax pellet that expands when heated and contracts when cooled down. This pellet is connected through a piston to a valve. When the pellet heats up, pressure is exerted against a rubber diaphragm that forces the thermostat valve open, allowing coolant flow. When the pellet cools down, the contraction of the pellet causes a spring to close the valve, cutting off coolant flow.

Cooling Fan

When a vehicle is moving, air is directed up and through the radiator so that the heated coolant can be cooled through the convection process. Cooling system fans are used to blow air through the radiator when the vehicle is stationary.

Some longitudinally mounted engines use mechanically driven cooling fans. Some of these mechanical fans use a thermostatically controlled viscous clutch but not always. This clutch is positioned at the hub of the fan, in the airflow coming through the radiator. This viscous clutch is much like the viscous coupling sometimes found in all-wheel-drive (AWD) vehicles.

Newer vehicles and all front-wheel-drive (FWD) vehicles use electric fans. The fans are controlled either with a thermostatic switch or by the engine computer. They turn on when the temperature of the coolant goes above a specific temperature. The fan turns back off when the temperature drops below that point. The cooling fan has to be controlled so that it allows the engine to maintain a constant temperature.

The electric engine cooling fan (left) is supplementing a viscous mechanical cooling fan (right). In most systems, you would use either an electric cooling van or a viscous clutch mechanical fan, but rarely both. (Photo Courtesy Jim Halderman)

The electric cooling fan circuit is operated by the engine management computer, in this case an engine control module (ECM). Based upon input from fan #1 and #2 control switches, along with the air conditioner high-pressure switch, the ECM will ground fan control relay #1 or both fan control relay #1 and #2 to turn on the cooling fans.

FWD vehicles use electric fans because the engine is mounted transversely, meaning the output of the engine points toward the side of the car. For all vehicles built after 1996 that use Onboard Diagnostics Generation II (OBD II), the ECM will use the engine coolant temperature (ECT) sensor to signal the fan.

Temperature Switch/Sensor

Early vehicles and those built for street or track may use a temperature switch or ECT sensor to operate a temperature gauge or a dash warning light. These display the engine’s temperature or alert the driver that the engine is overheating. In some cases, the sensor will turn on the electric cooling fan(s). Most all vehicles from about 1994 and all vehicles under 8,500 pounds gross vehicle weight after 1996 use an ECT sensor.

The ECT circuit is a variable ground that uses a voltage divider network. In the network, the voltage is divided between the sensor input and a sensor ground inside the computer (also known as the black box). The engine control module (ECM) or powertrain control module (PCM) provides a 5-volt reference signal to the ECT sensor.

When cold, the sensor provides high resistance, which the computer reads as high signal voltage. As the engine warms up, the thermistor sensor resistance becomes lower and the signal voltage drops, so that is the difference. You do not need to know all of the inner workings of the computer, just that when you check voltage on the yellow wire of a cold engine, you should read high voltage at 3 to 5 volts. At normal operating temperature, the signal voltage should be around 0.47 to 1.45 volts.

The engine coolant temperature (ECT) sensor is screwed into a water jacket somewhere on the cylinder head. It is the temperature-sensing device for all vehicles that use Onboard Diagnostics Generation II (OBD II). A 2.2L GM Ecotec 4-cylinder engine is shown here.

The ECT sensor provides the engine coolant temperature data to the engine management computer such as an engine control module (ECM). Problems in this circuit will turn on the malfunction indicator lamp (MIL) on OBD II built after 1996 or the check engine light on earlier systems. It will set a diagnostic trouble code (DTC).