Engine Cooling System: Operation, Components, Maintenance and Diagnosis

1. What is the cooling system?

The cooling system controls engine temperature during operation. Its purpose is not to keep the engine as cold as possible, but to make it warm up quickly, reach the temperature defined by the design, hold it steady and distribute it evenly.

On early engines the goal was simply to lower the temperature. As engines evolved, it became clear that they run better at a higher, constant temperature, at which the materials expand exactly as calculated by the engineers. That is why modern systems are designed for temperature management, not just cooling.

During combustion, gas temperatures can exceed 2,000 °C (3,600 °F). As a teaching reference, in a gasoline engine only about one third of the fuel’s energy becomes useful work at the wheels; a similar share leaves with the exhaust gases and another part must be removed by the cooling system. Exact proportions vary with the engine, the load and the cycle (gasoline or diesel).

Without proper thermal control, temperature can exceed safe limits and cause damage. Keep in mind that the aluminum of a cylinder head melts at around 660 °C. On the other hand, running permanently below the specified temperature also hurts efficiency, fuel consumption and emissions.

2. Why control temperature?

Engine components are made of different materials, with different expansion coefficients, and operate with carefully designed clearances. When heated, these materials expand. The cooling system helps keep that expansion within the intended conditions.

Proper thermal control helps to:

• preserve clearances between moving parts;
• reduce the risk of overheating;
• promote efficient combustion;
• allow the engine oil to behave as intended;
• reduce wear and deposit formation;
• help control fuel consumption and emissions;
• protect gaskets, cylinder head, block and seals;
• provide useful information to the engine management system.

2.1 The danger of overheating

When the ability to dissipate heat becomes insufficient, temperature can rise abnormally. Depending on intensity and duration, this can cause warping, gasket damage, oil breakdown and damage to pistons, rings, cylinders, valves and bearings.

An overheating indication must be treated as a warning sign. The engine should not keep running under conditions that may cause damage.

2.2 Why is an engine that runs too cold also a problem?

An engine that stays below its specified operating temperature may show higher fuel consumption, increased emissions, deposit formation and incorrect operation of management strategies. On fuel-injected engines, the control unit reads the engine as cold and enriches the mixture.

That is why removing the thermostat is not a technical solution for overheating. The cause of the problem must be identified and corrected.

The correct operating temperature is not the same for every engine. It depends on the manufacturer, the design, the fuel and the electronic strategy used.

2.3 Air cooling and liquid cooling

Some engines, such as many motorcycle engines, stationary engines and classic models like the air-cooled Volkswagen Beetle, are air-cooled: fins on the cylinders and cylinder head increase the area for heat exchange with the air. Most of today’s vehicles, however, use liquid cooling, the subject of this lesson.

3. Operating principle

In liquid cooling systems, the heat produced by the engine is transferred to the coolant flowing through the passages (water jackets) of the block and cylinder head.

The water pump drives the circulation. The thermostat helps control the coolant path and the heat exchange. When the hot coolant passes through the radiator, part of its thermal energy is transferred to the outside air.

The electric cooling fan helps move air through the radiator when natural airflow is not enough, according to the vehicle’s strategy.

3.1 Warm-up phase

With a cold engine, the system tries to promote gradual warm-up. In many designs, the thermostat limits flow through the radiator and the coolant travels only through a “small circuit” inside the engine, allowing it to reach its specified temperature faster.

3.2 Temperature control phase

As temperature rises, the thermostat changes the flow distribution. The radiator starts receiving the amount of coolant needed for heat exchange, forming the “full circuit”.

When the vehicle is moving, air flowing through the front of the car helps remove heat. At low speed or when stationary, the electric fan may switch on to boost that airflow.

3.3 The system works as a whole

A working pump, a clean radiator or a healthy fan are not enough on their own. Thermal control depends on the interaction between circulation, airflow, pressure, coolant composition and electronic commands.

It is also important to know that temperature is not uniform throughout the engine: areas farther from the pump outlet and closer to the combustion chambers are hotter. System pressure and proper circulation prevent vapor bubbles from forming at these spots, which would insulate the metal wall from the coolant.

That is why the same symptom — such as overheating — can have different causes.

4. Coolant: composition and function

Coolant should not be treated as plain “radiator water”. Besides carrying heat, the formulation specified for the vehicle contributes to the chemical protection and durability of the circuit.

Most coolants are based on ethylene glycol (or, in some products, propylene glycol). Mixed with water in the correct ratio, it raises the boiling point and lowers the freezing point of the solution. Its inhibitors help control corrosion, deposits, foaming and cavitation, and protect metal components and seals, including the water pump seal.

4.1 Correct concentration

There is no universal water-to-coolant ratio for every vehicle. Many systems use between 30% and 50% concentrate, but the concentration must follow the vehicle manufacturer’s specification and the product instructions. “Changing the color of the water” is not enough: the recommended concentration must be reached.

If the coolant is supplied ready to use (pre-mixed), it must not be diluted unless expressly stated. When a concentrate requires dilution, use the recommended type of water, which may be demineralized or deionized, depending on the formulation.

4.2 Why does the wrong water cause damage?

Water containing minerals and contaminants can promote scaling, corrosion and deposits. These residues reduce heat-exchange efficiency, coat sensors and temperature switches (changing their response) and can block narrow passages.

4.3 Does color identify the coolant?

No. Color does not guarantee chemical technology, specification or compatibility. Two coolants of the same color may be different, and incompatible mixtures can compromise corrosion protection and product stability.

4.4 How to check the concentration

Coolant concentration is checked by measuring a physical property of the solution. The most common instruments are:

• Refractometer: needs only a few drops and directly shows the freezing point or glycol percentage. It is the most practical and accurate method in the workshop.
• Hydrometer: measures the density of the solution, which increases with the amount of concentrate. The reading must be corrected for coolant temperature using the instrument’s chart.
• Test strips: some show concentration, pH and inhibitor condition. They are useful for quick screening.

Always take the sample with the engine cold and use the correct scale for the product’s glycol type (ethylene or propylene).

4.5 pH, corrosion and electrolysis

Over time, inhibitors are depleted and the coolant may become acidic or excessively alkaline. Degraded coolant conducts more electricity and promotes galvanic corrosion: metal particles are removed from components such as the pump, cylinder head and radiator and deposited elsewhere, restricting coolant flow.

A simple test helps detect this problem: with the engine cold, use a voltmeter to measure the voltage between the battery negative terminal and the coolant (probe dipped in the coolant without touching any metal parts). A widely used workshop reference is up to about 0.3 V. Higher values indicate degraded coolant or a grounding problem, and the system should be investigated, flushed and refilled with the specified coolant.

5. Water pump

The pump circulates coolant through the engine passages and circuit components by means of an impeller. Depending on the design, it may be driven by the accessory belt, timing belt, gears or an electric motor.

With electric pumps, engine management can control circulation according to thermal demand. With mechanical pumps, flow follows engine speed and depends on the corresponding drive system.

5.1 Possible faults

• leakage through the seal (check the weep hole in the housing);
• bearing wear, play or noise;
• damaged impeller or impeller loose on the shaft;
• corrosion and erosion;
• insufficient circulation;
• incorrect tension or wear of the drive belt;
• electrical or electronic failure on controlled pumps.

A pump can lose efficiency without any obvious external leak. Therefore, the absence of dripping does not prove that circulation is correct.

5.2 Cavitation

Cavitation involves the formation and collapse of vapor bubbles under certain pressure and flow conditions. This phenomenon can erode surfaces and damage the pump.

Restrictions, incorrect pressure, high temperature, depleted coolant and abnormal flow conditions can contribute to it. The investigation must take the specific system design into account.

6. Thermostat

The thermostat controls or directs coolant flow according to the engine’s thermal condition. In many systems, when the engine is cold, it restricts the path through the radiator to speed up warm-up.

As temperature rises, the thermostat progressively changes position, allowing more flow through the radiator. When the engine cools, it closes again.

Conventional thermostats use an expanding wax element, which grows in volume with heat and pushes the opening pin. Each thermostat is calibrated for the engine it is fitted to, with opening-start temperature, full-open temperature and minimum lift defined by the manufacturer. Other designs include electronic control (map-controlled thermostat) to adapt the thermal strategy to load and operating conditions.

6.1 Types and design details

• Bypass type: has a lower disc that closes the small-circuit path as it opens the way to the radiator.
• Insert (cartridge) type: in aluminum or plastic housings, some designs allow replacing only the thermostatic element; in others, the thermostat is integrated into the housing.
• Bleed hole or jiggle pin: a small passage in the thermostat that helps purge air pockets during filling. Respect the specified installation position.
• Electronic (map-controlled): a heating resistor controlled by the ECU opens the thermostat earlier under high load.

6.2 Bench test of the thermostat

1. Place the thermostat in a container of cold liquid, suspended so that neither it nor the thermometer touches the bottom.
2. Heat slowly (about 1 °C per minute), stirring the liquid to keep the temperature uniform.
3. Note the opening-start and full-open temperatures and measure the thermostat’s lift.
4. Remove the thermostat and let it cool in room-temperature water: it must close completely.
5. Compare everything with the manufacturer’s values. A thermostat that does not open, opens outside the range or does not close must be replaced.

Do not place the thermostat in liquid that is already hot, as that prevents you from seeing the start of opening. Not every temperature difference on the vehicle means a faulty thermostat: confirm using the manufacturer’s procedure.

7. Radiator and airflow

The radiator is a heat exchanger. Hot coolant flows through its tubes while air passes through the fins. This interaction transfers thermal energy from the coolant to the surroundings.

Older radiators were large, made of copper and brass, with vertical tubes. Today’s radiators, usually aluminum with plastic tanks, are thinner and cross-flow, which improves heat exchange and allows more aerodynamic front ends. In return, they are more sensitive to corrosion caused by the wrong coolant.

The most common problems include internal clogging, dirt between the fins, corrosion, leaks, deformation and insufficient airflow.

A radiator may look clean on the outside and still be restricted internally. Likewise, a radiator in good condition may not dissipate heat properly if air cannot pass through its fins.

7.1 The vehicle heats up when stopped but improves when moving

This pattern may indicate insufficient airflow at low speed, including problems with the electric fan, its control or external obstructions. However, the other conditions of the circuit must also be checked.

8. Electric cooling fan

The electric fan forces air through the radiator when needed. Depending on the vehicle, it may have one or more speeds, relays, resistors, an electronic module or PWM control.

Its activation may depend on sensors, engine management and integration with the air conditioning. Older systems use a thermal fan switch mounted in the radiator.

8.1 Thermal fan switch

The thermal switch uses a bimetallic disc that snaps instantly at a calibrated temperature: it closes the circuit and turns the fan on at one temperature and turns it off at another, lower one. It may be single-stage (two terminals, one speed) or dual-stage (three or four terminals, two speeds), common on vehicles with air conditioning.

Bench test: heat the switch in liquid together with a thermometer and connect a multimeter on the resistance range (or a test lamp) to the terminals. Below the switch-on temperature the circuit must be open; when it is reached, it must close; when it cools down to the switch-off temperature, it must open again. Use the values stamped on the part or given by the manufacturer, and never use both methods at the same time.

Before condemning the switch, jump its terminals on the vehicle: if the fan does not run, the fault is in the electrical circuit or the fan motor. When installing, use a new sealing washer with no other sealant and respect the specified torque, since overtightening damages the part and the radiator tank.

8.2 Correct fan diagnosis

When the fan does not work, do not immediately condemn the electric motor. Following the vehicle’s wiring diagram, check:

• fuses and relays;
• power supply and ground;
• harness and connectors;
• temperature sensor and thermal switch, if fitted;
• resistor or control module;
• ECU command and electronic communication;
• electric motor and mechanical condition of the assembly.

A scan tool can help observe the reported temperature and, on compatible vehicles, run actuator tests. An activation test must follow the specified safety procedure.

9. Coolant temperature sensor

The temperature sensor tells the electronic system the engine’s thermal condition. Depending on the design, this information is used for fuel injection, ignition, cold start, idle, oxygen sensor, fan, air conditioning and protection strategies.

Many vehicles use an NTC (negative temperature coefficient) sensor: its resistance is high when the engine is cold and drops as temperature rises. Typically, the gauge sender has one terminal and the injection sensor has two. There are also dual (duplex) sensors, which drive the dashboard gauge and switch on the overheating warning lamp or buzzer.

9.1 Testing the NTC sensor

With the sensor removed, immerse its tip in liquid and heat it slowly, measuring temperature with a thermometer and resistance with a multimeter. Compare each point with the manufacturer’s resistance × temperature curve. The sensor is faulty if it shows:

• infinite resistance (open circuit);
• resistance close to zero (short circuit);
• values outside the specified curve or jumps during heating.

On the vehicle, the test can be complemented by measuring the signal voltage at the connector and comparing the scan tool reading with the actual temperature. Each engine uses a specific sensor: check the application and do not exceed the installation torque.

9.2 Actual temperature versus reported temperature

A faulty sensor or circuit can report an incorrect temperature even though the engine is not physically in that condition. The opposite is also possible: the engine may be overheating while the reading does not reflect the problem.

10. Pressure cap and expansion tank

The radiator or pressurized reservoir cap is part of pressure control. It is not just a sealing part: it has a pressure valve, which relieves excess pressure, and a vacuum valve, which lets air or coolant from the reservoir return as the system cools.

Proper pressurization raises the coolant’s boiling point and helps prevent vapor from forming inside the engine. Rated pressure and opening and return characteristics are specific to each system.

An incorrect or faulty cap can contribute to coolant loss, air entry, collapsed hoses, pressure changes and abnormal thermal behavior.

The reservoir accommodates coolant expansion as temperature changes. Some systems have a tank that is part of the pressurized circuit, others have a recovery reservoir. The filling procedure depends on this design, and the level must stay between the MIN and MAX marks, checked under the condition specified by the manufacturer.

11. Hoses, fittings and clamps

Hoses endure heat, pressure, vibration, contact with coolant and aging. With the engine cold, squeeze them by hand and look for cracks, dryness, swelling, hardening, abnormal softness, deformation and signs of leakage.

Also check clamps, fittings and proximity to moving parts. A hose that looks intact on the outside may be deteriorated inside.

12. Trapped air and bleeding

After a coolant change, repair or opening of the circuit, air pockets may remain. They can impair circulation and heat transfer.

Possible signs include temperature fluctuation, abnormal heating, noises and erratic heater operation.

12.1 General sequence after a repair

1. Confirm the specified coolant and concentration. If you need to mix it, do so in a clean container outside the vehicle.
2. Identify the filling points and procedure.
3. Fill slowly, opening the bleed screws where fitted and closing them in the order in which coolant comes out without bubbles.
4. Bleed the system according to the manufacturer.
5. Monitor temperature and circulation.
6. Observe fan operation where applicable.
7. Check for leaks and heater behavior.
8. After the engine has fully cooled, recheck the level using the correct procedure.

There is no single bleeding procedure valid for all vehicles. Some systems require special tools, such as vacuum filling, activation of electric pumps or electronic routines.

12.2 Flushing the system

When the coolant is contaminated, degraded or full of deposits, simply replacing it is not enough. Drain the entire circuit (radiator, block, hoses and reservoir), fill with water and the recommended cleaning product, run the engine for the time indicated by the product manufacturer, drain again and rinse until clean water comes out. Only then fill with new coolant and bleed the system.

13. Pressure and boiling point

Circuit pressure and coolant composition influence boiling conditions. At sea level, pure water boils at 100 °C; with the system pressurized and the correct coolant concentration, the boiling point rises well above that. That is why the cap, coolant and concentration must match the manufacturer’s specifications.

Do not use a generic test pressure or a higher-pressure cap to try to solve overheating. This can hide symptoms and cause damage.

14. Cooling system pressure test

A pressure tester lets you pressurize the system, with the engine cold, within the specified limit to look for coolant loss. The procedure must be carried out with compatible equipment and adapters and under the conditions indicated by the manufacturer.

Points to check include the radiator, hoses, fittings, reservoir, pump, thermostat housing, seals and other circuit components.

14.1 What does a pressure drop mean?

A pressure drop may indicate a leak, but the diagnosis must consider the tester’s own connections, correct adaptation and test conditions. Locate the cause before replacing components.

14.2 Cap test

Using the proper adapter, fit the cap to the tester and apply pressure until the valve opens. Check that it opens within the specified range and that it then holds a residual pressure. If the pressure drops to zero or the valve opens outside the range, the cap must be replaced.

15. External and internal leaks

External leaks may leave moisture, stains, crusts, dry residue and a characteristic smell. Some only appear when the system is hot and pressurized; others are small enough to evaporate before forming puddles.

When the level drops repeatedly with no visible external leak, investigate possible internal losses and components connected to the circuit. During oil changes, look for signs of coolant in the oil.

15.1 Head gasket and combustion gases

A sealing failure between a cylinder and the cooling circuit can let combustion gases in. Possible signs include abnormal pressurization, coolant being expelled, persistent bubbles under inappropriate conditions and overheating.

A single symptom does not prove a blown head gasket. Depending on the case, the diagnosis may require a chemical combustion-gas test, pressure test, compression test or leak-down test.

15.2 Does oil in the reservoir mean a blown head gasket?

Not necessarily. Some engines and transmissions use heat exchangers connected to the cooling system. An internal failure in these components can cause cross-contamination between fluids.

You need to know the vehicle’s design and prove the source of the mixing.

16. Additional circuit components

16.1 Heater core

In many vehicles, a small heat exchanger uses hot coolant to heat the cabin. Leaks can cause interior dampness, odor, fogged windows and coolant loss.

16.2 Oil coolers

Heat exchangers can transfer heat between oil and coolant. Their operation and possible failures must be considered during diagnosis.

16.3 Turbocharged engines

Some turbochargers are also coolant-cooled. On these engines, the turbo connections are part of the circuit and must be considered during inspections and repairs.

17. Overheating: a symptom is not a diagnosis

Overheating can result from low level, leaks, wrong coolant, trapped air, a faulty thermostat, an inefficient pump, a loose belt, a clogged radiator, an inoperative fan, an incorrect cap, a sensor fault or mechanical problems.

The analysis must begin by confirming the symptom and describing the conditions under which it occurs.

18. Diagnosis by temperature difference

Comparing temperatures at suitable points can help identify circulation and heat-exchange patterns. However, a difference between the radiator inlet and outlet should not be interpreted in isolation.

The result depends on load, engine speed, coolant flow, airflow, thermostat opening and measurement location. Infrared thermometers also require attention to emissivity and the surface being measured.

19. Diagnostic tools

20. Workshop case studies

Case 1 — Overheats only in traffic

Symptom: temperature rises when the car is stopped and improves at speed.

Investigation: check fan activation, available speeds, power supply, control and airflow through the radiator. Also confirm circulation and coolant level.

Lesson: behavior at different speeds helps direct the tests, but does not prove the cause on its own.

Case 2 — Engine takes too long to warm up

Symptom: temperature stays below the expected range under normal operating conditions.

Investigation: compare the sensor reading with the actual temperature and check thermostat behavior according to the manufacturer’s procedure.

Lesson: running too cold is also a thermal fault.

Case 3 — Level drops with no puddle on the floor

Symptom: the reservoir needs frequent topping up.

Investigation: look for dry residue, check fittings and cap, carry out appropriate tests and consider internal losses if no external ones are found.

Lesson: coolant can escape in small amounts and evaporate before dripping.

Case 4 — Overheating after a coolant change

Symptom: the system worked normally and started showing fluctuations after a service.

Investigation: confirm coolant, concentration, level, bleeding procedure, connectors and component assembly, including the position of the thermostat bleed hole.

Lesson: the timing relative to the service performed is an important clue.

Case 5 — Abnormal pressure and coolant being expelled

Symptom: hoses become excessively pressurized and coolant is expelled.

Investigation: check the cap, temperature, circulation, trapped air and the possibility of combustion gases entering the system.

Lesson: do not diagnose a head gasket based on a single sign.

Case 6 — Rough idle and corrosion in the system

Symptom: a fuel-injected engine shows intermittent faults; the coolant is dark and there are deposits in the reservoir.

Investigation: measure the voltage between the battery negative and the coolant, check coolant concentration and condition, inspect engine and body grounds and evaluate sensors and connectors affected by corrosion.

Lesson: degraded coolant affects not only heat exchange, but also component life and the reliability of electrical signals.

21. Common workshop mistakes

• removing the thermostat to mask overheating;
• mixing coolants just because they are the same color;
• using unsuitable water without checking the specification;
• adding concentrate “by eye” without measuring the concentration;
• replacing the cap with one of a different pressure rating;
• replacing the fan without checking power and control;
• overtightening sensors and thermal switches;
• condemning the head gasket without testing;
• skipping bleeding after opening the circuit;
• opening the reservoir with the system hot;
• repeatedly topping up coolant without investigating the loss.

22. Preventive maintenance

Preventive maintenance must follow the manufacturer’s interval and procedure. There is no universal coolant change interval for all vehicles: older coolants often required yearly replacement, while long-life coolants may last several years.

23. Knowledge check — 12 questions

Select the correct answer and check the commented answer key at the end.

24. Conclusion

Mastering the cooling system requires understanding how heat is produced, absorbed, carried and dissipated. It also requires recognizing that temperature depends on a set of mechanical, hydraulic, chemical and electronic components.

A good diagnosis does not start with replacing the pump, the radiator or the thermostat. It starts by confirming the symptom, identifying the system design and performing tests capable of proving the cause.

Technical note: temperatures, pressures, concentrations, maintenance intervals, bleeding procedures and electrical tests vary by vehicle. The values mentioned in this lesson are teaching references. Always consult the manufacturer’s technical documentation.

References: MTE-Thomson, Sistema de Arrefecimento — Tecnologia e precisão em controle de temperatura; vehicle manufacturers’ repair manuals.