Your Guide to Automotive Cooling Systems

Posted on December 2nd, 2013 by GEM Motoring Assist

The cooling system tends to be overlooked until a fault develops and so Rob Marshall explains how to avoid car overheating.

Cooling system

As an internal combustion engine wastes so much energy, the cooling system is charged with dissipating large amounts of excess heat. Coolant (a solution of water and antifreeze) is pumped around the engine to a radiator, which is chilled by air forced through it. An electric fan, operated by a thermo electric switch, increases the airflow through the radiator, making engine cooling more effective.

Typical cooling system componentsTypical cooling system components, designed to prevent car overheating.

The cooling system also includes a mini-radiator within the passenger compartment, known as a heater matrix. On many modern cars, the cooling system might extend to include transmission and engine oil coolers as well.

To raise the boiling point of the coolant and reduce the possibility of car overheating, the cooling system is pressurised. Removing the pressure cap, when the system is hot, could result in the coolant boiling instantly and cause a scalding injury. Therefore, only attend to the cooling system when it is cold.

As an engine is more efficient when it is hot, a thermostat is built into the cooling system, which prevents the coolant from circulating to the radiator prematurely. Thermostats can fail to open, which can result in the engine overheating, but they are more prone to sticking open and causing overcooling. This will affect the interior heater’s performance and increase both fuel consumption and engine wear.

ThermostatA faulty thermostat, compared with a new one


The cooling system not only has to promote engine cooling, the liquid coolant also has to prevent the engine’s internals from corroding. Just as cholesterol blocks human arteries, corrosion clogs the cooling passages, reducing their efficiency. As the special additives, contained within antifreeze, diminish over time, the coolant should be replaced periodically.

 Car cooling system

 You will need to bleed out any excess air, if you are renewing the coolant. Air bubbles, which persist after prior overheating could indicate a blown cylinder head gasket. 

 Cooling sensors

Additional coolant temperature sensors might be fitted to not only operate the dashboard’s engine temperature gauge but they may also feed the engine’s ECU with information to influence the fuel mixture, ignition timing and exhaust emission systems. Therefore, failing cooling system temperature sensors can also cause poor engine cooling and uneven running.

Electrical sensors fail over time, which can cause erratic engine performance


Prevent any chance of a leak from occurring. Coolant leaks, found within the footwells are likely to emanate from the cooling system’s heater matrix. Replacing these on most cars is an extremely involved and expensive task.

Rubber hose

Rubber hoses deteriorate with age, so inspect them carefully for any cuts or splits, because they could burst under pressure.

Apart from checking the level of the coolant, when the engine is cold, inspect the cooling system’s hoses and radiator carefully. Older cars that are filled with the ‘traditional’ type of antifreeze, which is often (but not always) coloured blue, should have bi-annual coolant changes. Organic Acid Technology (OAT) antifreeze, which is often (but not always) coloured red, can withstand up to five years in a typical cooling system. Generally, OAT antifreezes should not be used in classic cars. Never leave antifreeze unattended; its ingestion can cause rapid kidney failure.

Mix concentrated anti-freeze in a 50/50 ratio with either soft or deionised water, which will reduce the risk of lime scale forming in the cooling system.

As most modern cooling systems’ water pumps are driven by the engine’s cam belt, consider replacing them at every cam belt change. Some models, especially those of theVW/Audi group, have water pumps that use plastic impellers that can become detached and so it is prudent to change these parts as a matter of course.

Coolant cap

 Ensure that the rubber seal, fitted to the pressure cap, is in good condition.

 Damaged radiator

Damaged or corroded radiator fins will affect the unit’s ability to dissipate heat and reduce the effectiveness of the engine cooling system.

With only minor maintenance being required, most of which are visual checks, the risk of your car overheating will be reduced dramatically. Not only will your car’s life be extended but you will also save money in the longer term.