The Truth About Engine Coolant: It's More Important Than You Think

Among the various reservoirs, pipes and dipsticks under your car’s bonnet, the coolant system isn’t one that should be overlooked. Most drivers know to keep their engine coolant level between the “min” and “max” lines, but too many are unsure what this mysterious liquid actually is and why it’s so critical. Engine coolant plays a central role in maintaining safe operating temperatures, protecting internal components, and ensuring the long-term reliability of vehicles across all categories. A properly managed cooling system helps prevent overheating, resists freezing in cold climates, and safeguards against corrosion and scale buildup.

I’ll give you all the info you need to know about engine coolant, covering its purpose, types, maintenance, and even how to service your car’s cooling system properly.

Combustion engines generate a huge amount of heat. Only a portion of the energy from burning fuel is converted into mechanical output. The remainder becomes heat that must be managed. Without a cooling system, this heat would quickly build up and cause metal components to expand beyond their limits, leading to warping, seizing, or complete engine failure. On the other end of the scale, if the engine runs too cold, it won’t reach its ideal operating conditions, resulting in poor efficiency and increased wear over time.

A properly functioning cooling system helps the engine maintain an optimal temperature for efficient combustion. When temperatures are stable and within the intended range, fuel is burned more effectively, which can improve fuel economy and reduce emissions.

If the system is neglected, the engine may run too hot or too cold. Both conditions can lead to poor fuel consumption, increased emissions, and long-term mechanical issues. An overheating engine is also more prone to engine knock, as excessive combustion chamber temperatures can cause the air-fuel mixture to ignite prematurely. Regular maintenance helps avoid these problems and keeps the engine operating as efficiently as it was designed to.

Let’s start with the reason for the cooling system. Why it even exists.

While almost all modern cars are water-cooled, not every engine uses liquid coolant. Some engines, particularly in classic or specialist applications, are air-cooled. These engines rely on finned surfaces and airflow, rather than coolant circulation, to dissipate heat. Though simpler and often lighter, air-cooled designs are less capable of maintaining consistent operating temperatures and are more vulnerable in hot, slow-moving traffic.

Liquid cooling systems, despite their additional complexity, offer superior thermal regulation, especially in race engines. They allow for precise control through thermostats and enable cabin heating by repurposing engine heat.

Engine coolant is a mixture of antifreeze and water that circulates through a vehicle’s engine to regulate temperature. The antifreeze component (usually a glycol-based chemical like ethylene glycol) lowers the freezing point of the water and raises its boiling point.

In other words, mixing antifreeze with water allows the coolant to remain liquid in extreme cold and not boil away under high heat. For example, a typical 50/50 coolant mix of glycol and water might freeze around –37 °C and boil around 106 °C, whereas pure water freezes at 0 °C and boils at 100 °C. This widened temperature range is crucial because it ensures the engine stays within safe operating temperatures on a frigid winter morning or a scorching summer afternoon.

There are more benefits beyond boiling point elevation and freezing point depression. Engine coolant contains additives that prevent corrosion inside the engine and radiator, and it can even lubricate the water pump seals, contributing to the longevity of cooling system components.

When the engine is running, the coolant continuously circulates through channels in the engine block and cylinder head. It absorbs excess heat and then flows through the radiator, where the heat is released to the outside air. By carrying thermal energy away from the engine and dissipating it, coolant keeps the engine from overheating. At the same time, in cold weather, the antifreeze in the coolant prevents the liquid from freezing solid when the engine is off. Essentially, engine coolant ensures the engine can operate efficiently across a wide range of temperatures.

Nope. This is a common misconception stemming from the two terms often being used interchangeably. Antifreeze is a concentrated chemical (typically ethylene glycol or propylene glycol based) that, on its own, lowers the freezing point and raises the boiling point of water. It’s usually sold as a bright green, orange, or pink liquid. Coolant, on the other hand, refers to the mixture of antifreeze and water that we put into the cooling system. In other words, antifreeze is an ingredient, while coolant is the final product made by mixing antifreeze with water (usually in a 50:50 ratio, though mixtures like 60:40 are used in more extreme climates). Water itself is an excellent base for cooling due to its high heat capacity, but it freezes at 0 °C and can boil off at engine operating temperatures. The antifreeze additive solves those issues and includes corrosion inhibitors, but pure antifreeze on its own would not cool as effectively as water. Thus, when combined, water + antifreeze = coolant.

Once mixed correctly, the coolant in your car’s system will provide year-round protection against freezing, overheating, and corrosion. So while people might say “add antifreeze” in winter or “check the coolant,” they are usually talking about the same fluid – just remember that proper coolant is the combination of antifreeze chemical and water, not one or the other alone.

It’s worth noting that coolant is sold pre-mixed (ready to use) or as a concentrate that you must dilute with water. If you’re mixing it yourself, always use distilled or de-ionised water rather than tap water. This prevents mineral deposits or limescale from building up inside the radiator and engine.

During all my years in the auto industry, it’s unfortunate that so many workshops don’t know the long-term effects. They buy antifreeze concentrate in bulk, dunk the hose in a container and that’s it. Mixture complete.

I know it isn’t free, but if I were a customer and had the choice, I’d pay the difference for pre-mixed or de-ionised water for a car I truly cared about.

Using tap water in your coolant mix might seem harmless, but it can introduce minerals, especially calcium and magnesium, that cause limescale. These minerals deposit themselves onto hot surfaces inside the engine and radiator as the water evaporates and cools. Over time, these deposits form a hard, crusty layer that lines internal passages, just like in a kettle or home boiler.

Limescale is a poor conductor of heat, so even a thin coating can drastically reduce heat transfer, making your engine run hotter than it should. In severe cases, limescale buildup can clog narrow cooling channels, restrict flow, and lead to overheating or damage to components like the water pump or thermostat.

Modern engines often combine different metals such as aluminium cylinder heads and cast iron blocks. Without proper corrosion inhibitors in the coolant, this mix of dissimilar metals can set up an electrochemical reaction known as electrolysis. Stray electrical currents or even minor grounding issues can accelerate this process.

Electrolysis corrodes metal from the inside out, often targeting softer aluminium components. It can lead to pitting, premature leaks in radiators and heater cores, and the formation of coolant passage blockages. Once this type of internal damage begins, it’s difficult and expensive to reverse.

Maintaining the correct coolant with active corrosion inhibitors is essential for preventing electrolysis. So is timely replacement, since old coolant becomes more conductive over time and loses its protective properties. For vehicles with mixed-metal engines, ignoring coolant quality can cause more than just overheating – it can quietly eat away at your engine’s internals.

Coolants are categorised by their additive technology. Each is tailored to the materials used in different engines and the corrosion inhibitors required to protect them:

• Inorganic Additive Technology (IAT): Traditionally green. Designed for older vehicles. Requires replacement every 2 years or 30,000 miles.
• Organic Acid Technology (OAT): Found in many newer vehicles. Often orange, pink, or red. Typically lasts up to 5 years or 100,000 km.
• Hybrid Organic Acid Technology (HOAT): A combination of IAT and OAT features. Found in various European and American vehicles. Lifespan similar to OAT.

Coolant colour is not a reliable indicator of type. Always confirm the specification through the vehicle manual or product labelling.

Matching the coolant to your vehicle’s specifications is essential. Owner’s manuals include references to standards such as VW G12 or Ford WSS-M97B51, which correspond to specific coolant types. Using the wrong formulation may lead to inadequate protection or chemical incompatibility with existing fluid, resulting in sludge or reduced corrosion defence.

When topping up, use the same coolant already in the system. If the fluid is unidentified or a change is required, flush the entire system before refilling with a new, correct type.

Premixed coolants simplify maintenance by eliminating the need to dilute concentrate. As previously mentioned, if mixing yourself, use only distilled or de-ionised water to prevent limescale and mineral deposits.

Coolant degrades over time, losing its corrosion-inhibiting properties and becoming less effective. Replace it at the intervals recommended by the vehicle manufacturer, often every 5 years or 60,000 miles.

Basic steps to flush and refill coolant include:

1. Cool the engine completely.
2. Drain the old coolant using the radiator drain plug or by removing the lower hose.
3. Flush with distilled water if needed.
4. Refit components and fill slowly with the new coolant.
5. Bleed the system to remove trapped air.
6. Check levels again after the engine has cycled through a full warm-up and cooldown.
7. Dispose of used coolant at a licensed waste facility.

Trapped air inside the cooling system can lead to overheating, poor heater function, or erratic temperature readings. Bleeding removes this air, ensuring the coolant circulates freely.

Techniques include:

• Opening bleed valves at high points in the system.
• Running the engine with the heater set to maximum.
• Gently squeezing radiator hoses to encourage air movement.
• Topping up the coolant as the system stabilises.

From personal experience, back when I was at JBM Performance, I can attest that thorough bleeding makes a big difference. At the garage, we regularly used an airline-powered vacuum bleeding tool to service cooling systems. This tool attaches to the filler neck and uses compressed air to create a vacuum in the entire cooling system. When the vacuum is released, it sucks new coolant into every nook and cranny of the engine, with zero air pockets. Using this method, we could refill and bleed the system in one step. The vacuum draws the coolant in so completely, that no manual “burping” is needed.

For home mechanics, careful attention to the bleeding process is usually sufficient. Be methodical, check the system after a few drives, and top up as needed.

Most people who work on their own car at home won’t have a vacuum fill tool, but it’s good to know the equipment exists (many professional shops have them, and it might be worth the service if you’ve had persistent air bleeding issues). Bleeding the cooling system on modern cars in particular requires patience and careful following of the bleeding steps will usually do the job. The goal is simply to ensure only liquid coolant is circulating through your engine and radiator, with no lurking air bubbles to cause trouble.

Once the cooling system is properly bled and sealed up, you should verify that the engine maintains normal temperature during operation and that the cabin heater works when requested (a good sign that coolant is circulating fully through the system).

Water-cooled systems have been around since the beginning of the internal combustion engine, and have been refined through years of motorsport innovation and development. Engines are abused and pushed to the limit where older cooling technologies simply wouldn’t suffice.

In motorsport, keeping coolant temperatures stable under pressure is critical. A simple but often overlooked upgrade is the use of a fan shroud. It’s a simple method that helps the radiator fan draw air across the entire core, improving efficiency at low speeds or during drifting where natural airflow is limited. In motorsport disciplines like Formula Drift or GT endurance cars, shrouds are paired with powerful brushless fans and pressure flaps that open at speed to prevent airflow restriction.

Ducting and sealing are equally important. Directing air cleanly into the radiator and preventing it from escaping around the sides significantly improves cooling. Many teams, from grassroots time-attack builds to factory GT3 efforts, use foam seals or fabricated panels to box in the radiator. Sometimes the radiator is mounted at an angle and hot air is vented through bonnet louvres, reducing under-bonnet pressure and aiding aerodynamic stability.

While most vehicles use front-mounted radiator setups, mid- and rear-mounted radiators in race cars rely on proper ducting and airflow management to function efficiently. Even for your own track-day car, getting cool air in and hot air out—while managing pressure zones—is key to maximising cooling performance.

If you’re a car enthusiast; someone who loves automobiles. I hope you take care of your car. Whatever it is you drive. Regular maintenance is good to keep it reliable and prolong the life of your car. But I get the feeling most just check the tires, top up the oil or coolant when it gets a bit low, and that’s about it. By having a better understanding of engine coolant and maintaining it well, you’ll be able to preserve your engine’s health and performance in the long run. With the correct coolant, the proper mixture, and a bit of care in maintenance, your car will be hassle-free all year round, for many years to come. So it can tackle the highs of summer, the lows of winter and if you’re feeling adventurous, a couple of track days a year.