Coolant carries heat away from your engine, prevents freezing in cold weather, and protects internal metal surfaces from corrosion. It circulates through passages in the engine block, absorbs the intense heat generated by combustion, then flows to the radiator where that heat dissipates into the air. But heat management is only one of several jobs coolant performs. A proper coolant mixture also raises the boiling point of the fluid, lubricates seals, and prevents the metal inside your cooling system from slowly eating itself.
How Coolant Moves Heat Out of the Engine
An internal combustion engine converts fuel into motion, but roughly a third of the energy produced becomes waste heat. Without a way to remove that heat, engine temperatures would climb past the point where metal warps, gaskets fail, and oil breaks down. Coolant solves this by flowing through narrow passages cast directly into the engine block and cylinder head. As it passes through these channels, the fluid absorbs heat from the surrounding metal.
A water pump pushes coolant through the system in a continuous loop. When the engine is cold, a thermostat valve stays closed, keeping coolant circulating only within the engine so it warms up faster. Once the engine reaches operating temperature, the thermostat opens and allows coolant to flow out to the radiator. There, the hot fluid passes through thin tubes with large surface area while air flows across them, pulling heat away. The now-cooled fluid returns to the pump and starts the cycle again.
Pure water actually absorbs heat better than any coolant mixture. Water’s thermal conductivity is more than twice that of pure ethylene glycol, the main chemical in most coolants. A 50/50 blend of water and ethylene glycol conducts heat about 30% less efficiently than pure water. So why not just use water? Because water alone freezes, boils too easily, and offers zero protection against corrosion. The trade-off in heat transfer efficiency is worth the gains in every other category.
Raising the Boiling Point and Lowering the Freeze Point
A standard 50/50 mix of ethylene glycol and water freezes at roughly minus 37°C (minus 35°F) and boils at about 106°C (223°F) at normal atmospheric pressure. Pure water, by comparison, freezes at 0°C and boils at 100°C. That wider operating range is critical. Frozen coolant expands and can crack an engine block, while boiling coolant creates steam pockets that block heat transfer and cause localized overheating.
The cooling system adds another layer of protection through pressure. Your radiator cap is a calibrated pressure valve, typically rated at around 15 psi. Pressurizing the system raises the boiling point by about 3°F for every pound of pressure. A 15-psi cap adds roughly 45°F to the boiling point, pushing the effective boiling threshold of a 50/50 mix well above 250°F. This gives the system a comfortable margin even in extreme heat or under heavy load like towing or climbing steep grades.
Corrosion and Metal Protection
Your cooling system contains a mix of metals: aluminum in the cylinder head and radiator, cast iron in the engine block, copper or brass in some heater cores, and steel in various fittings. When different metals sit in contact with a water-based fluid, electrochemical reactions cause corrosion. Left unchecked, this corrosion eats through radiator tubes, clogs passages with rust particles, and weakens critical components.
Coolant contains chemical inhibitors specifically designed to prevent this. These inhibitors form a thin protective layer on metal surfaces, blocking the chemical reactions that lead to rust and pitting. The challenge is that inhibitors effective for one metal can sometimes accelerate corrosion in another. Coolant manufacturers balance their formulas to protect all the metals in a given system simultaneously. Fresh coolant maintains a pH between 7.5 and 11.0. When coolant ages and its inhibitors deplete, the pH drops toward acidic levels, and corrosion accelerates.
Preventing Cavitation Damage
In heavy-duty diesel engines especially, coolant plays a less obvious but important role: preventing cavitation erosion. Cylinder liners vibrate rapidly during combustion, creating tiny vacuum bubbles in the coolant film against the liner wall. When these bubbles collapse, they release enough force to pit and erode the metal surface over time. Without protection, this process can eventually punch holes through cylinder liners.
Coolant additives, particularly those based on nitrite compounds, combat this by forming a passive protective film on metal surfaces. This film doesn’t stop the physical force of the collapsing bubbles, but it prevents the corrosion that would otherwise accompany and accelerate the erosion. Research on diesel engine liners has confirmed that in the presence of nitrite-based additives, damage is limited to mild physical erosion, while liners running without these additives suffer combined erosion and corrosion that progresses much faster.
Types of Coolant and How They Differ
Not all coolants use the same chemistry, and using the wrong type can cause problems. The three main categories are defined by their corrosion inhibitor packages.
- IAT (Inorganic Additive Technology) uses mineral-based inhibitors like silicates and phosphates. This is the traditional green coolant found in older vehicles. It works well but depletes relatively quickly, requiring replacement every 2 years or 30,000 miles.
- OAT (Organic Acid Technology) uses organic acids instead of silicates or phosphates. These inhibitors last significantly longer. Passenger car formulas are typically rated for 5 years or 150,000 miles, while heavy-duty versions can go up to 600,000 miles or 12,000 hours of operation.
- HOAT (Hybrid Organic Acid Technology) combines organic acids with a small dose of inorganic inhibitors, aiming for the long life of OAT with the fast-acting protection of traditional additives.
Your vehicle’s manufacturer specifies which type to use because the formula is matched to the metals and gasket materials in that particular cooling system. Mixing incompatible types can cause the inhibitors to react with each other, forming a gel-like sludge that clogs passages and reduces cooling efficiency.
Ethylene Glycol vs. Propylene Glycol
Most automotive coolants use ethylene glycol as their base. It’s effective and inexpensive, but it’s also toxic. The lethal dose for an adult is roughly 100 mL, about the volume of a small cup. It has a sweet taste, which makes it dangerous around children and pets. The EPA sets a drinking water guideline for ethylene glycol at just 7 parts per million.
Propylene glycol is the alternative. It performs similarly as a coolant but is far less toxic, which is why it’s used in applications where accidental ingestion is a concern or where environmental regulations are strict. The trade-off is slightly lower heat transfer efficiency and a higher cost. Some vehicle manufacturers specify propylene glycol-based coolants, but for most passenger cars, ethylene glycol remains standard.
When Coolant Stops Doing Its Job
Coolant doesn’t last forever. Over time, the corrosion inhibitors get consumed through the chemical reactions they’re designed to facilitate. Silicate-based inhibitors are particularly prone to depletion, and once they’re gone, aluminum components lose their protective coating with no residual benefit from the prior exposure. The fluid’s pH drifts downward, and the coolant shifts from protector to contributor to corrosion.
Testing coolant condition is straightforward. Inexpensive test strips measure pH and the concentration of key inhibitors. If the pH has dropped below 7.5 or the glycol concentration has shifted from the recommended 50/50 ratio due to water additions or evaporation, the coolant needs to be replaced. Neglecting this allows corrosion debris to accumulate, which clogs radiator tubes, accelerates water pump seal wear, and reduces the system’s ability to transfer heat, all of which compound into expensive failures.