The "freezing point" of engine coolant refers to the temperature at which it begins to freeze. It is a critical metric for determining whether the coolant can operate reliably in sub-zero or extreme low temperatures, and whether the concentration of its active ingredients meets specified standards.
Coolant is typically formulated by blending ethylene glycol (or propylene glycol) with deionised water. As the concentration of ethylene glycol increases, the freezing point of the mixture decreases accordingly.
Standard ratio: A 50:50 ratio of ethylene glycol to water generally provides a freezing point of around -37°C, which is the most widely available premix specification on the market.
Take note: A higher ethylene glycol concentration is not always better. Once the concentration exceeds 70%, the freezing point actually rises, while heat dissipation efficiency and fluid viscosity are negatively affected. As such, never add pure, undiluted coolant concentrate indiscriminately.
The freezing point of your chosen coolant should be lower than the historical extreme minimum temperature in your area, with a recommended safety margin of roughly 10°C:
Southern and temperate regions: Coolant with a freezing point of around -25°C is generally adequate.
Northern and alpine regions: It is recommended to choose a coolant rated between -35°C and -45°C.
Extremely cold regions (such as Northeast China, Inner Mongolia, etc.): Dedicated products with a freezing point below -50°C are required.
Many motorists find that after two to three years of use, the coolant's freezing point shifts. The main reasons include:
Water evaporation and top-ups: During routine operation, a small amount of water in the system evaporates. If the car owner frequently tops up with pure water, the ethylene glycol concentration is diluted, causing the freezing point to rise (deteriorate).
Additive depletion: Chemical additives such as corrosion inhibitors degrade over time. Consequently, even if the freezing point remains acceptable, the anti-rust and anti-corrosion protection will no longer be sufficient to safeguard the engine.
Never rely solely on visual inspection to judge whether the coolant is still usable. The most precise way to test is with dedicated diagnostic tools:
Coolant refractometer (Highly recommended): A professional tool standard in most workshops. With just a drop or two of coolant, it accurately reads the freezing point through optical refraction.
Hydrometer (Freezing point tester): A basic float-type gauge that estimates the freezing point based on float buoyancy. Its readings are easily affected by temperature, making it less accurate than a refractometer.
Inspect before every winter: Especially for vehicles driven in cold climates, it is advisable to visit a workshop every November for a complimentary freezing point inspection.
What if the freezing point fails the test?: If the tested freezing point is higher than the expected minimum winter temperature in your area, do not attempt to "correct" it by mixing in high-concentration concentrate. Blending coolants from different brands or with differing chemical formulations risks forming sludge and precipitation. The correct procedure is to perform a complete flush and refill of the entire cooling system.
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