What temperature does 50/50 antifreeze boil at?
A 50/50 ethylene glycol–water antifreeze mixture typically boils at about 223–226°F (106–108°C) at atmospheric pressure. In a modern pressurized automotive cooling system with a 14–16 psi radiator cap, the boiling point rises to roughly 265–270°F (129–132°C). Propylene glycol–based 50/50 mixtures are similar, usually within a couple of degrees of these values. Actual boiling temperature varies with system pressure, mixture accuracy, altitude, and the specific coolant formulation.
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Why the boiling point changes
Antifreeze raises the boiling point of water through colligative properties, and a sealed cooling system raises it further by increasing pressure. The radiator cap maintains system pressure—commonly 14–16 psi in passenger vehicles—which substantially elevates the boiling threshold, helping prevent vapor lock and overheating under load or in hot weather.
Typical boiling points for common setups
The following figures outline typical boiling points for a 50/50 ethylene glycol–water coolant under different pressures you might encounter in real-world vehicles.
- At atmospheric pressure (0 psi gauge): approximately 223–226°F (106–108°C)
- With a 7 psi radiator cap: approximately 240–245°F (116–118°C)
- With a 13–16 psi radiator cap (common OEM spec): approximately 262–270°F (128–132°C)
- With a 20–21 psi cap (some heavy-duty/performance systems): approximately 274–279°F (134–137°C)
These ranges are typical rather than absolute. Exact temperatures depend on the brand and chemistry of the coolant, the accuracy of the 50/50 mix, and the condition of the pressure cap and system.
Ethylene glycol vs. propylene glycol
Most passenger vehicles use ethylene glycol–based coolants, though some applications use propylene glycol. At a 50/50 ratio, both have similar boiling points. Propylene glycol mixtures at 1 atm generally boil around 221–225°F (105–107°C) and around 260–265°F (127–129°C) with a 15 psi cap—slightly lower or comparable to ethylene glycol, depending on formulation.
Other factors that affect boiling point
Beyond pressure and base chemistry, several practical factors determine the real boiling point in your vehicle.
- Mixture accuracy: Deviations from 50/50 (too much water or glycol) can lower or raise the boiling point—and can also affect freeze protection and heat transfer.
- Altitude: Higher elevations reduce atmospheric pressure, lowering the boiling point if the system isn’t fully sealed or is depressurized.
- Radiator cap condition: A weak or failing cap won’t hold rated pressure, dropping the effective boiling point.
- System health: Air pockets, contamination, or oil/corrosion in the coolant can change behavior under heat.
- Coolant technology: OAT/HOAT formulations and additives can produce minor variations in boiling characteristics.
Considering these variables can help you interpret temperature readings and diagnose potential overheating or pressure-related issues more accurately.
How to find your vehicle’s exact spec
Check the owner’s manual or service documentation for the recommended coolant type and system pressure. The radiator cap is typically stamped with its pressure rating (e.g., 16 psi). For mixture accuracy, a refractometer or hydrometer can verify concentration; aim for a true 50/50 unless the manufacturer specifies otherwise. If temperatures seem high, confirm cap integrity, bleed air from the system, and verify fans and thermostat operation.
Summary
A 50/50 antifreeze mix generally boils at about 223–226°F (106–108°C) at atmospheric pressure and roughly 265–270°F (129–132°C) in a typical pressurized cooling system with a 14–16 psi cap. Small variations arise from coolant chemistry, pressure, altitude, and system condition, so always consider your specific vehicle’s specs and state of maintenance.
