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Difficulty: HardCooling System

During an engine diagnostic on a vehicle with a properly mixed 50/50 ethylene glycol and water solution, coolant begins boiling and purging into the overflow reservoir at approximately 216∘F216^\circ\text{F} (102∘C102^\circ\text{C}), even though the water pump, thermostat, and electric cooling fans are verified to be functioning properly. A cooling system pressure test confirms that the system fails to hold its rated 15 psi pressure, despite having no external fluid leaks. Which of the following is the most probable cause of this condition?

  1. A
    A thermostat stuck in the fully open position allowing rapid coolant circulation
  2. A defective radiator pressure cap failing to maintain system pressureAnswer
  3. C
    An excessively lean air-fuel mixture elevating combustion cylinder temperatures
  4. D
    An alternator overcharging the battery and causing erratic temperature sensor resistance

Answer

A defective radiator pressure cap failing to maintain system pressure
The radiator pressure cap is designed with a calibrated spring-loaded valve that pressurizes the cooling system up to approximately 14-16 psi. For every 1 psi of pressure added, the boiling point of the coolant increases by about 3∘F3^\circ\text{F}. An unpressurized 50/50 coolant mixture boils at roughly 216∘F216^\circ\text{F} at sea level. If the pressure cap spring weakens or its rubber seal deteriorates, the system cannot maintain pressure, causing the coolant to boil at standard operating temperatures and vent into the expansion tank.

Step-by-Step Solution

1
Analyze the relationship between pressure and the boiling point of automotive coolant.
Under atmospheric pressure (0 psi), a 50/50 coolant mixture boils at approximately 216∘F216^\circ\text{F} (102∘C102^\circ\text{C}), whereas under a typical 15 psi operating pressure, its boiling point is elevated to approximately 265∘F265^\circ\text{F} (129∘C129^\circ\text{C}).
Each pound per square inch (psi) of pressure increases the boiling point of water/coolant by approximately 3∘F3^\circ\text{F}.
2
Correlate the diagnostic symptoms with cooling system components.
The coolant boils at 216∘F216^\circ\text{F} and the system fails to hold pressure despite zero external leaks, indicating pressure loss past the relief valve.
The radiator pressure cap contains a spring-loaded pressure relief valve that seals the system; if the spring is weak or the seal is degraded, pressure vents to the reservoir at atmospheric levels.
3
Conclude the root mechanical failure.
The radiator pressure cap is defective and must be replaced.
The radiator cap is the sole mechanical component responsible for maintaining system pressurization independently of engine load.

Key Concept

Radiator Pressure Cap Operation and Boiling Point Elevation
Estimated Time:1m 30s
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Topics

Cooling System

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