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Difficulty: MediumCooling System

An automotive technician inspects a vehicle that boils over during normal highway driving despite having a clean radiator and a properly functioning water pump. A cooling system pressure test reveals that the radiator cap pressure spring is weak and fails to hold pressure. Which of the following best explains why a weak radiator cap causes this condition?

  1. A
    The loss of system pressure prevents the thermostat from opening mechanically.
  2. The reduced internal pressure lowers the boiling point of the coolant, allowing it to boil at standard operating temperatures.Answer
  3. C
    The lack of pressure causes the air-fuel mixture in the cylinders to become excessively lean and generate extreme heat.
  4. D
    The water pump impeller requires static pressurization to create the centrifugal force necessary to circulate coolant.

Answer

The reduced internal pressure lowers the boiling point of the coolant, allowing it to boil at standard operating temperatures.
Pressurizing the cooling system raises the boiling point of the coolant mixture roughly 3°F per pound of pressure. A weak radiator cap fails to hold the specified pressure, allowing the boiling point to drop close to atmospheric levels. As a consequence, the coolant boils into steam at normal engine operating temperatures and boils over.

Step-by-Step Solution

1
Identify the primary function of the radiator pressure cap.
The pressure cap seals the cooling system and maintains an internal pressure typically between 12 and 16 psi above atmospheric pressure.
Maintaining pressure raises the boiling point of the liquid coolant by approximately 3°F (1.7°C) for every 1 psi of pressure.
2
Evaluate the mechanical effect of a weak pressure cap spring.
The pressure relief valve opens prematurely, keeping the system at or near ambient atmospheric pressure (0 psi gauge).
A weakened spring lacks the tension required to hold expanding hot coolant within the closed engine passages.
3
Relate the lower pressure to engine overheating symptoms.
The coolant boils at normal engine operating temperatures (195°F to 215°F), creating steam pockets and boilover.
At atmospheric pressure, standard coolant boils around 225°F, whereas under 15 psi it will not boil until approximately 265°F.

Key Concept

Radiator Pressure Cap and Coolant Boiling Point Elevation
Estimated Time:1m 0s
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Topics

Cooling System

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