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Difficulty: MediumCooling System

When an internal combustion engine cools down after operation, the contracting coolant creates a partial vacuum inside the sealed cooling system. If the upper radiator hose collapses inward under this vacuum instead of drawing coolant back from the expansion tank, which component has failed?

  1. A
    The engine thermostat stuck in the closed position
  2. B
    The water pump impeller slipping on its drive shaft
  3. The radiator pressure cap vacuum relief valveAnswer
  4. D
    The engine coolant temperature sensor sending erratic signals

Answer

The radiator pressure cap vacuum relief valve
The radiator cap serves a dual-valve function. When the engine operates and coolant expands, the primary pressure spring allows excess fluid to vent into the recovery reservoir. When the engine is turned off and cools down, the coolant volume shrinks, generating a partial vacuum. The small vacuum return valve in the center of the radiator cap is designed to open under this negative pressure, drawing coolant back from the reservoir into the radiator. If this vacuum valve sticks closed or becomes clogged, the vacuum cannot be relieved, and outside atmospheric pressure crushes the flexible upper radiator hose flat.

Step-by-Step Solution

1
Analyze the physical state of the cooling system during cooldown
As hot coolant cools down after the engine shuts off, its volume contracts, reducing pressure inside the sealed radiator and hoses below atmospheric pressure (creating a vacuum).
Thermal contraction of liquids and trapped gases in a closed vessel inevitably creates a negative pressure differential.
2
Identify the mechanism designed to relieve cooling system vacuum
The radiator pressure cap features two valves: a large spring-loaded pressure valve that opens under excess pressure, and a small central vacuum relief valve that opens under negative pressure.
The vacuum valve allows coolant to be siphoned back into the radiator from the recovery tank, restoring system volume without pulling the flexible rubber hoses flat.
3
Determine which component failure directly produces a collapsed hose symptom
If the vacuum valve in the radiator cap sticks shut, atmospheric pressure on the outside of the upper radiator hose crushes it inward, causing the hose to flatten.
A stuck-closed vacuum valve prevents pressure equalization, leaving the hose unable to withstand external atmospheric pressure.

Key Concept

Dual-valve operation of automotive radiator pressure caps under thermal expansion and contraction
Estimated Time:1m 0s
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Topics

Cooling System

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