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Difficulty: Very hardCooling System

Match each advanced cooling system diagnostic symptom to its most likely mechanical root cause.

  • Coolant boils in the reservoir during hot-engine shutdown, but normal temperature is maintained while driving.Radiator pressure cap failing to maintain specified system pressure.
  • Continuous stream of pressurized bubbles observed in the radiator filler neck immediately upon a cold start.Failed cylinder head gasket allowing combustion gases into the water jacket.
  • Engine fails to reach normal operating temperature at highway speeds, resulting in weak cabin heat.Thermostat mechanically stuck in the fully open position.
  • Lower radiator hose collapses and flattens entirely when engine RPM is significantly increased.Missing or deteriorated internal wire support spring in the lower hose.

Answer

The correct diagnostic pairings map each specific thermal, pressure, or flow symptom to its corresponding mechanical failure: boil-over after shutdown maps to the pressure cap; cold-start bubbles map to the head gasket; failure to warm up maps to a stuck-open thermostat; and lower hose collapse maps to a missing support spring.
Each symptom strictly correlates with the physics of the cooling system. Pressure drops lower the boiling point (cap failure), combustion leaks introduce non-condensable gases (head gasket), unregulated flow prevents heat accumulation (thermostat), and pump suction physically deforms unsupported hoses (missing spring).

Step-by-Step Solution

1
Analyze the boil-over symptom during heat soak.
Identified as a loss of system pressure, pointing directly to a failed radiator cap.
System pressure is required to raise the boiling point of the coolant; without it, the residual heat after shutdown causes boiling.
2
Evaluate the continuous bubbles upon a cold start.
Identified as combustion gas intrusion, pointing to a failed cylinder head gasket.
Since the engine is cold, boiling is impossible. The bubbles must be external gas (compression/combustion) entering the cooling jacket.
3
Assess the overcooling condition at highway speeds.
Identified as continuous, unregulated coolant flow to the radiator, pointing to a thermostat stuck open.
The thermostat must restrict flow to build engine heat; if open, highway airflow removes heat faster than the engine generates it.
4
Examine the mechanical collapse of the lower hose under high RPM.
Identified as a suction-side failure due to a missing internal structural spring.
The water pump pulls coolant from the lower hose. High RPM creates extreme suction that will flatten a hose unless supported by an internal wire spring.

Key Concept

Cooling System Diagnostics and Fluid Dynamics
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Topics

Cooling System

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