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Difficulty: HardCooling System

An engine rapidly overheats shortly after reaching operating temperature. A technician inspects the cooling system and finds that the upper radiator hose remains cold and flexible, yet the passenger compartment heater core produces strong, hot air. Which of the following is the most likely cause of this condition?

  1. A
    The thermostat is stuck in the fully open position.
  2. B
    The water pump drive belt has snapped, stopping all coolant circulation.
  3. The thermostat is stuck in the closed position.Answer
  4. D
    The radiator pressure cap spring is weak and venting pressure prematurely.

Answer

The thermostat is stuck in the closed position.
The thermostat acts as a temperature-sensitive flow control valve between the engine block and the radiator. When it fails in the closed position, coolant is trapped in the engine block and bypass/heater circuit, allowing the cabin heater to blow hot air while completely preventing coolant from reaching the upper radiator hose and radiator, leading to rapid engine overheating.

Step-by-Step Solution

1
Analyze the heat output from the passenger cabin heater.
Hot air from the heater confirms that the water pump is circulating coolant through the engine block and heater core circuit.
The heater core is connected to the engine block circuit before the thermostat; functioning cabin heat proves coolant is moving internally and reaching high temperatures.
2
Evaluate the thermal condition of the upper radiator hose.
A cold, flexible upper radiator hose indicates that hot coolant is not entering the radiator assembly.
The thermostat sits between the engine cooling passages and the upper radiator hose, acting as a temperature-controlled valve that opens to allow coolant to reach the radiator.
3
Synthesize the symptoms to isolate the failed component.
The engine block is hot and circulating coolant internally, but coolant is blocked from entering the radiator for heat rejection.
A thermostat stuck closed isolates the radiator from the cooling loop while allowing internal engine and heater core flow, directly resulting in severe overheating with a cold radiator hose.

Key Concept

Coolant Circulation Pathways and Thermostat Thermal Regulation
Estimated Time:1m 15s
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Topics

Cooling System

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