ASE G1 Auto Maintenance and Light RepairEngine SystemsEasy
A vehicle owner reports that their engine frequently overheats when idling in traffic, but the temperature returns to normal once the vehicle is moving. What is the most likely cause of this issue?
- AA restricted heater core
- BA faulty water pump
- CA defective radiator fan or circuit
- DA stuck-open thermostat
Show answer & explanationAnswer & explanation
Correct answer: C. A defective radiator fan or circuit
When a vehicle is moving, airflow through the radiator helps cool the engine. When idling, the radiator fan is crucial for pulling air through the radiator. If the fan or its circuit is defective, the engine will overheat at idle but cool down when moving.
Why the other options are wrong
- A. A restricted heater core would primarily affect cabin heat and potentially cause some flow restriction, but is unlikely to be the primary cause of engine overheating only at idle.
- B. A faulty water pump would cause overheating at all times, not just at idle.
- D. A stuck-open thermostat would cause the engine to run too cool, not overheat.
Radiator Fan Failure
Causes engine overheating at idle or low speeds due to insufficient airflow through the radiator, while cooling improves with vehicle speed.
- Fan pulls air through radiator when vehicle is stationary or moving slowly.
- Failure can be electrical (fuse, relay, motor) or mechanical (broken blades).
- Engine temperature often normalizes at highway speeds.
Memory trick: Overheating: Idle or Go, the fan's the show!