ASE G1 Auto Maintenance and Light RepairEngine SystemsMedium
A vehicle owner reports that their engine frequently overheats when idling in traffic or driving at low speeds, but the temperature returns to normal once the vehicle is moving at highway speeds. Which of the following is the MOST likely cause?
- AClogged heater core
- BLow engine oil level
- CStuck-closed thermostat
- DMalfunctioning radiator fan
Show answer & explanationAnswer & explanation
Correct answer: D. Malfunctioning radiator fan
When a vehicle is idling or moving slowly, there isn't enough natural airflow through the radiator to cool the engine. The radiator fan is crucial in these situations to pull air through the radiator. If the fan malfunctions, overheating occurs at low speeds/idle but dissipates once highway speeds provide sufficient airflow.
Why the other options are wrong
- A. A clogged heater core would primarily affect cabin heating and might slightly impede coolant flow, but it's not the primary cause of overheating specifically at low speeds/idle that resolves at high speeds.
- B. Low engine oil can contribute to overheating, but it wouldn't typically cause a condition that resolves at highway speeds and is specific to low-speed/idle. It's a general overheating cause.
- C. A stuck-closed thermostat would cause overheating at all speeds, as it restricts coolant flow regardless of vehicle speed.
Radiator Fan Failure Symptoms
Signs that the engine's cooling fan is not operating correctly, leading to insufficient airflow through the radiator, especially at low vehicle speeds or while idling.
- Engine overheats when idling or in slow traffic.
- Engine temperature returns to normal at highway speeds.
- Fan may not turn on when commanded or may operate weakly.
Memory trick: Slow 'TRAFFIC' makes the engine 'HOT', the 'FAN' is forgot!