ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A vehicle owner reports that their engine frequently overheats when idling in traffic or driving slowly, but the temperature returns to normal once the vehicle is moving at highway speeds. The coolant level is full, and the thermostat has been recently replaced. Which of the following is the most likely cause?
- AA clogged heater core
- BAn inoperative cooling fan
- CA faulty engine coolant temperature (ECT) sensor
- DA faulty radiator cap
Show answer & explanationAnswer & explanation
Correct answer: B. An inoperative cooling fan
Overheating at low speeds or idle, but not at highway speeds, is a classic symptom of an inoperative cooling fan. At highway speeds, sufficient airflow cools the radiator, but at low speeds, the fan is needed.
Why the other options are wrong
- A. A clogged heater core would primarily affect cabin heat, not cause engine overheating, and certainly not in this speed-dependent manner.
- C. A faulty ECT sensor would give incorrect temperature readings, but not necessarily cause actual overheating only at low speeds.
- D. A faulty radiator cap could lead to coolant loss or reduced boiling point, causing general overheating, but typically not specific to low speeds only.
Cooling Fan Malfunction
The cooling fan draws air through the radiator to dissipate heat at low vehicle speeds. A malfunction leads to overheating when airflow from vehicle movement is insufficient.
- Crucial for cooling at low speeds/idle
- Inoperative fan causes overheating in traffic
- Engine cools down at highway speeds if fan is the issue
Memory trick: Hot in traffic, cool on the road? Fan's not doing its job, load!