ASE A1 Engine RepairLubrication and Cooling SystemsHard
A vehicle owner reports that their engine frequently runs hot, but the temperature gauge sometimes drops to cold while driving, then returns to a high reading. The coolant level is full. What is the most likely cause of this intermittent temperature gauge behavior?
- AA stuck-open thermostat
- BA faulty engine coolant temperature (ECT) sensor
- CAn airlock in the cooling system
- DA failing water pump impeller
Show answer & explanationAnswer & explanation
Correct answer: C. An airlock in the cooling system
An airlock in the cooling system can cause the ECT sensor to be exposed to air instead of coolant. Air heats up and cools down much faster than liquid coolant, causing rapid fluctuations in the sensor reading, leading to the gauge jumping from hot to cold and back.
Why the other options are wrong
- A. A stuck-open thermostat would cause the engine to run cold or warm up slowly, not typically overheat and then show cold intermittently.
- B. A faulty ECT sensor could cause incorrect readings, but usually it would be consistently high, low, or erratic, not typically this specific hot-to-cold-and-back pattern with a full coolant level.
- D. A failing water pump impeller would lead to consistent overheating due to lack of circulation, not intermittent drops to cold on the gauge.
Airlock in Cooling System
An airlock is a pocket of air trapped in the cooling system that can impede coolant flow and cause erratic temperature readings, especially when the ECT sensor is exposed to air instead of liquid coolant.
- Intermittent temperature gauge fluctuations (hot to cold)
- Engine overheating despite full coolant
- Poor heater performance
Memory trick: Gauge jumps? Air in the pumps! (or sensor's dumps)