CASE 008
Engine Overheating in Traffic
intermediate
20 min
Diesel
Vehicle
- ModelTraining Sedan 1.6
- Engine1.6 HDi
- Mileage156,000 km
Customer complaint
Temperature rises in slow traffic and returns to normal on the open road. The coolant level drops slowly.
History
Coolant topped up several times. No visible puddle under the car.
Live data
- Coolant temperature, idle in trafficRising above normal
- Cooling fan commandNot activating when hot
- Coolant temperature sensor vs infrared readingAgreement
Measurements
- Cooling fan direct power testFan runs at both speeds
- Fan relay control signalAbsent when hot
- Coolant pressure testSlow pressure loss
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Diagnostic case
Next step — Reasoning.
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Confirmed cause
Cooling fan not commanded by the control circuit (relay/control fault), plus a small coolant leak.
Reasoning
Overheating only in traffic with normal open-road temperature is characteristic of insufficient airflow at low speed — the fan. Directly powering the fan proves the motor works, so the fault is in its command. The coolant pressure test reveals an additional slow leak that must be found and repaired.
Recommended repair
- Repair the fan control circuit (relay, fuses, wiring or control module).
- Locate and repair the coolant leak.
- Refill, bleed and confirm normal temperature in traffic.
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.