Highway-Only Overheating With a Large Radiator Temperature Drop
2013 2.0 L four-cylinder diesel, 185,000 km, overheats only at steady 100–110 km/h; around town the gauge stays at 90 °C. Coolant is full, no external leaks, and the thermostat and housing were replaced two weeks ago. The radiator fan engages when idling with A/C on. A pressure test holds 1.4 bar for 30 minutes. An infrared thermometer at idle shows 92 °C at the radiator inlet and 58 °C at the outlet; at 2,500 rpm the outlet rises only to 62 °C. The lower radiator hose is cool to touch while the upper is hot. The heater blows warm but not hot. Combustion gas test fluid stays blue. The water pump belt is intact and the pulley turns; coolant concentration is 50%. The owner reports the temperature climbs on long inclines and recovers when he turns the heater on.
1. Interpret the radiator inlet/outlet temperatures and heater performance. What do they rule in and out?
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At idle, 92 °C in and 58 °C out is a 34 °C drop; at 2,500 rpm it is still about 30 °C. A healthy radiator at operating temperature typically shows a much smaller drop, often around 5–15 °C, because flow is high. A large drop means coolant is spending too long in the radiator or too little heat is being carried into it-low flow. The cool lower hose and warm-but-not-hot heater support reduced coolant flow. The fan is irrelevant at 100 km/h, and the new thermostat is unlikely to be the cause unless it is stuck partly closed. A failed water-pump impeller or a restricted radiator can both reduce flow. The negative combustion gas test and stable 1.4 bar pressure test make head gasket less likely, but not impossible; a cold pressure test can miss a hot-only leak.
2. What tests would you run to separate a restricted radiator from a failed water pump?
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Use an infrared thermometer to scan the radiator core: a restricted radiator will show cold bands or a large inlet-to-outlet gradient across a small area, while a pump problem tends to give a uniformly hot top and cooler bottom with weak heater flow. Check the heater hoses: if both are hot but airflow is weak, the heater core may be air-bound; if both are lukewarm, flow is low. Pressure-test the system and test the cap; a cap that releases early lowers boiling point but does not explain a 30 °C radiator drop. Scan live data for coolant temperature and fan command. If the water pump is driven by the timing belt, inspect the impeller only if accessible; otherwise a flow test or pump removal may be needed. A combustion gas test should be repeated hot and under load.
3. The owner authorizes one repair. What do you recommend and why?
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On the evidence, I would not start with another thermostat or a fan. I would recommend a radiator flow test or replacement only if core scanning shows cold bands or a measured restriction; otherwise inspect the water pump impeller. The 30–34 °C radiator drop with a cool lower hose and weak heater is classic low flow. If the radiator is original at 185,000 km, internal blockage is plausible, but a failed pump impeller is equally plausible and cheaper to verify on some engines. Do not authorize both blindly. A hot combustion-gas test and a cooling-system pressure hold should be repeated first. If the pump is belt-driven and the impeller is accessible, inspect it; if it is timing-belt driven, the decision changes because pump replacement adds labor. The repair must restore flow, not just increase fan operation.