Overheat at Highway Speed on a Transit Bus
A 2016 low-floor transit bus with a 6.7 L inline-six diesel and a charge-air-cooled cooling package reports intermittent overheating. The complaint only occurs on grades above 4% at road speeds over 55 mph with the A/C on. At low speed and idle, coolant temperature (ECT) stays at 195-200 F. On the grade, ECT climbs to 228 F within three minutes while the fan clutch command from the ECM reads 92% and the fan is audibly engaged. The coolant is 50/50, the surge tank cap holds 15 psi on a bench tester, and the radiator fins show road grime but no bent fins. Inlet air temperature to the CAC is 110 F and outlet is 165 F. Ambient is 95 F. The driver reports no coolant loss.
1. List the most likely causes of this highway-only overheat and explain why each fits or does not fit the symptoms.
Read the worked answer
The pattern is high heat rejection with high airflow demand, so think in terms of heat-transfer capacity rather than a simple coolant leak.
- Fan clutch/command: ECM commands 92% and the fan is audibly engaged, so the fan is likely doing its job. Confirm with a stall test or tach reading, but it is not the prime suspect.
- Radiator airflow restriction: Road grime between the CAC and radiator is a classic highway-only cause. At low speed the engine makes less heat; at highway speed, heat rejection is high and a fouled core cannot shed it. Inspect and clean the core stack.
- CAC heat load: 110 F in, 165 F out is a 55 F rise, which is plausible under load but adds heat to the radiator. A partially restricted CAC or high boost could push more heat into the radiator.
- Thermostat: A thermostat stuck partly open would cause low-speed overcooling, not a highway-only overheat. A thermostat stuck closed would overheat at all speeds. Less likely.
- Head gasket/combustion gas: Possible, but no coolant loss and stable low-speed temps make it less likely. A combustion gas test in the surge tank would rule it in or out.
- Water pump/flow: A failing impeller can cause high-load overheat. Verify flow and belt condition.
2. You clean the radiator core and the overheat persists on the same grade. What is your next diagnostic step and why?
Read the worked answer
Move from airflow to heat rejection and coolant flow. First, verify actual fan speed and clutch slip with a tach or stall test, because an audible fan can still be slipping under load. Second, measure the temperature drop across the radiator (inlet vs outlet) with an infrared gun or contact probes at stabilized 228 F. A small drop (under about 10-15 F) points to poor coolant flow (water pump, restricted radiator, or air lock); a large drop with high inlet temp points to insufficient airflow or excessive heat load. Third, perform a combustion gas test on the surge tank and a pressure test at operating temperature. Finally, check the CAC for restriction and confirm boost and charge-air temps against spec. Document each step so the repair is evidence-based rather than parts-swapping.