Diesel Engines
Overheating on a hill — what to check
Heavy trucks overheat because of airflow, coolant flow or a fan that is not commanded on. Work outside-in — the cheap checks are all on the outside.
The answer
Roadside checks
- Radiator and charge air cooler fins packed with bugs, dust or oil film.
- Fan clutch not engaging — listen for the fan roar when temperature climbs.
- Coolant level low from a leak, or a leaking water pump weep hole.
- Shutters or seals missing so air bypasses the core.
Shop checks
- Thermostat stuck partially closed.
- Water pump impeller eroded — flow drops without any external leak.
- Combustion gases in the coolant from a head gasket or cracked liner.
- Air trapped in the system after a service, which shows as wild temperature swings.
Keep driving?
Back off the throttle and drop a gear before you shut down hot. If temperature keeps climbing at reduced load, stop and let it idle down rather than shutting off immediately.