Short answer
Diesel pushers mount the engine and radiator at the rear, where road dirt and oily grime pack into the radiator and charge-air cooler. Dirty cores, a fan or fan drive that does not reach full speed, and low coolant are the common causes. On grades, downshifting to keep engine speed up helps the fan and pump.
Safety first. Diesel injection systems run at extremely high pressure; a pinhole leak can inject fuel through skin. Exhaust and turbo parts stay very hot long after shutdown. Never open a hot cooling system. If you are unsure, stop and see a qualified diesel technician.
Start with the easy checks
- Clean the radiator and charge-air cooler gently with a regular hose and mild detergent; avoid high-pressure washing that bends fins.
- Check the coolant level and condition.
- Inspect belts, hoses and the fan drive.
- Downshift on grades and keep engine speed up.
Likely causes, most common first
- Dirty stacked cores
- Most common.
- Fan drive or controls
- Fan never reaches full speed.
- Coolant, thermostat or pump
- Normal cooling suspects.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
When to stop and see a diesel tech
- Still overheating after cleaning.
- Fan drive problems.
Questions
How do I clean a diesel pusher radiator?
Owner guidance favors a regular garden hose and mild detergent from the correct side, never high-pressure washing that bends fins.
Related symptoms
Cooling
Why does my diesel run hot when towing?
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Troubleshoot →Where this comes from
- Family RVing: Rear radiator cleaning
- Cummins: Motorhome maintenance and operation, L9/ISL9/ISC8.3 (owner-forum copy, PDF)
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.