Cummins/Onan — Cooling System Drain, Flush, and Fill
Maintenance
From a Cummins/Onan owner's manual covering: B3.9, B4.5, B5.9. Check it applies to your exact model before use.
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Before you start
- Do not remove the pressure cap from a hot engine; wait until coolant temperature is below 50°C [120°F].
- Coolant is toxic; keep away from children and pets and dispose of in accordance with local environmental regulations.
- Never use water alone for coolant.
Procedure
- Open the drain valve on the radiator and remove the plug in the bottom of the water inlet.
- Check for damaged hoses and loose or damaged hose clamps; replace as required.
- Fill the system with a mixture of sodium carbonate and water and operate the engine 5 minutes above 80°C [176°F].
- Shut the engine off and drain the cooling system.
- Fill the cooling system with clean water and operate the engine 5 minutes above 80°C [176°F].
- Shut the engine off and drain; repeat until the drained water is clean.
- Fill with a 50-percent water and 50-percent ethylene glycol or propylene glycol antifreeze mixture.
- Install the pressure cap.
- Operate the engine until it reaches 80°C [180°F] and check for coolant leaks.
Tools required
- Drain pan with a capacity of 19 liters [5 gal] Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Design fill rate 19 liters per minute [5 U.S. gallons per minute]
Source: Cummins/Onan owner's manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.