Cummins/Onan — Cooling System - Drain/Flush/Fill
Maintenance
From a Cummins/Onan owner's manual covering: 4B3.9, 4BT 3.9, 4BTA 3.9, 6B5.9, 6BT 5.9, 6BTA5.9, B3.9, B5.9, C8.3. Check it applies to your exact model before use.
Applies to: Cummins/Onan 4B3.9, Cummins/Onan 6BTA5.9, Cummins/Onan B5.9 engine
Before you start
- Do not remove the pressure cap from a hot engine. Wait until the coolant temperature is below 50 C [122 F].
- Coolant is toxic. Keep away from children and pets.
- Never use water alone for coolant.
Procedure
- Drain the cooling system by opening the drain valve on the radiator and removing 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 (0.5 kg [1.0 lb] sodium carbonate per 23 liters [6.0 gal] of water).
- Operate the engine for 5 minutes with the coolant reaching above 80 C [176 F].
- Shut the engine off and drain the cooling system.
- Fill the cooling system with clean water; operate engine 5 minutes above 80 C [176 F]; drain again until water is clean.
- Fill with a mixture of 50-percent water and 50-percent ethylene glycol antifreeze.
Tools required
- Drain pan with 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
- Cooling system design fill rate of 19 liters [5 gal] per minute
- Fill slowly to prevent air locks; vent the engine and aftercooler
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.