Cummins/Onan 20W-20 — Testing Thermostat
Testing & diagnosis
For the Cummins/Onan 20W-20, from the Cummins/Onan service manual.
Applies to: Cummins/Onan 20W-20, Cummins/Onan RDJF engine
Procedure
- Remove thermostat from cylinder head.
- Heat a pan of water to approximately 150 F.
- Suspend thermostat in water; it should start to open.
- Remove from hot water and allow to cool; it should close in a short time.
- Replace if it sticks or does not operate properly.
- Always install a new gasket when replacing the thermostat.
Tools required
- Thermometer Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Thermostat opens at 150 F +/- 2 F
Full service-parts list: Cummins/Onan 20W-20 →
Source: Cummins/Onan service 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.