Cummins/Onan — Compression Test
Testing & diagnosis · Engine
From a Cummins/Onan service manual covering: 12.0MDJC, 15.0MDJF, 3.0MDJA, 6.0MDJB, 7.5MDJE. Check it applies to your exact model before use.
Applies to: Cummins/Onan 3.0MDJA, Cummins/Onan MDJA, Cummins/Onan MDJC, Cummins/Onan MDJE, Cummins/Onan MDJF, Cummins/Onan MDJA engine, Cummins/Onan MDJB engine, Cummins/Onan MDJE engine, Cummins/Onan MDJC engine, Cummins/Onan MDJF engine
Procedure
- Run engine until thoroughly warm.
- Stop engine and remove all injection nozzles.
- Insert compression gauge in one nozzle hole.
- Crank engine and note reading.
- Squirt SAE 50 oil into cylinder and repeat check.
- Increase in compression indicates piston blow-by.
Tools required
- compression gauge Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 300-350 psi prior to Spec P; 350-400 psi Spec P and later.
Full service-parts list: Cummins/Onan 3.0MDJA →, Cummins/Onan MDJA →, Cummins/Onan MDJC →, Cummins/Onan MDJE →, Cummins/Onan MDJF →
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.