Cummins/Onan — Testing Compression / Leakdown
Testing & diagnosis · engine
From a Cummins/Onan service manual covering: 5.5 HGJAA, 5.5 HGJAB, 5.5 HGJAC, 5.5 HGJAD, 5.5 HGJAE, 5.5 HGJAF, 7.0 HGJAA, 7.0 HGJAB, 7.0 HGJAC, 7.0 HGJAD, 7.0 HGJAE, 7.0 HGJAF. Check it applies to your exact model before use.
Applies to: Cummins/Onan 5.5 HGJAD, Cummins/Onan 5.5 HGJAE, Cummins/Onan 7.0 HGJAD, Cummins/Onan 7.0 HGJAE
Procedure
- Warm engine ten minutes if it will start.
- Disconnect negative battery cable and remove spark plug.
- Rotate engine to top dead center on compression stroke.
- Connect leak down tester to shop air and set calibration.
- Screw air fitting into spark plug hole and attach tester.
- Read percent leakdown; listen for leakage points if over 30 percent.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 0-10 percent excellent; 10-20 percent normal; 20-30 FridayParts $58.50 percent near service limit; over 30 percent may need major service
Full service-parts list: Cummins/Onan 5.5 HGJAD →, Cummins/Onan 5.5 HGJAE →, Cummins/Onan 7.0 HGJAD →, Cummins/Onan 7.0 HGJAE →
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.