Cummins/Onan — Checking Engine Cylinder Compression
Testing & diagnosis · Cylinders
From a Cummins/Onan owner's manual covering: G 12, G28, G855, GTA12, GTA19, GTA28, GTA855. Check it applies to your exact model before use.
Applies to: Cummins/Onan G 12, Cummins/Onan G&855, Cummins/Onan GTA855
Before you start
- Verify that your batteries have a full charge before starting this check.
Procedure
- Locate and shut off the gas fuel valve and tag the valve with a DO NOT OPEN tag.
- Ground the ignition G lead to prevent any ignition spark.
- Remove spark plug probes from the spark plug wells. Check for oil or water contamination and clean before continuing.
- Remove the spark plugs from ALL cylinders using a proper spark plug removal tool.
- Block the engine throttle wide open.
- Install a compression gauge in #1 cylinder.
- Crank the engine with the starter until the engine has rotated 6 to 8 revolutions; the gauge should be at a stable reading.
- Record the reading in pounds per square inch. Release the gauge pressure and remove the gauge.
- Repeat for each cylinder. All cylinder pressures should be within a 10 percent variance.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- All cylinder pressures within a 10 percent variance
Full service-parts list: Cummins/Onan G 12 →, Cummins/Onan G&855 →
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.