Cummins/Onan GTA38 / GTA50 — Engine Cylinder Compression Check
Testing & diagnosis · Engine cylinders
For the Cummins/Onan GTA38 / GTA50, from the Cummins/Onan owner's manual.
Applies to: Cummins/Onan GTA38 engine, Cummins/Onan GTA50 engine
Before you start
- Cylinder pressure between engines may vary by as much as 40 psi.
Procedure
- Verify batteries are fully charged; shut off and tag the gas fuel valve.
- Ground the ignition G lead to prevent ignition spark.
- Remove spark plug probes; clean any oil/water contamination.
- Remove spark plugs from all cylinders using a proper tool.
- Block throttle wide open; install compression gauge in #1 cylinder.
- Crank engine until it has rotated 6 to 8 revolutions and gauge is stable.
- Record the reading in pounds per square inch.
- Repeat for each cylinder.
- Average all cylinder pressures; all cylinders must be within +/-5% of average.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Within +/-5% of average
- Typical 160 psi (low) to 200 psi (high)
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.