When an excavator boom or stick cylinder is rebuilt, the mechanic refills it with clean hydraulic oil. However, if the cylinder is mounted with the rod facing upward, a massive air pocket is trapped at the rod-end of the cylinder. If the operator starts the machine and cycles that boom rapidly, the pressurized air inside the cylinder compresses violently, generating immense localized heat.
This is called hydraulic dieseling. When air is compressed rapidly, the temperature skyrockets-just like in a diesel engine. If there is atomized hydraulic oil mist mixed with that air, it can actually ignite inside the cylinder. This micro-explosion creates a localized thermal spike that instantly burns and carbonizes the polyurethane seals on the piston, turning them into hard, black ash. The cylinder will immediately begin to bypass internally and drift down.
To prevent this, you must purge the air from a rebuilt cylinder before applying full system pressure. The correct procedure is to start the machine at low idle. Gently extend the cylinder, leaving an inch of cushion before the physical stop. Then, crack open the bleed valve on the rod-end of the cylinder (or slightly crack the hydraulic fitting) until clean, air-free oil weeps out. Retighten the fitting, then slowly retract the cylinder. Repeat this slow cycling process three or four times to dissolve the remaining air bubbles into the oil before running the system at high pressure. Skipping this step guarantees you will be rebuilding that cylinder again next week.
