A contractor reported that a Hitachi ZX350LC-6 excavator had a strange issue when lowering the boom. The boom would lower slowly for a second, then drop rapidly, then slow down again, creating a "bouncing" motion. The problem was worse with a heavy bucket attached. The operator also noticed a loud "gurgling" sound coming from the hydraulic tank.
The symptoms pointed to a regeneration circuit failure. When an excavator lowers its boom, the cylinder rod side (head end) fills with oil, but the piston side (rod end) creates a void as the rod retracts. To prevent cavitation, the regeneration valve opens and allows oil from the piston side to flow back into the rod side. If this valve fails, the rod side starves for oil, causing the jerky motion.
The technician removed the main control valve cover to access the boom regeneration spool. Upon disassembly, the regeneration spool was found stuck in the closed position due to varnish buildup from overheated oil. Additionally, the centering spring behind the main boom spool had snapped, allowing the spool to vibrate and create inconsistent flow paths. The "gurgling" sound was the cavitating cylinder drawing air into the system.
The technician cleaned the regeneration spool with fine emery paper and replaced the broken centering spring. The anti-cavitation check valve was also inspected and found to have a stuck ball bearing, which was cleaned. The hydraulic oil was changed, and the system was bled. The boom then lowered smoothly and consistently, and the gurgling noise disappeared completely.
