Fault Phenomenon: Zoomlion ZLJ5530THB boom positions accurately when cold. While holding a posture (operating temperature), the boom automatically drifts, the speed increases, and eventually becomes uncontrollable.

Troubleshooting Process:
– Checked counterbalance valve: good sealing.
– Checked cylinder internal leakage: pressure holding test normal.
– Checked hydraulic lock: functions normally.
– Measured counterbalance valve control pressure at operating temperature: found control pressure leakage.

Root Cause: Wear of the counterbalance valve pilot spool due to heat causes poor sealing.

Required Parts: Boom counterbalance valve assembly.