A contractor reported that a Hitachi ZX330LC-5G excavator would stall the engine whenever the operator tried to swing and lift the boom simultaneously. The engine ran fine at idle and under single-function loads, but as soon as a heavy load was placed on the hydraulic system while swinging, the engine would bog down and die.
Initially, the fuel filters were checked and found to be clean. The technician then checked the fuel tank breather cap. The breather was completely blocked with dried mud, creating a vacuum in the fuel tank. This was restricting fuel flow to the injection pump under high demand. The breather was cleaned, but the stalling persisted, though it was now less severe.
The technician then focused on the engine timing. The excavator uses a gear-driven timing system. The injection pump gear is held to the shaft by a key and a locking bolt. Upon inspection, it was found that the locking bolt was slightly loose. The keyway on the shaft had worn slightly, allowing the injection pump gear to "retard" (rotate slightly backward) under heavy torque loads.
When the hydraulic load increased (swinging and lifting), the engine needed maximum fuel. However, the retarded timing meant the fuel was being injected too late, resulting in poor combustion and low power. The loose gear was also causing erratic timing.
The repair involved re-timing the injection pump, replacing the worn key, and applying a high-strength thread locker to the gear bolt. The fuel tank breather was replaced as a precaution. The engine now held steady RPM under full hydraulic load, and the stalling issue was resolved.
