A site prep contractor reported that a Sumitomo SH210-5 excavator's boom would slowly sink under load, and the engine felt "sluggish" (heavy) even when the controls were in neutral. The exhaust temperature was high.
The technician first checked the "Load Holding Valves" on the boom cylinders, which were functioning. He then focused on the "Main Hydraulic Pump." In neutral, the pump should "destroke" (reduce displacement) to minimum flow. He connected a pressure gauge to the pump's case drain line. The case drain flow was massive, indicating severe internal leakage.
He removed the "Pump Regulator" housing. The "Feedback Lever" connecting the regulator to the swashplate was seized solid due to rust. Because the feedback lever couldn't move, the regulator was stuck in the "Max Displacement" position. The pump was continuously forcing high-pressure oil through the internal leakage paths of the pistons back to the tank, bypassing the system relief valve. This constant high-pressure bypass caused the boom drift (pressure couldn't build) and the engine lug (pump was stuck at max stroke, loading the engine constantly).
Upon disassembling the pump, he found the "Swashplate" journal was deeply scored. The scoring was caused by the seized regulator preventing the swashplate from tilting smoothly, causing metal-to-metal galling.
The repair involved replacing the main pump rotating group, the swashplate, and the regulator assembly. The hydraulic tank was drained and flushed. After reassembly, the engine idled freely, and the boom held its position solidly.
