A contractor reported that a Kobelco SK350LC-10 excavator felt very sluggish when lifting the boom. The engine sounded like it was under a heavy load (bogging down) even when the machine was idling with no hydraulic function activated. The black smoke from the exhaust was excessive.
The technician first checked the engine fuel filters and air intake, assuming the engine was the issue. The engine was running well, so the focus shifted to the hydraulics. The machine uses a dual-pump system with load-sensing regulators. A pressure gauge was connected to the pump delivery ports. At idle, the system pressure should be around 300-500 psi (standby pressure). Instead, the pressure was reading 2,500 psi.
This indicated that the hydraulic pump was stuck in a high-displacement state, pushing high pressure against a closed control valve. The main pump regulator controls the angle of the swash plate based on the load-sensing signal from the main control valve. The regulator cover was removed from the rear main pump. The servo piston that controls the swash plate angle was seized in the full-angle position due to contaminated oil that had turned the hydraulic fluid into a varnish-like sludge.
Furthermore, the swash plate itself had deep scoring marks. Because the servo piston was stuck, the pump was trying to push maximum flow constantly, causing the engine to work hard just to turn the pump shaft. The excessive pressure was bypassing through the main relief valve, generating the heat and the bogging sensation.
The repair involved replacing the main pump (as the scoring on the swash plate was too deep to hone) and flushing the hydraulic tank. The source of the contamination was found to be a cracked hydraulic tank breather cap that had allowed fine dust into the system. The breather was replaced, and the system was refilled with fresh ISO 46 hydraulic oil. The standby pressure returned to normal, and the engine idle smoothed out immediately.
