A mining company reported that a Doosan DX300LC-5 excavator's boom cylinder would drift in (retract) when the engine was idling, and a loud "squealing" noise was heard from the main hydraulic pump.
The technician first checked for cylinder leakage. He isolated the boom cylinder lines, and the drift stopped, indicating the cylinder seals were likely fine. He then checked the "Main Control Valve." He found that the "Regeneration Check Valve" (located in the boom control section) was stuck open. The regeneration circuit is designed to assist the boom down-speed by redirecting return oil. If the check valve sticks open, the oil pressure from the head side of the cylinder can bypass the valve and leak into the tank line when the load is static, causing the drift.
The "squealing" noise was a separate, critical issue. He checked the suction line pressure. The gauge showed a slight vacuum. He inspected the "Hydraulic Tank Breather." The breather cap was plugged with mud. This created a vacuum in the tank. When the pump tried to suck oil, the vacuum caused the oil to "boil" (cavitation) at the pump inlet. The bubbles imploded inside the pump housing, destroying the piston shoes and creating the noise.
The repair involved cleaning the tank breather and replacing the hydraulic suction filter. He replaced the main pump (as the internal components were damaged by cavitation) and cleaned the stuck regeneration check valve in the control block. The hydraulic system was flushed with new ISO 46 hydraulic oil. The drift stopped, and the pump noise disappeared.
