A construction site reported that a Volvo EC220D excavator's boom was weak when lifting. The machine would stall the engine if the operator tried to lift the boom quickly, but it worked fine if the lever was pulled slowly.
The technician checked the main relief valve pressure, which was normal (350 bar). He then focused on the "Boom Circuit." The Volvo system uses a "Boom Regeneration Valve" to speed up the lowering. He removed the regeneration valve cartridge. The "Check Valve Spool" inside was stuck in the "Closed" position by a small piece of Teflon tape.
When the regeneration valve is stuck closed, it blocks the flow of oil from the rod side of the cylinder. When the operator tries to lift the boom, the piston side fills with oil, but the rod side cannot evacuate oil through the regeneration path. This creates "High Backpressure" in the cylinder, fighting against the lift force. This backpressure caused the engine to stall (hydraulic lock).
Furthermore, he found the "Pilot Filter" (located under the pump deck) was completely clogged. The clogged filter restricted the pilot pressure flow to the main control valve, delaying the response time.
The repair involved cleaning the regeneration valve spool and replacing the pilot filter element. The Teflon tape debris was flushed from the hydraulic tank. After reassembly, the boom lifted smoothly and powerfully without stalling.
