When an excavator is weak and slow under load, the default diagnosis is often a worn-out main hydraulic pump. But tearing down a main piston pump is an expensive, multi-thousand-dollar mistake if the problem is actually a fatigued main relief valve. You can differentiate between the two using a simple hydraulic stall test and a pressure gauge.
Install a 6,000 PSI gauge into the main system test port. Have the operator start the machine and extend the boom cylinders until the arm hits the mechanical stop, and then hold the joystick in the extend position (this is a stall condition). Watch the gauge.
If the pump is worn, it is internally bypassing fluid across the damaged swashplate or valve plate. The gauge will slowly and smoothly climb, but it will peak out well below the system's rated pressure-for example, it might only build 3,500 PSI before the gauge stops climbing, and the engine won't lug down. The pump simply cannot move more oil at higher pressure; it is leaking internally.
If the main relief valve is failing, the gauge will spike instantly to a specific pressure (say, 3,500 PSI) and hold there like a brick wall. The engine will heavily lug down, and the relief valve will scream as it dumps high-pressure oil back to the tank. A relief valve popping off too early feels exactly like a weak machine, but it means your pump is actually making full pressure-the relief valve is just throwing it away. If the gauge spikes instantly to a hard limit, adjust or replace the main relief valve before you ever touch the pump.
