A common, maddening issue on excavators is a hydraulic pump that runs hot and whines loudly even when the machine is idling and all joysticks are in neutral. The pump should destroke to zero flow at idle, reducing the load on the engine. If it's whining, it's pumping oil over the main relief.
The problem is almost always a leak in the load-sense signal network. Closed-center hydraulic systems use a tiny "load-sense" line that connects each control valve spool to the pump regulator. When a joystick is moved, the spool sends a pressure signal down this line, telling the pump to stroke up. When the joystick is centered, the signal drops to zero, and the pump destrokes.
If a load-sense line is chafed and leaking, or if the tiny orifice inside a control spool is blocked by debris, the signal pressure cannot bleed back to tank when the joystick is released. The pump thinks a function is still being commanded, so it stays on full stroke, dumping 5,000 PSI oil over the relief valve and generating massive heat. To diagnose it, crack the load-sense line at the pump one at a time. When you crack the line that is holding the trapped signal, the pump will instantly quiet down and destroke. Follow that line to the faulty spool.
