Had a tech with 30 years experience come out to diagnose my machine's problem of low speeds in all motions. He first found a spring broken in one of the relatively new pump control solenoid valves. Electrically it was good and in spec, but it had failed mechanically internally. Correcting that allowed 3,800 PSI out put from the main hydraulic pump versus 2,000 PSI before, but was still well below specs.
Further investigations and testing showed the output of the engine injector pump was inadequate to develop full power. He says he has seen that one a number of times and in his opinion it is caused by dirty fuel causing excessive wear internally to the pump. I had been looking for issues in the hydraulic system and the PVC system for some time. I ask him how he knew to open the solenoids to inspect them internally. He said "experience" based upon all functions slow and low pump pressure output. When the defective solenoid was replaced, the PVC computer system tested as having no faults. The machine would not develop specified main hydraulic pump pressure, and again, based upon experience, he pressure tested the injector pump output to determine it was producing only 1/6 the specified pressure for proper operation and full power delivery from the engine. It was cranking easily, running smoothly and spooling up on command. I had never suspected the problem was engine related.
My fuel supplier has a filter in the piping that fills my tank. I have a filter on the fuel tank used to refuel the machine complete with water separator. I regularly change the engine mounted fuel filters with those supplied by JD. The machine has 6,500 hours, so perhaps the injector pump just wore out internally due to use, but I defer to the Tech's opinion because he obviously knows more about this machine than I do.
This was an expensive experience. I had replaced the computer's sensors, replaced the PVC computer, removed the main hydraulic pump and took it to a pump shop to be be torn down and inspected. We spent many hours looking for electrical faults I could never find. All relief valves and control valves were removed and inspected (except the pump control solenoids which I knew to be recently new). It took 6 weeks to get the Tech here from the time of my request. I spent many hours looking for phantom faults, defective wiring or defective connections. Meanwhile the machine was parked and unusable now approaching the third month.