We have a FDP-series CAT 420D (circa 2003) that is losing power when the hydraulics are engaged. In other words, every time we use any of the hydraulic controls, the engine bogs down. In the past, this would only happen when water was present in the fuel -- especially in the winter -- but we've changed both fuel filters now and we still have power loss: even in warmer weather. We're also hearing an intermittent audible sound coming from the underside of the machine when the hydraulics are being used. Needless to say, this has us worried about the hydraulic pump, but the hydraulics are still strong, so I'm really scratching my head now. In other words, the loader bucket and the hoe will still lift the machine off of the ground -- at idle -- so the pump still has some guts. Anyway, this is how I was taught to test for a bad pump, but that was a very LOOONG time ago, so I assume there are more sophisticated ways to check for a bad pump now. The machine has over 3,000 hours on the timer now, so I don't expect it to be pristine. I'm just wondering if anyone else on the board has either experienced and/or heard of the same CAT 420D issue. Thank you for your time.
Did you drain the fuel tank for water and/or sediment when you were changing filters..? Also did you blow through all the fuel lines and fittings all the way between the fuel tank and the injector pump.? There are often banjo bolts in weird places in fuel lines that have a small screen in them, no idea if that applies to your machine.