On the flywheel side of the engine. On the deck, beside the head you will see two pumps flanking a PWM valve on a pot metal pedestal. Those two pumps are driven by the camshaft. They are slaves to the PWM valve in the fuel control.
So you understand what you are playing with, we'll start from the beginning. Starting at the fuel tank, through the sender/standpipe up to the fuel/water separator. Here's the first chance for problems. Aftermarket filters, for some reason, don't fit right and introduce higher restriction than factory. Issue 2, the primer check in the filter head. Often overlooked. If you remove the filter head, there's a large plastic allen plug, behind that is a large check ball. Sometimes debris holds this open allowing a drain-back condition. Moving on, up to the lift pump. The excavators have the longest living belt driven pump of the bunch. As mentioned, the paint at the inlet was a problem on brand new machines. The rule, for me, if there's moist dirt collected around the pulley, it's junk. That said, I have replaced only one excavator fuel pump and that was foreign object damage. Now we leave the lift pump and head towards the secondary fuel filter and press sensor. No real issues there aside from external leaks which almost never occur. From secondary filter it goes to the FCV (fuel control valve) the FCV serves several functions. It has a fuel pressure regulator on the bottom, this maintains your low side pressure of 70-75 psi. It distributes the fuel to either tank or the two high pressure pumps. This is done with the metering valve on top. So now we're getting serious. Your fuel pressure has just risen from 75 psi to at least 8k psi. This high pressure is fed directly into the common rail. So the theory is that the fuel is dead-headed in the rail. There should be no leak-off. At the end of the rail is a pressure sensor, there is also a safety valve. I forget the specific value, but it's around 25k psi. The valve unloads and dumps to tank. This is calculated by the computer. When the pressure is recorded above the threshold value, it times it. I believe its 5 seconds, then it wants a reset from a dealer service tool. That keeps the common rail from rupturing. When the desired fuel pressure cannot be achieved in spite of raising mA to the FCV, the computer thinks that there is an external fuel leak . Once you get "Fuel system failure" you usually have 30 seconds of run time before she shuts down. It's for safety. You don't want vaporized fuel spraying around a hot turbo. So, we've got our high pressure fuel in the rail, the pipes carry it to each injector which is waiting to do its thing. If one injector has etching of the pintle and seat, your fuel will leak to tank via return path. The problem with the early common rail is all the return piping are rigid pipes stacked on one another with hollow screws. Not ideal for troubleshooting.
In my experience, there have been rare instances where the high pressure pumps cam follower broke and damaged the block. Water intrusion is the biggest killer. The system has zero tolerance for any moisture. The high pressure etches the machined surfaces of the pressure pumps and injectors. Most often, the failure lies in the injectors. I have welded fuel lines shut to make canceling plugs for the rail. It proved to be a waste of time. If I was able to find which injector was causing issue and replace it, I would be back within a month changing out the rest, since they all eat the same fuel.
My apologies for the long-winded description but you gotta know what you're up against before diving in. The raised fuel pressures are dangerous. Injection hazards are real and must be respected. Please proceed with caution. Good luck
