Re: Return diesel fuel to the tank?
Because the low pressure side of the fuel pump system ALWAYS sends more to the high pressure side than it needs.
For a direct example my westerbeke 30b pumped 25 gallons per hour, and ran full time.
The engine, at full RPM, consumed 1.1 gallon per hour.
The rest (23.9 gallons) returned to the tank via the fuel return hoses.
The same is true whether it's a mechanical fuel pump (cam operated) or an electrical pump on the low pressure side.
The high pressure side of the system is cam driven and boosts the PSI to 800 or so (engine dependent), that amount of pressure is needed to overcome the springs in the injectors and get good spray into the cylinder.
You would not want the entire fuel system at 800 PSI for obvious reasons.
Because of that you can literally, depending on filter placement, clean your fuel just by running the pump (electrical) or running the engine.
Any professional should in fact know that.
Lessons learned are opportunities earned.