On a Cummins, the "lift pump" has a single purpose. It "lifts" fuel from the fuel pump to the engine. It is there because most fuel tanks on trucks and motorhomes are below the engine. Since it can be a long run from the tank to the engine it is likely that some fuel may drain back to the tank when the engine is not running. That fuel will be replaced by air. In other words, their can be air in the fuel line from the tank. The "real" fuel pump and injector system can't stand having any air in it. So, the "lift pump" runs for long enough to flush fresh fuel up from the tank, past the "real" fuel pump and back to the tank via a return line. Then, when the engine starts, the "real" pump sees nothing but fresh fuel. When running, the "real" pump does it all and the "lift" pump is useless.
It is mounted on the engine. Mine was replace once a few years ago when it began leaking. The theory at the time was that the leak was caused by the switch to ULSD caused O-rings to shrink.
Since I'm typing, I'll offer an opinion which may be worthless to you. It is to not screw around with the engine. The Cummins installation in the Monterey has been bullet proof. It never runs hotter than 185 on the hottest days over the steepest hills. On ours, we've need only the lift pump and an exhaust manifold gasket in 123,820.5 miles. After having tested the lowly 350 HP engine on a 35 mile run up and down three 6%+ hills against a similar Beaver with a 455HP Cat, I was only "behind" by a minute or two in a 5 hour day. I just don't see any point in trying to push up the performance.
Bill
