Just came across an application today that made me pause. HPCR pump, normal looking Denso 2 cylinder, with no lift pump. Just a single fuel filter before the HPCR pump. Lift from tank to pump inlet about 2-3 feet. Never seen it done like that, would have thought it was not recommended/allowed. I have not had too much trouble priming these fuel systems after filter change but I wonder where the air is supposed to go. It has a primer pump on the filter head, I filled the filter with it partially off to relieve air and then screwed it down when full, but when I pump the primer it gets hard but I am at a loss to where the remaining air can go, the only line goes to the HPCR pump inlet. In this case the engine did start and run with no problems.
I don't know but I'll tell you a story. Back in 08 when the 6.4 diesel came out we changed fuel filters and the thing would shut off from air. It wouldn't run bad and die it would just shut itself off. I looked in the owners manual and it said to key cycle it 6 times. Boss said where is the air going to go, I said they probably designed it to go somewhere. He calls the mechanic and that guy says to leave the filter cracked and key cycle it. 20 minutes later and several gallons of diesel on the ground he finally listens to me and two minutes later engine is running.
A lift/transfer pump eases the strain on the high pressure fuel pump. Many of our common rail engines will start and run just fine without the lift/transfer pump but it ain’t good for them.
Not sure about that Denso, but some Delphi at least have a gear pump built in.
Don't know what you're working on, but like ozarkag said, usually there is a gear pump running up to the HPCR, whether mounted separately or built into the side of the IP.
The cat skidloaders are a fine example of this it seems till you change the filters