So I too am not quite understanding the concept.
The fuel pressure from the LP is helping to hold open the shut off solenoid ?
Fuel Solenoid at the back is not the Fuel Shutoff Solenoid which stands up on the front. So... No
The DS4 Service Manual is hard for many people to understand.
The way to understand it is the Fuel Solenoid is the throttle. The rest if the IP is the carburetor. When there is insufficient fuel pressure to keep the fuel bowls fuel when the accelerator pump goes to squirt fuel it cannot but it tries tries to stay open so a venturi effect keeps pulling what fuel it can from the bowl to keep the engine running.
In the case of the DS4 it keeps making what pressure it can and trying to operate the injectors by supplying whatever pressure it has. The Fuel Solenoid does this from the ecm telling it that it needs a specific rpm or the accelerator pedal is calling for increased fuel injection for increased rpm. The PMD does this by increasing the duty cycle to the Fuel Solenoid.
For a new pmd it will keep this up for quite a while. For an older pmd it can cause the power transistors to thermally and electrically stress and be unreliable as a switch or just fail.
An IP that has max pressure at the inlet will operate 100% as designed
The concept is simple if it can take 18 psi - then feed it 18 psi - that is what will operate it best, but in theory it is statically tested at 5 psi which is the minimum it should operate at with sufficient fuel volume but WillL at post 162 truthfully told us that the inlet for volume is too small from what GM did trying to get the OEM solenoid pump to work. Pressure and Volume when restricted by line size means you need higher pressure and volume to meet the needs of the DS4 IP design.
There should be enough bleed from the internal parts to cause a housing pressure at high speed and WOT to keep pressure in the housing to keep the spec circulating which originally to keep the pmd cool which does not get hot at high inlet pressures anyway.
But the truth is it all went to hell in a hand basket when gm put on a lift pump too small, that did not work, and did not really push pressure and volume of fuel for a new electronic design which relies on pressure to work right.
I mentioned in an earlier post how I tried AN unregulated Holly Blue knock off, and it proved what I am saying. As I a created and the fuel pump came to push maximum pressure the6. 5 ran like a rocket for a diesel at 14 psi. When I stopped at idle the alternators let the fuel pump rpm and pressure drop slightly, and so too came a loppy idle. Took it off the same day and put on the regulated system and a gauge under the hood also at the ip and have not looked back since.
In my posts are enough clue to build something yourselves, if you know the features of the OEM lift pump.
There are several ways the inlet volume and pressure is not used exclusively for injection. But when there is plentiful fuel the pressure itself causes idle without pmd involvement and to accelerate the engine minimum electrical power is needed energize the throttle as the fuel is ready for increased injection rates and tends to push through on its own. A spring and plate is setup in such a delicate balance that requires a crush ring and dial indicator to set it up.
The concept is no different from a more powerful fuel injection pump to feed more fuel to larger fuel injectors on a gas engine.
Try to run say a vortec on 30 psi of fuel pressure and see how well it runs when the injectors require 45 or 60 psi to work. DS4 usually cannot idle with insufficient pressure and fuel volume. It often will shut off because the fuel solenoid is at 100% duty cycle and cannot maintain 600 rpm.
Ask yourself where is the throttle cable on a DS4?? OF COURSE it is called drive by wire. There are several places in the DS4 Service Manual that the difference between it and a DB and you need to understand what this means in how engine speed and injection is controlled.
This where car sales people and mechanics do not try to explain engines and vehicle systems - people try to understand it the way they want to, and after several times being told no that is wrong, they get mad because they themselves don't understand.
IF there is someone in a math class trying to tell the class a wrong answer is correct - well we watched the last president do that and so no, that person is not here to help at all.
I think the OP has plenty of information to complete his project, and I am retired and this fun for a while, but now I am not helping the OP.
This has turned into a challenge for who can argue about nothing, so it can go on without me
I know what I am doing and I do not have problems with my 6.5. I think it is time I am done sharing.
