300 HPDI fuel pressure issue

[Wide_Open]

I've got a 300 HPDI thats given me years of trouble free operation that started to act up on the last trip above ~4200rpms. Engine would not reach full rpm(~5600-5800rpms trimmed all the way up) and would miss/surge above 4200rpms. Engine would not bog down and die, just had a clear and distinct miss above 4200rpms and would occasionally surge. No problems at all from idle to 4200rpms. I ran boat with a fuel pressure gauge on the VST and found good pressure at key on(52psi), idle to 4200 rpms (48-50 psi), above 4200rpms fuel pressure drops further with increased rpm ( ~40 psi at 4500rpm, ~35 psi at 4800rpm, ~30 psi at 5000rpms, ~25 psi at 5200 rpm). I ran the same gauge on another identical boat/motor combo to rule out gauge problems.

I have done the following with no change:
-Ohm out spark plug boots(all within spec)
-Change spark plugs
-Change 10 micron fuel/water separator in boat
-Change clear cannister filter under cowling
-Change mid-pressure metal cannister filter
-Change VST filter/screen and o-ring on fuel pump outlet(top side)
-Clean / Check VST twice and VST regulator & screen on top of tank
-Change both Low Pressure Fuel Pumps
-Pumped primer ball when problem occurs
-Ran engine on separate 5-gallon tank and hose direct to clear cannister filter inlet.

No debris was found anywhere within the fuel system and I have always used non-ethanol fuel. All gaskets(VST) and orings(clear cannister filter) are new and installed properly with no air/fuel leaks.

Short of replacing the mid-pressure electric fuel pump, I am at a loss. Other idea, although a stretch and probably unlikely, is electric fuel pump relay maybe causing low voltage at fuel pump at high rpms. Also, have not ran a vacuum gauge either, but I can't imagine a restriction in the short distance of fuel hose between the new clear cannister filter to new LPFPs and new LPFPs to the VST. I didn't even bother to check high pressure fuel system since the medium pressure fuel system isn't up to par.

Also, I'm aware of the "mystery basket filters", but those are all on the High Pressure fuel system side.

HPDI gurus, any ideas, is there anything I may be missing?

[rodbolt17]

yep
your missing the fact that the medium pressure is dumpimg.
dump medium pressure and High pressure dumps.
dump high pressure and the motor leans out and pops pistons.
either you cant keep the VST full or the medium pressure filter is clogged.

[greasyshaft]

Have you checked the quadrophonic isotopes?

[rodbolt17]

if the medium pressure side cannot maintain 50 PSI the high pressure side cant maintain its 1125 psi.
if it cant maintain about 1000 psi it falls on its face.

[Wide_Open]

Rodbolt,

I follow all that and agree 100%. I know I am dumping medium pressure at high rpms, but short of what I have tried already in the original post, what else could it be besides a weak medium pressure fuel pump in the VST. Have you had a case before where the problem was the VST pump and act like this where it would maintain good pressure until high rpm range and fall off after that as opposed to one that just worked or it didn't?

Medium pressure filter is new but it's after VST so I don't think it would cause low VST pressure.

I thought about not being able to keep VST full, but everything downstream is new and have tried pumping primer ball when low VST pressure problem is occurring with no change.

[rodbolt17]

itspossible to have a voltage issue, not likely.
but easy to test.

[Wide_Open]

Tomorrow, I'm gonna hook up to YDS and go run it. Also, I'm gonna make up a harness to monitor voltage at the electric fuel pump. We'll see what happens then and I'll report back.

"Mystery basket filters" are on the list to change but I don't see how those could cause low VST fuel pressure?

Thanks for all the help so far and keep the ideas coming.