Slow loss of RPM at near WOT

[DennisG01]

duplicate duplicate

[DennisG01]

Fuel pressure gauge took a dump. Only reading 10psi all of a sudden. Engine runs strong so there must still be normal PSI at the engine. I double checked and plumbed a PSI/Vac guage between the HP pump outlet and fuel rail. It only reads to 10PSI but the needle swung all the way around.

More research and learning (I think!) leads me to believe it can't be the O2 sensor or I would have running issues at all RPM's.

Hoses between the HP pump/rail/return at VST all seem solid - no soft spots to indicate an interior restriction.

Ran it for a couple minutes at WOT and it "dialed down" to 5,000RPM as expected... but I left it there and after about 30 seconds it climbed back up to 5,600. I ran it like that for another 30 seconds and then idled down and went back to the dock.

Maybe an injector issue? Filter basket?

I wonder... maybe I can pressurize/vacuumize (like that terminology!) the vac line that is attached to the pressure regulator. I'm not sure of the other end and whether or not it might be "open" at certain times?

[99yam40]

guessing on what the rail pressure is doing will not work.
get a good gauge and monitor it.
if pressure is dropping when RPM drops, you have to find out what is causing the pressure to drop

[DennisG01]

guessing on what the rail pressure is doing will not work.
get a good gauge and monitor it.
if pressure is dropping when RPM drops, you have to find out what is causing the pressure to drop

I'm not actaully guessing - see my first post for the details.

Yes, trying to figure out why the pressure is dropping is the goal. Any ideas?

[99yam40]

I'm not actaully guessing - see my first post for the details.

Yes, trying to figure out why the pressure is dropping is the goal. Any ideas?

I believe I mentioned pressure regulator bypassing too much fuel so rail pressure drops or lift pumps not keeping up,
or sucking air, not much but enough to not keep up with the fuel need some times.
I also mentioned shutting off the key as soon as the RPM drops so you can measure the amount of fuel in the VST

does this motor have a fuel cooler that could be restricted?

[DennisG01]

I believe I mentioned pressure regulator bypassing too much fuel so rail pressure drops or lift pumps not keeping up,
or sucking air, not much but enough to not keep up with the fuel need some times.
I also mentioned shutting off the key as soon as the RPM drops so you can measure the amount of fuel in the VST

does this motor have a fuel cooler that could be restricted?

I don't completely understand what you mean by recycling/bypassing too much fuel. Can you please explain? It seems like the FPR might be the issue - even though is "appears" to test fine - but I'm still working on understanding the FPR so I'm not totally following what you're saying.

I can't see how the VST would be low on fuel - if it was, air would be sucked up by the HP pump and the performance of the engine would be drastically different (more info about that in my first post).

No, there is no fuel cooler on this engine.

[99yam40]

how does the pressure regulator control the rail fuel pressure.
from what I understand it dumps off the excess fuel pressure, so it sends it back to be pumped again.

if level gets low enough and it gets just a little air I could see pressure dropping.
as I said I am just trying to think of ways the pressure could drop

as I said only way to verify the level in the VST when the problem shows up is do what I mentioned.
if it stays full, that eliminates that.
then the only thing I can think of would be the pressure regulator is not doing its job.
or you can replace the PR to see if that fixes your problem