Optimax fuel starvation

[copano]

I have a 2006 225 optimax in the shop, complaint is loss of power at WOT


Tests performed..
Suction side less than 2 in hg
Low pressure 30 psi running
High pressure fuel 90 psi running
Air pressure 80 psi running.


Water test...
Loss of power after a couple of minutes at WOT
Low pressure fuel drops to 10 psi.
High pressure fuel drops to about 85 psi.
Pulling back throttle restores normal pressures.


One odd thing is shutting down engine pressure will slowly build on suction side to 5 psi, obviously able to do so because of the check valves in primer bulb.
I replaced the diaphragm pump thinking the internal check valves were defective but replacement pump has same result.

Did a comparison flow test on both high and low pressure pumps.
both flow a pint in 9 seconds using new cleaning solvent, would perhaps be more with gasoline, but used solvent for safety issues..
VST internals are fairly clean
Pump inlet screens are clean


I'm at a bit of a loss to why fuel pressure is dropping at WOT.
Grasping at straws I wonder if this could be an issue with the fuel/air regulator diaphragms and maybe explain the fuel inlet pressurization I see on shut down

I don't work one many mercs, we're not in an area where mercs are popular.



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[copano]

Oh, and what should the float level be set at?
I find no reference to it in the service manual.
Inverted the float on this is about 15 degrees higher on the free end, is that about right?

[copano]

Also there is a non replaceable check valve in the VST, what is its purpose?
Testing with my Mityvac it acts more like a restrictor as it takes some time to leak down under vacuum or pressure, but it was hard to get a seal on it down in the tank

[XstreamVking]

One thing you didn't mention is the fuel lines. Also if dealer rigged they sometimes put a connector on the fuel line INSIDE the rigging tube. Also fuel line of some types can collapse internally. You could eliminate all the above by trying a small portable fuel tank and different lines.

[copano]

I may run a portable tank tomorrow to double check, but I ran a compound gauge at the inlet for the pulse pump with a clear hose, never observed vacuum over 2 in hg or any air intrusion

[flabum1017]

Have you replaced the filter in the VST?

[Bruce2]

You test fuel pump under full throttle load..

[copano]

VST filter reported to have been replaced at another shop, visual inspection confirms its like new, (white pleats and no foreign matter)

However, the observation of fuel pressure building on the inlet side of the pulse pump (between the pump and primer bulb) is concerning and I'm sure not normal.
If its trying to push fuel back to the tank it would seem to have a problem pushing fuel to the injectors.
And thats why I was asking about the check valve inside the VST.

That said, running pressures are fine except after a few minutes at WOT when low pressure drops indicating the VST is running dry.

I'll put it back together tomorrow and run it on a portable tank, as I'm second guessing myself if my suction side vacuum test was valid if the compound gauge didn't have a tight connection.
Only other thing I noticed was the rubber seal at the VST outlet to the port fuel rail had a small cut in it possibly allowing it to fold over and partially restricting flow.
Again, I'll replace that tomorrow and retest.

And before I forget, can someone chime in on where that float should be set at? for the life of me I can't find that spec in several service manuals

[copano]

You test fuel pump under full throttle load..

Listed in first post under water test at WOT

[flabum1017]

Check the pulse hose to the lift pump for leaks.... Float should set level with needle seated in the seat (hold vst up-side-down ).It should not pass the gasket surface, otherwise, it will hit the cover and the needle won'rt seat.

[flabum1017]

423506


423507

[Motv18]

What did the air side drop to didn’t see it listed. The regulator is air psi +10

if air stayed 80 and fuel dropped to 85 it’s starved if air dropped to 75 then its air starved.

Amp draw the low pump It’s a fairly common replacement.

[copano]

Problem solved, put VST back together and ran boat on extended water test, ran perfect with no fuel pressure drop.
I wish I could put my finger on the cause, but all I did was slightly reset the float higher, replaced the rubber seal at the high pressure outlet fitting and re zip tied the pulse hose at the block fitting..
Many thanks to all that replied.

[Motv18]

Cool could have been all three just a little from each. Or a turn it off and back on ha.

[vnemous]

Did you re adjust the float height?

[copano]

Did you re adjust the float height?

Yes, raised it a little bit as old setting looked like fuel level in VST was low, though even then there should have been enough fuel in tank to keep LP pump submerged.

The pulse hose to block fitting was slightly loose, perhaps just enough that a small leak there prevented the pulse pump to meet WOT fuel demand.

The output seal from VST to fuel rail was cut in such a way it could partially occlude fuel flow, but I dismiss that as a cause as it would effect high pressure readings but not low pressure where I was seeing the pressure drop