F225 fuel system issue stumps Yamaha Tech

[Ljs3@mindapring.com]

Yamaha 2002 225 TXRA. Motor starts fine, runs fine for 2-4 minutes then stalls. Yamaha master Tech ran diagnostics and eventually gave up.
Squeeze the primer bulb (new) and it keeps running. Same symptoms on remote tank. I can hear the brand new LP pump cycling on and off.
F filter going into VST is new and hoses are clear. Clamps are tight.
Motor stalls at 700 rpm or 2000 rpm, no difference. Have replaced check relief valve and cleaned VST. Something is keeping fuel from filling VST I think as manual pump via bulb keeps it going. VST pump was replaced two years ago. VST filter is clean. VST input valve and float appeared okay when I cleaned it but not sure. Is there anything in the pump driver or ECU that could be causing it? Here is an odd thing, motor will start and run with same symptoms with fuse removed. Does that mean anything?
Have not replaced air vent check valve but I cleaned it, but would that keep VST from filling?
Have had no boat since 7/4. Unhappy wife and kids...tnx!

[boscoe99]

If the motor will start and keep running just fine all of the time when using the primer ball (with fuel pressure is being maintained on the FI rail), but not when the lift pump is operating, then I would suspect a vacuum leak somewhere between the primer ball and the inlet to the lift pump. The presumption is that the new pressure relief valve is working correctly and that the engine is stalling because of and when pressure on the FI rail is lost.

Did the mechanic install a vacuum gauge and a section of clear fuel hose at the inlet to the lift pump to see what the situation is?

[Ljs3@mindapring.com]

He put a vacuum gauge on it and it showed 40 lbs then slowly dropped to zero before motor began to stall. He didn't put a clear section on. He said I had fuel getting to the LIft pump.
The primary fuel filter yellow bowl stays full.
If it happens even when on a remote tank plugged directly into lift pump inlet, what might it be?
If the pump driver was bad, pump wouldn't work I guess.
Inlet valve inside of VST? It appeared ok when I took apart and cleaned VST.

[Ljs3@mindapring.com]

relief valve on fuel overflow loop was replaced with new Yamaha OEM part.
Did not replace check valve on VST vent line but blew air through it.
It seems like a vacuum/fuel flow issue though as opposed to vent right?
Thanks.

[Nautical]

Did you check the VST needle valve/seat?

[Ljs3@mindapring.com]

Not sure...the little post valve from the top down into vst? I cleaned the input and it appeared ok. I also replaced the rubber grommet on the top of HP pump at outlet.

[Nautical]

It would be inside the VST connected to the float assembly. If the needle valve sticks the HPump will not get a fresh supply of fuel to the rail.

[zenoahphobic]

Fuse

Just on that fuse. You may have a separate issue as well. If pulling a fuse makes no difference then I'd check that fuse and it's connection. May well have nothing to with current symptoms but I would check.

[rodbolt17]

ok
stop and tink wid de ole tinker.

using the primer bulb to keep the VST full and it runs.

now we know the VST is doing its thang, no further checks are nessasary there.

you claim the lift pump cycles and fuel stays in the engine fuel cup.
now here is the rub.
that fuel level in the cup should change everytime the pump cycles.

my guess is an air leak on the lift pump suction side or the pump portion of the lift pump has failed.

you have not had a vacuum guage on the intake.
you need to.
we have a boxed device that we can watch flow and vacuum and check for leaks. its part of the Yamaha essential tool kit.

[Ljs3@mindapring.com]

Thanks. The lift pump is brand new. The fuel loop check valve is new.
I agree that I seem to be getting air into the LP pump. Just can't determine where it's coming from..
could it be coming from the VST vent loop or is that one way?

[boscoe99]

My gut feel is that the loop line will be pressurized and not a likely source of a vacuum leak.

Every mechanic should have a vacuum pump in his tool box.

Mityvac Vacuum Pump - Save on Mityvac Pumps at Harbor Freight!

Clamp off the fuel line just upstream from the primer ball. Using the vacuum pump apply a vacuum to the fuel line at the hose that is normally connected to the inlet of the fuel filter downstream from the lift pump. See if the system will hold the vacuum. If not, you have a leak. Find and fix that leak.

[Ljs3@mindapring.com]

Sounds like a plan. Where could the leak be though? If I have same symptoms with remote tank connected directly to inlet T at bottom of LP, it has to be between output of LP and input of VST correct?
Sounds like Have we ruled out the fuel loop and the vapor loop?

[Nautical]

If you pressurize the suction side of the lift pump via the primer ball, if there was a vacuum /air leak would it not leak fuel or smell of fuel . Remove the output hose from the LP and see if you get flow from the pump when its on.

[Ljs3@mindapring.com]

Good point. I have no fuel leaks/smell anywhere when squeezing primer bulb or when pump is running.
i can put a clear hose on the output it I guess..
I still don't understand why a brand new LP pump with no noticeable fuel leaks in the system cannot keep the VST full.

If it was electrical, wouldn't the 5 amp fuse blow? Never has.

[boscoe99]

When the motor is not running the pressures are all the same. Output fuel pressure, input fuel pressure and air pressure. AKA ambient pressure.

When the lift pump is running the output fuel pressure is higher than ambient air pressure. The input fuel pressure is lower than ambient air pressure. What some will call a vacuum.

If there is a leak on the output (high pressure) side of the lift pump then fuel will come out since the fuel pressure is higher than the ambient air pressure.

If there is a leak on the input (low pressure) side of the lift pump then air will leak into the fuel system since the ambient air pressure is higher than the inlet fuel pressure. You can see this in the form of air bubbles in the fuel system if a clear hose is installed or in the form of a lower than normal vacuum reading if a vacuum gauge were to be installed at the inlet to the lift pump.

If air gets into the fuel system then the lift pump is pumping fuel and air to the VST. Quite possibly insufficient fuel to run the motor.

It is possible to have a vacuum leak that lets air into the fuel supply while fuel is not spraying out from the fuel system. Some leaks will do both. Some leaks will only do one.

Once again, if the motor can stay running at all RPM's when the primer ball is doing the job of supply fuel to the VST, but the motor won't stay running with the lift pump running and trying to do the same job, then I suspect that you have an air leak in the fuel system on the inlet side of the lift pump.

On a 13 year old fuel system I am betting on an air leak in the system.