F150 TXRD losing rpm till you pump bulb

[gunz@pc]

I' ve had an issue for about a week but the readers digest version is idles fine hit about 5K rpm and it starts to shutter then loses power and rpm, pump the bulb it will pick back up so measured fuel rail pressure at 39 psi, pressure and neg pressure test on both mechanical pumps passed cleaned VST tank, Screen and injectors. Isolated it from the boat fuel tank and lines by bypassing the front filter and hooking up a 6 gal tank straight to mechanical pump and it still does it. Somebody mentioned a check valve, and was suppose to get back with me (4 days ago), that holds the pressure somewhere but couldn't find one on the parts breakdown has anyone heard of this mystery valve or have any other suggestion.

thanks for the help in advance
Gunz

[Nautical]

Whats the fuel pressure at the point it bogs down ?

[gunz@pc]

just a little lower then 38 hard to tell exactly on the water my needle jumps around a bit.

[rodbolt17]

if you can pump the primer and keep it running then go fix the fuel pumps.

[gunz@pc]

Capt Solo I bypassed the Racor by hooking up a 6 gal tank to the fuel pumps and it still did it. Was wondering where this "check valve" is at you referred to...

Rodbolt the fuel pumps passes both the positive pressure and neg pressure (vacuum) test according to the service manual do you another way to test them???

Thanks guys,

[rodbolt17]

dude.
if your attaching a separate tank directly to the lift pumps intake and it runs if you pump the primer bulb, then it has no choice but to be fuel pumps.
if your description is correct the primer bulb is being used as a fuel pump.
well actually that's what the primer bulb is.
double check the oil for fuel contamination.

[Nautical]

You say you have 38 psi at the time it bogs, but needle bounces.. Does it bounce all the time when running good? When you tested the low psi lift fuel pumps did you test them under back pressure? You can bypass the lift fuel pumps and use a 12 volt electric pump to test.. Be careful hooking things up and running with a jury rig..Gas is explosive!risk of fire .. Rod is right! but if you are having doubt to confirm try that temporary set up .

[gunz@pc]

Took both fuel pumps off my motor (that runs like a rape ape) put them on his motor does the same thing. This one has me scratching my &^%. Check oil after each time no sign of H2O in it. Tested gas it has about 1% of the bad stuff. 6 gal tank is fresh non ethanol also. Machined cleaned injectors too.

[IRONDRAGON]

I had a similar problem, water in main tank. Tried to run on a portable 6 gal. tank. The 1/4" fuel line was not large enough to feed the V-6. Changed the line to a 5/8" and runs fine.

[gunz@pc]

Thanks Irondragon I wish it was that but it had 3/8 fuel line, had to call the folks at Mother Yamaha on this one, I think we stumped them too because they still haven't gotten back to me and that was about 9am.

V/r
Gunz

[boscoe99]

Are you saying that by pumping the primer ball fuel pressure maintains at 39 psi in the injector rail but when not using the ball that fuel pressure is lost?

[gunz@pc]

No Sir saying that is the initial rail pressure and it runs great till ya get to around 4900 sometimes and then sometimes it will start at 3800 rpm then it falls off and if you go pump the bulb it will go back up to whatever rpm you want as long as you are pumping the bulb. Thanks

[boscoe99]

Need to put a pressure gauge on the rail.

Does the pressure start to fall and then the motor loses RPM's at some point?

Does the pressure come back on the gauge if the primer ball is pumped?

[gunz@pc]

Yes Sir when you hit that magic RPM you can see it start to fall to about 20 + or - a little then when you hit the bulb it will start to climb again.. and run "normal" again,

Gunz

[Nautical]

If you replaced the low psi lift pumps, and you ran it on a portable tank,cleaned vst , replaced all fuel lines, replaced all the filters, cleaned the fuel injectors..What about the pressure regulator??It seems like its on the low pressure side because you manually fill the vst ( your high pressure pump is starving) Clean out all the VST and small vents and fuel lines going to the rail including the needle valve and seat. If that doesn't solve the problem...deep six it ...lol
I still might try running an electric fuel pump to the vst and eliminate the low pressure pumps just for shits and giggles..