2005 Yamaha F250 stalls at idle, VST vent check valve relief assembly

[thedillybar]

Motor has run fine on a few trips, but stalled twice when hot and >15 minutes of idling/low-speed (<1200 rpm where LP pump runs intermittently -- 10sec on, 20sec off).

After it stalled, drained the VST the next day and only got 4-5oz out. Yes I used the Schrader valve when draining. Makes me think it's a fuel issue on the low pressure (LP) side. Either VST vent to atmosphere is plugged, LP pump relief valve open, or LP failed.

I can barely blow through breather (check-valve) in VST vent to atmosphere. Suspect this is normal? Cannot blow through in reverse direction.
Relief valve leaks a little (can blow in both directions, but very restricted). Can't fully open it by blowing, but can easily move ball off-set with screwdriver. Spring & ball appear intact.

After re-installing, priming, and idling for a couple minutes...I drained the VST and got >12oz out. Seems the LP system is capable of filling VST at idle, just not when motor is hot??

Also discovered white residue on both fittings of the pressure relief valve. When scraped off, very fine powder. Only visible on fittings & inside of line where fitting was in contact.
I suspect this calls for replacing the F filter (inlet of VST), and cleaning screen inside VST tank. Can this be done spraying carb cleaner thru drain? Or Intake manifold and VST has to come off?

relief.jpg

[99yam40]

I would take apart and clean properly

[thedillybar]

Done, this is not a difficult job. Intake manifold is 10 bolts, easy. Gaskets stay attached to the manifold and would be difficult to damage. Then easy access to everything.

Hardest part was removing the bolts that hold the two halves of VST together. Impact driver + big hammer.

Didn't find much. Easy to blow through the F filter, slight discoloration but no visible debris on the filter inside the VST. Generally looked very good.

I do think I had a small air leak near the fuel/water separator (in the boat) that is now corrected.

1560602572608711.jpg

[99yam40]

now it is time to try it out to make sure all is OK again.

that is some nasty looking stuff on that fitting.
the tip of that metal connection looks like it was corroded away some, was it?

[panasonic]

At least you know the VST is now clean and not part of the problem.

How did you determine that had and air leak that you mentioned?

When this happened originally did you happen to pump the primer bulb? Or notice if it gotten real soft?

I hope that none of the white stuff is in your lines. Maybe someone had those lines apart and that was something that they put on to help it go together?

I would be tempted now to pull apart some other joints and see if that is anywhere else....or not.

[thedillybar]

The barb still had a tip on it. Didn't see the white crap anywhere else in the lines or VST, just where the barbs were. Had several joints apart.

I'm not sure if there was an air leak, or how bad. I put a fuel pressure gauge on the output side of the LP pump with a T and some clear plastic hose. Noticed bubbles in the hose, but also had some fuel leaking around the T I had added. I tightened the fuel/water separator another 1/4 or 1/2 turn by hand. One of the hose clamps on that separator was so corroded it fell apart when I touched it, so I added a hose clamp there as well.

Engine ran fine yesterday. But it usually does right up until it quits.