No pressure at schrader valve

[jackmanjls]

Engine: F90TLR, 2006, 1100hrs.

I put a test gauge on the schrader valve, turned the key on and didn't get a reading. Gauge needle didn't budge.

When I turn the key on I could hear the VST HP motor turn on/off. I immediately attempted to release pressure on the schrader valve and didn't hear any pressure release (pssst).

I started the F90 and still didn't see any pressure on the gauge.

I then removed the schrader valve, connected gauge and hose and manually tested the pressure...it worked as expected. There's no issue with the valve and gauge.

The F90 starts and idles as expected but I don't get any pressure at the valve.

Any ideas on what might be the issue?

[rejesterd]

I would guess the needle valve in the VST is stuck closed. You can try replacing it, but you already made that workaround to bypass it. As I understand, that's all that needle valve is for: diagnostic testing. As long as it stays stuck-closed, then you don't really have an issue. Do you have any bad behavior that encouraged you to test the fuel pressure?

[99yam40]

sounds to me that the test gauge set up does not press the pin on schrader valve

[fairdeal]

Some - far from all - Yamaha 4-stroke models have a Schrader valve on the fuel rail,
which facilitates measuring fuel rail pressure.

Some Yamaha 4-stroke models have a Schrader valve on the top of the VST -
its only purpose is to allow air to enter the VST when manually draining it -
- there would not be any measurable pressure there.

I'm fairly sure the 2006 F90 does NOT have a valve on the fuel rail,
but rather requires a special Yamaha tool inserted between the supply hose and rail.

[TownsendsFJR1300]

sounds to me that the test gauge set up does not press the pin on schrader valve

+100 ^^^^^^^

[99yam40]

I was thinking as fairdeal also on just letting air in when draining, but the op said
"I then removed the schrader valve, connected gauge and hose and manually tested the pressure...it worked as expected."

that made me think he saw pressure, but am not sure

[jackmanjls]

Fairdeal was right. The valve is on top of the VST. That explains why there's no pressure. I reread the manual and it does say that's a pressure release. The proper way to check pressure is to put gauge at the output of VST...what FDeal was referring to.

99yam40, as far as the schrader valve I did verify that the needle was pressed so that pressure could pass. When I removed the valve I connected valve and hose together and manually blew through it, the air passed freely.

The reason I'm doing this is b/c I want to monitor rail pressure when the motor runs at >30mph. It will spits/sputter after a few minutes. I pull it down to idle and throttle back up and it repeats after a minute or so. Below 30 runs fine. This points to the filter in the VST...gas starved. From my research, a common problem with the VST. Last time I cleaned was 7yrs ago so it's time.

[fairdeal]

when the motor runs at >30mph. It will spits/sputter after a few minutes. I pull it down to idle and throttle back up and it repeats after a minute or so. Below 30 runs fine. This points to the filter in the VST...gas starved.

Points to gas starved, agreed;
but perhaps not so much the VST screen.

It's a nice big one in that motor - lots of surface area -

also, a restriction of the pump intake screen would have an immediate effect -
the pump itself is starved -
as soon as the fuel starts flowing out of the rail,
the pump can't keep up the pressure.

whereas a failure of the low pressure system to keep the VST full at high rpms
would have a delayed effect -

starting with a full VST, you would get some time at high fuel use,
before the VST runs low.

[panasonic]

Mr Fairdeal nailed it right on the head I think...go looking for your fuel supply problem to the VST.

Have you had someone pump the primer bulb like a madman when this is happening?

Still not a bad idea to clean the VST anyway.

[jackmanjls]

Can the VST pump be bench tested for pressure? If so, how do I do that?

If I'm going to pull the VST I'd like to have a reasonable idea that the pump is functioning properly before I put back together.

[fairdeal]

Can the VST pump be bench tested for pressure?

not as easily, IMO, as you can now, before you take things apart -

you just have to McGyver an adapter for your gauge
to fit that hose connector for the fuel rail.
Something perhaps like 1/4" - 3/8" copper plumbing tubing, IIRC

remove the hose form the top of the fuel rail -
you'll see the diameter of the barb on the rail that it mates to

so you need something to slide into the hose connector
which seals against its internal o-ring.
Doesn't need a 'barb' -
you just keep it from blowing off when you test.

[boscoe99]

Can the VST pump be bench tested for pressure? If so, how do I do that?

If I'm going to pull the VST I'd like to have a reasonable idea that the pump is functioning properly before I put back together.

It is all explained in your service manual.

[fairdeal]

It is all explained in your service manual.

a mere $155.80 from our gracious host....

https://www.boats.net/product/yamaha/90890-06942-00

[boscoe99]

Such a good deal. If that was an aviation part it would cost three times as much. At least.

[jackmanjls]

Thanks...B99 and FDeal.

I got the idea. My plans are to start working on tomorrow. If I come up with anything earth shattering I'll post pcs+comments.

I'm usually fairly proficient (and lucky) at making A work with B.