Brand new F25 LWTC, 'bogs down"

[bjacobs@sourceones.com]

I have a brand new (now 140 hours) F25 (with EFI). I installed a new Moeller 10 micron fuel/water separator when I installed the engine. It is perfectly clean. If I set the throttle at say 3400 RPM, it can run fine for an hour, then drop to ~2800 RPM, then come back up to 3400 after a few minutes. Then it will drop down again after awhile. I've checked that the fuel tank is well vented (so no vacuum being created): no problem there. I thought maybe the priming bulb was restricting the flow, so I inverted it so fuel pump doesn't have to pull against the one way valves in the priming bulb. That didn't fix it. There are no kinks in the fuel line. The fuel connector at the engine looks good. The engine stalls if I throttle it down too quickly. I have only used non-ethanol fuel. Any ideas are appreciated! Brian

[rodbolt17]

my bet would be on cleaning the VST filter.

[bjacobs@sourceones.com]

Hi Rod! Thank you for responding. With more testing, it consistently stalls at ~2250 RPMs. Will run at higher RPM ~3500, but drops down from time to time. Will idle poorly at 1000 RPM. Is this also indicative of a VST problem? For an EFI engine, there is only one fuel inlet, the injector, correct? No idle jet like on a carburetor? So the injectors must be working... do I have that right? Is there any diagnostic I can run that will clarify this more? This is our main engine on our sailboat and we'll travelling, so we don't have a lot of ways to test things while underway. We are grateful for any info, hints, suggestions! Brian

[99yam40]

monitoring fuel pressures should tell you if they get out of spec when the RPM drops.
no need to chase fuel problems if they are not the issue

But then it may be easier to just take the vst off and clean it all up if you do not have a way to monitor the rail pressure

[walleye1]

If you have clean fuel and it's under warranty take it back to the dealer.

[bjacobs@sourceones.com]

99Yam40: I have no way to monitor fuel pressure out here. So I'll remove and clean the VST and keep my fingers crossed. Thank you for the fuel pressure note! If I can't fix it, I'll know what a tech should be looking at. Do you monitor fuel pressure by plugging into a diagnostic computer, or attach a gauge somehow?
Walleye1: Boy, I wish I could. The dealer is now 950 miles away. I understand Yamaha doesn't cover anything fuel related (if that's what it is) anyway.

[99yam40]

I am sure a gauge is needed to be hooked up,

brand new motor does not say much, what is the Model # and serial #
how long did it take to put those hours on the motor?
when was it bought
did it sit for long periods of time between running

[bjacobs@sourceones.com]

99Yam40: Gauge: okay, thanks. Model F25LWTC 6FMKL S/N 1013746. Purchased new June 8, 2018. Splashed the boat July 1. It has been idle for about a week at the most. I've only used non-ethanol gas. 10 micron fuel/water filter/separator installed before day one. If it helps in diagnosis, I just now took the paper fuel filter off and examined it (because it is accessible!) and it ran perfectly for one minute, then started stalling again. I'm in the Great Dismal Swamp, ~15 miles south of Norfolk, VA, heading N to Deltaville on the Chesapeake. I doubt it is relevant, but one of the condensors failed at about 100 hours (near Surf City, NC). A simple replacement and we were off again.

[99yam40]

might be a small air leak that under higher RPMs cause the VST to run out of fuel.
lower RPMs do not use as much fuel so it can keep the VST filled enough.
The new filter seal and all line connections before the pump could be a possible small air leak spot.

140 hours in less than 2 months means you are not using the sails much doing that loop

friend of mine, his wife, and her cats are in Charleston SC right now heading up the coast slowly taking in the sights

Just reread you posts and the air leak would not make the idle get bad, so I do not know what could affect it at high and low RPM unless there is more than one problem.
bad injector maybe( leaking or plugging) along with an air leak?

have you pulled the plugs to see if that shows anything different on they way they are burning fuel?

I am sure you could find a Yamaha deal along the way

Also did it run well for some time before acting up?

[bjacobs@sourceones.com]

The problem is overwhelming at medium and low RPMs, and less so at higher RPMs. Correct: we've been on the inside route taking in sights and motoring most all of the way, just like your friends in Charleston :-). I did get a spare set of injectors, just in case. Unfortunately I've never heard of a VST before. There is no brass screw visible to drain the VST housing. Both spark plugs are equally, and very sooty. I replaced them. The engine did run well in the beginning. Please comment on my plan: 1) Replace Moeller filter. 2) Replace injectors (wait, no, both plugs are equally sooty, so it would not be one bad injector?). 3) Replace VST filter. Does that sound like the correct priorities and sequence? It is looking like we might have to use the dingy 8 HP 2 stroke outboard for the next 25 miles.... Thank you for any help! Brian

[TownsendsFJR1300]

Part #20 is the VST filter: https://www.boats.net/catalog/yamaha...injection-pump

The high pressure pump is what it attaches to (and then to the injector rail and injectors)

There is no VST drain shown in the parts diagram (kinda unusual).

[bjacobs@sourceones.com]

TownsendsFJR1300: Do the symptoms indicate that the high pressure pump is bad? It wouldn't run at all if it was, would it?

[99yam40]

Black and sooty plugs make me think of over rich condition.
hard to believe boh injectors would go bad at once
Maybe too high of a rail pressure dumping too much fuel.
where is Rodbolt, maybe he could chime in again.

Just thought of something,
have you checked the oil level to make sure it is not making oil.
breaking in a 4 stroke properly is important to get the rings o seal properly.

diluting the oil with gas to the point that the level gets high enough and dumps the thinned oil into the intake might make the plugs look black and sooty

try and find a Yamaha service at a harbor boat place on the water close to you.
Rodbolt works at one but he is back down in NC

[bjacobs@sourceones.com]

99yam40: "check oil level to make sure it is not making oil." Can you clarify: what does "making oil" mean?
The oil level is correct: 2/3 of the way up the hatch marked area on the dipstick when fully inserted.
The oil is about like I'd expect after 50 hours, semi dirty, but not heavy black.
I was alert and careful in the break in period, cycling through low, medium and then high RPMs for short periods.

[TownsendsFJR1300]

Do the symptoms indicate that the high pressure pump is bad? It wouldn't run at all if it was, would it?

That pump is "new", I wouldn't expect any problems from it. Now a clogged filter going to it, that would hamper it's capacity.

And HP pumps can cause erratic running to no running. Again, yours is new, not likely that's bad this soon.

The plugs burning differently IS odd, IF the VST filter is the culprit. I would think they'd be the same.

Looking at the parts diagram; https://www.boats.net/catalog/yamaha...tc-0117/fuel-2 (I don't have your shop manual), doesn't appear to be too bad. If your not mechanically inclined, might want to wait until a tech can get to it. PIA but it is under warranty, you don't want to mess that up..