Fuel Starvation at low rpm - 2014 Yamaha 250, Model F250XA

[Lower Keys Joe]

I am hours old to this forum and apologize in advance for my mechanical ignorance. My mechanical work is being done by a local Yamaha mechanic who has been very engaged and supportive. That said, I have a reoccurring and intermittent problem that appears related to fuel starvation. My motor runs without issue at higher (3500 to 4500) rpms. However with extended *****ing speed (800 to 1000 rpm) after about 1 hour the motor will begin to stall. Sometimes I can get to the ball and pump up the pressure and sometimes no matter how hard I pump it will stall out. At times it has emptied the fuel water separator of gas before stalling. If I let it sit for a few minutes I can get it started. It can be a bugger if the fuel water separator is empty but I have learned to shut down the motor if it starts to struggle for fuel so this doesn't happen.

I took it to my mechanic and he found a bad pulley that had caused the serpentine belt to get chewed a bit and this was replaced. He changed the bulb and fuel line and removed the anti-siphon valve. I am able to blow air through the gas tank vent and checked the vent screens. He tells me the VST is fine.

I read that another member with similar problems reported: "...without the fuel cooler able to function, the bypass fuel is constantly being recycled thru the heat producing fuel pump until the boiling point at the fuel pump pickup (vacuum) is being reached, thus creating the vapor lock situation inside the pump itself." He reported "I took the elbow off the hose ( #17) and rerouted it thru the hole in the lower cowling to give me a straight shot thru the hose, and ran a piece of mechanic's wire thru the tube. I felt it when I hit the hose nipple, then a twist or two and it went into the nipple. I could feel some restriction there but whatever it was gave way."

This apparently solved his issue. I am due to take the motor back to the mechanic who is unsure what to do next (I did share the above solution with him via email but we haven't discussed.)

Any ideas on what should be explored?

Many Thanks in Advance.

[fairdeal]

after about 1 hour the motor will begin to stall. Sometimes I can get to the ball and pump up the pressure and sometimes no matter how hard I pump it will stall out. At times it has emptied the fuel water separator of gas before stalling. .... I have learned to shut down the motor if it starts to struggle for fuel so this doesn't happen.

Joe, the engine runs off its own little fuel tank, that VST.
And it has a little electric pump that is supposed to suck fuel from the boat tank, to keep that VST full.

You should also be able to keep the VST full, with your 'hand pump' - the "primer bulb"

Now if the VST is kept full, and the engine doesn't run right - that's a problem with the engine.

But if you can't pump fuel from the boat tank with the primer bulb, to keep the VST full -

then the problem is not with the engine.

[Lower Keys Joe]

Thanks for your insight Fairdeal. I will check it out. The mechanic has told me there is no problem with the VST but I will double check for myself and report back.

[Lower Keys Joe]

Fairdeal now I get what your saying whether or not it is the VST, monitoring the fuel in the VST will locate the source of the problem (motor vs. fuel source). Brilliant!

[TownsendsFJR1300]

You can confirm it's the BOATS FUEL system if you run off a separate, portable fuel tank rigged direct to the engine too..

[Lower Keys Joe]

Thanks Fairdeal and Townsends for you feedback. Yesterday I ran with the engine cowling off to monitor the VST. After running at 900 rpm for 90 minutes the engine started to sputter and stall. There was fuel in the VST. I was able to restart with much sputtering and missing until the motor suddenly picked up rpm, smoothed out and ran without issue at 3500. It seems like the issue is the motor and not fuel starvation. Any ideas what to consider next. Tomorrow I will follow up with my Yamaha mechanic. Thanks in advance for any help you offer.

[99yam40]

how did you prove that there was plenty of fuel in the VST when the problem occurred?
Monitor the fuel rail pressure maybe to see that it never dropped?

[Lower Keys Joe]

Thanks 99yam40. I have zero mechanical skills and will take the motor back to the mechanic. I was referring to information provided by Fairdeal's response in this posting chain. He recommended monitoring the clear fuel canister that I believe may be providing fuel to the VST? Again this is my first outboard and I am trying to get an algorithm to discuss with the mechanic. Per my original post, the mechanic found no problems other then a bad pulley that was chewing up the serpentine belt. These were replaced. He also changed out the bulb and fuel lines. He also removed the anti-siphon valve. The motor runs fine at 3500 to 4500 but at low rpm (800 to 1000) will sputter and stall after about 90 minutes. I had thought the problem my have been an air leak between the fuel water separator and tank perhaps the fuel vent but I was able to clear that without effort by blowing through it and the filter screens are clean. I appreciate your feed back and recommendations.

[99yam40]

I went back and read the whole thread .
sorry for not doing that well enough before chiming in.

your statement about the water separator going empty has me scratching my head.
but maybe if the fuel cooler is not cooling as it should the vapor pressure is overcoming the fuel pressure to the VST and is sending vapor back the line.

just thinking out loud on this.

making very sure the cooler is getting the best flow of water thru it would be the best thing for now.

Just to remark on this some,
Removing the anti-siphon valve could drain gas into the bilge if there was a leak in the fuel line lower than the fuel level in tank, which could be very bad.
they put them in there for a reason

was it an idler pulley that went bad?
hard to think a motor not that old would have that happen.

[Lower Keys Joe]

99yam. Thanks for you additional thoughts. I agree about the anti-siphon valve and once this is figured out I will have it re-installed. It was done without my input. It was the tension pulley that failed. The bill was submitted to Yamaha warranty. Waiting on the results of that claim. I went fishing yesterday (no *****ing!). Being that I am stuck in karmic hell, my grapple anchor would not release and while pulling at it with the motor had similar fuel related stall out. I had a bugger of a time getting it started and called for a buddy to come with my dive gear and to give me a tow. By the time he arrived (40 minutes) I was able to re-start the motor. I think this may be related to your notion that the fuel cooler is not functioning and when the motor cools down it will re-start. (By the way I was able to make the 50 foot dive to retrieve my hook). I am off the the mechanic for a chat and will advise of next steps considered. I very much appreciate you comments.

[Lower Keys Joe]

Met with my Yamaha mechanic who wants to take a look at the "charcoal filter" in the fuel line (?) although he has never had to repair one. In another forum I found: "These Yamaha's (F150) frequently have an inline fuel filter between the low pressure fuel pump and the vapor canister. This is a very fine filter and can plug up easily. Back-flowing through it will usually remove most debris. Yamaha reports: "Then there is an F shaped fuel filter located between the fuel pump assembly and the vapor separator tank. Once again, your Yamaha LF250 owner manual will tell you when this fuel filter should be inspected or replaced.
Finally, there is a fuel filter located in the vapor separator tank on your LF250. Disassembling a VST should be left to your Yamaha dealer, as it requires special tools and expertise."

I am not sure what filter the mechanic is considering but I hope he finds the root cause of this problem!

[Masonmobilemarine]

Met with my Yamaha mechanic who wants to take a look at the "charcoal filter" in the fuel line (?) although he has never had to repair one. In another forum I found: "These Yamaha's (F150) frequently have an inline fuel filter between the low pressure fuel pump and the vapor canister. This is a very fine filter and can plug up easily. Back-flowing through it will usually remove most debris. Yamaha reports: "Then there is an F shaped fuel filter located between the fuel pump assembly and the vapor separator tank. Once again, your Yamaha LF250 owner manual will tell you when this fuel filter should be inspected or replaced.
Finally, there is a fuel filter located in the vapor separator tank on your LF250. Disassembling a VST should be left to your Yamaha dealer, as it requires special tools and expertise."

I am not sure what filter the mechanic is considering but I hope he finds the root cause of this problem!

Special tools for a VST?

[pstephens46]

Special tools for a VST?

Who knows? Jap screwdrivers or an impact driver? Those sorry little screws can be a btch.

[Lower Keys Joe]

Sorry about the long delay in reporting the final fix. The motor finally died right after the last trip to the mechanic. I was able to nurse it home by squeezing the primer bulb. The problem was the fuel pump. This was my (and many others) original opinion but the mechanic told be that the problem would not be intermittent if it was the fuel pump and would not be affected by engine temperature. After his calling Yamaha he was told, in fact, there are cases of intermittent fuel pump failure before it eventually catastrophically fails. The mechanic made the repair for free including costs of the pump. He was very apologetic. I have been fishing off shore several times a week with no problems. Thanks to everyone who offered ideas and support.