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2003 Yamaha F115 cutting in and out at higher RPM's

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  • 2003 Yamaha F115 cutting in and out at higher RPM's

    I have a 2003 Yamaha 115 4-stroke on a 24' pontoon boat. This past weekend we took the kids tubing. Motor ran great, we pulled them around for a few hours with no issues at all. Then, as we were making it back to the launch, we accelerated, and at around 4,000 RPM's, the engine cut way back, to about 1500 RPM's. I immediately pulled the throttle back and let it sit for a minute. Tried to accelerate again, and the same thing happened...accelerated smoothly, but when it got up to the higher RPM range, it dumped back to the mid 1000's. This time, I didn't pull the stick back, and it shot straight back up to the high RPM's after a half second or so, and then back down, then back up, back down, like a bucking bronco. After a couple cycles of that, we decided to limp back to the launch. It runs smoothly, no sputtering, it's like it's on or off, on or off, not like it is running on fumes. It should also be noted it didn't appear it was doing it at one specific RPM, and dumping back only to one specific RPM...it was just dumping from a higher RPM (in the plus or minus 4,000 range) to the mid 1,000's range.

    Anyway, because we had been out all day, I was hopeful I was just running low on fuel, or maybe had a cracked fuel pick-up, or something along those lines. So I pulled the pickup tube, and it was solid. It was somewhat low on fuel, so I filled the tank and took it back to the river to try it again in hopes it was just sucking air when we were accelerating. Long story short, it is still doing the same thing. Posted a link to a video of it. https://youtu.be/HmkkZSSFo0o

    LIke I said, it isn't sputtering or acting like it's running out of gas, it's like it is on or off, on or off.

    Any ideas?

  • #2
    need to monitor rail pressure to see if it is dropping out of spec.
    if the VST is running low on fuel and causing HP pump to suck air instead of fuel ,I think it would act like that

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    • #3
      I went ahead and pulled out the VST pump to inspect it. The filter screen was relatively clogged, and the rubber damper at the top of the pump was dry and cracked...I'm hoping that servicing it with a new damper and filter will take care of it. Question...does anyone know if the metal intake manifold gasket is re-usable, or do I need to be getting a new one?

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      • #4
        Well, Yamaha Service Manual says that gasket is "Not reusable" -
        but it says that for just about every gasket...

        Personally, I might risk it re-using it....

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        • #5
          I did replace the VST filter. It was fairly clogged, so I hoped that would fix the issue. Took the boat out again, and basically the same thing boat revs smoothly up and then hammers back and forth down hard to about 1500 RPM, back up to roughly 4k RPM, back and forth, back and forth, back and forth. It isn't sputtering as if it were slowly running out of gas, it if full go or full off. Wondering if it was some sort of bad sensor, I bought a YDS connector and software. There are no trouble codes stored. I'm not terribly good with the software, as I have never used it before, but engine only has about 380 hours on it. Given the description of what is happening, is it more likely the low pressure full pump not filling the VST tank fast enough, or is it the fuel injection pump failing, or is it something completely different? Any ideas, particularly if something like this has happened to your Yamaha before, would be greatly appreciated.

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          • #6
            as I said before monitor the rail pressure to see if that is dropping and picking back up when problem shows up.
            that should tell you if the VST is running out of fuel at higher RPMs

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            • #7
              99yam40...would that tell me if the VST not being fed enough fuel from the low pressure fuel pump, or would that tell me that the high pressure pump is just failing to work properly? Is there a way to isolate the problem to the high pressure fuel pump, or to the low pressure fuel pump not filling the VST fast enough?

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              • #8

                maybe turn off the key as soon as the motor starts to cut out.
                that will turn off the lift pump so it cannot put anymore fuel into the VST.
                , and then drain the VST to see how much fuel was in there

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                • #9
                  Hmmm. That sounds like a good idea. Here's another potential clue. I ordered a Yamaha Diagnostics cable and software, and I finally got around to hooking it up this afternoon. While monitoring the system, I pushed the throttle forward until it started doing what it does, and immediately as it started bucking, I got a message on the YDS that the ECM and PC had lost communication. Is it possible that the issue isn't a mechanical issue, and more of an electrical issue? If electrical, how would I go about isolating it? FYI, there are no error codes stored on the ECM.

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                  • #10
                    a bad connection could cause all sorts of problems.
                    only thing I can think of is monitoring voltage at different places to see if voltage drops when this happens.
                    the battery, fuel pumps, injectors, main relay.
                    not sure what all to check

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