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2002 F115 fuel pressure dropping to 20psi at 4300 RPM

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  • 2002 F115 fuel pressure dropping to 20psi at 4300 RPM

    Hey all,

    I recently bought a boat with a clean 2002 F115. The boat fired right up and would run at 6000 RPMs with no issues. During a fishing trip I had the boat idling and/or barely in gear to fight the current I noticed a faint gas smell and then a few minutes later the boat was starving for fuel and if left unattended it would eventually starve out. What I found was I could not get the motor to hold prime. After a few squeezes of the ball all was well. My initial thought was I must be sucking in air. So i went to the tank and check to make sure the vent was not clogged. Then I pulled each hose off and reattached with two hose clamps. I then moved on to the 10 micron filter and prime ball and replaced both of them. The problem proceed. I then got my 3 gallon dingy tank and bypassed everything and problem proceed. After this I discovered that when I trimmed up I would see oil or gas in the water. Further inspection found fuel leaking out of intake silencer. I assumed the motor was flooding. Furthermore, I noticed that it was "making oil" . I thought this played into the flooding narrative quite well but I replaced the thermostat too for good measure.

    After reading this forum I knew that VST tanks are infamous for casing all sorts of problems and I figured it was a good maintenance step to do on a new to me motor. My suspicion was the needle in the VST was bad and I was getting too much fuel in the VST. As a result, I was flooding the motor at low speeds. I then changed the inline filter on the motor, cleaned the VST tank, new vst filter and gasket, and replaced a noticeably warn needle and set. I then drop the boat in the water and it idles great. That issue was gone. However, I after I took the boat out for a test run I found that I no longer had any top end. At around 4300 RPMs the motor would bog down and starve for fuel. So I fixed my first issue and now I have a top end issue. So I pulled the VST back off again inspected and reattached thinking I must've hooked something up wrong, left something unplugged ect. Of course no change.

    My next idea was that the low pressure pump was bad and causing the "making oil" issue and the holding prime issue. I replaced it with a $219 genuine OEM fuel pump again confident that this would solve my issue. The pump had no effect on the rpm loss however I don't seem to be "making oil" now. (The caveat to that is I haven't put much time on the boat with these problems occurring. Since the boat runs great at low RPMs I am only going outside the no wake zone to apply load and returning to the dock.) Sooo my next step was to clean injectors. I used a battery and compressed air to move a combo of carb cleaner and seafoam through them and did not see any large particles coming out. Everything looked clean but still no power up top.I am now getting tired of throwing parts at this thing,

    ** I then bought a harbor freight pressure tester (probably not too precise but good enough)

    Key on first time 30ish
    key on second time up to 40-44
    Low RPMS runs from 36-44
    4200-4300 the pressure drops to 20 PSI

    MAIN QUESTION: So does this drop in pressure signal a bad high pressure pump? bad fuel pressure regulator, VST starving for fuel, motor going into limp mode, bad temp sensor, vacuum leak?

    I can pull the vacuum line off the pressure regulator and it run rich at cold idle but once I tried it once the motor was warm after a test run the motor would bog down and eventually die.



    I really need your help on this one, I feel like I am just slapping parts on as I go and Im stumped....


    Long time user first time poster. I thank you for your help

  • #2
    What does the pressure regulator do? Why it regulates fuel pressure of course. But it is tied to the intake manifold so that as manifold pressure rises and falls, with the throttle being opened and closed, the fuel pressure rises and falls accordingly.

    The pressure regulator is plumbed to the intake manifold. When the manifold pressure is high (throttle opened with the motor running or with the motor not running) the fuel pressure will be increased. When the manifold pressure is low (throttle closed) the fuel pressure will be lowered.

    More throttle, more power, more fuel. Less throttle, less power, less fuel. Makes sense.

    At key on (before motor start) the fuel pressure should be more or less 44 psi. Once the motor is started and running the fuel pressure should drop to more or less 38 psi. It should remain steady as the motor runs at an idle RPM. As the throttle is opened up the fuel pressure will rise.

    Sounds to me like your pressure regulator is kaput.

    Comment


    • #3
      Thank you Boscoe99, you specifically is who I wanted to chime in. I have read several of your posts and really respect and appreciate your opinion. You contribute a lot to this forum! Thank you for your input.

      I apologize for the lengthy post. I just didn't want to be one of those guys that fails to provided enough information and has to go back and forth with posts. Posting here is my LAST resort.

      I just bought new regulator and will report back to help the community.

      *** Side note : To my knowledge its only the five digit part number that matters when it comes to buying parts. The first three digits are simply to specify the model. Is this correct?

      Linked my source

      https://www.yamaha-enduros.com/index...esign%20change).

      If this is true we should make a sticky ( or it may already be). This could save the community a lot of money. My part number 68V-13906-00-00 on boats.net at. $264.94

      Part I found:
      Boats.net 5SL-13906-00-00 at $105.32



      Am I missing something ?

      Comment


      • #4
        Like some lives, most all digits in a part number matter. Only the last two might can be ignored. Even then it might be a crap shoot.

        An F115TXRA used part number 68V-13906-00-00 regulator. Changed to part number 68V-13906-01-00.

        Comment


        • #5
          When I look up you motors pressure regulator and click on it the web site shows the motor that it is for
          I do not see that when I punch in that other part number you posted, so I have no idea what motors that is for

          Comment


          • #6
            Originally posted by 99yam40 View Post
            When I look up you motors pressure regulator and click on it the web site shows the motor that it is for
            I do not see that when I punch in that other part number you posted, so I have no idea what motors that is for
            The 68V- prefix pressure regulator is the one for the OP's F115.

            The 5Sl- prefix regulator is probably for some other Yamaha product. A motorcycle for instance.

            Comment


            • #7
              Originally posted by Hedgedfun View Post

              I just bought new regulator and will report back to help the community.

              *** Side note : To my knowledge its only the five digit part number that matters when it comes to buying parts. Nope.The first three digits are simply to specify the model. Is this correct? The first three digits indicate the model but the part from model to model can be vastly different.

              Am I missing something ? Yes.
              Take a look at the pressure regulators shown here https://www.boats.net/search?q=5SL-1...0&ui=typeahead

              Note how different they can be physically. A 68V regulator (F115) will not replace a 6AW (F350) regulator.

              Comment


              • #8
                zsl.png

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                • #9
                  zsl.png

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                  • #10
                    those last 2 posts are too blurry to read anything, but looks to me like it shows what that regulator would fit on in that box maybe.
                    never mind I found it on the web and it shows that part is for motorcycles
                    Last edited by 99yam40; 6 days ago.

                    Comment


                    • #11
                      Something is wrong with my screen resolution and capture program. Operator induced I am afraid.

                      Comment


                      • #12
                        Nice to know the method to the part number madness.

                        Always good to do a bit of detective work and save a bunch of money aa well. Looks like the same pressure regulator off a R6 motorcycle. I have ridden a couple of those things...rocket ships!!

                        Comment


                        • #13
                          Originally posted by panasonic View Post
                          Nice to know the method to the part number madness.

                          Always good to do a bit of detective work and save a bunch of money aa well. Looks like the same pressure regulator off a R6 motorcycle. I have ridden a couple of those things...rocket ships!!
                          I am not so sure that one for a bike would work on his f115, if that is what you are meaning

                          Comment


                          • #14
                            Hey all, quick update.

                            I replaced the fuel pressure regulator, dropped in new plugs, and bypassed the inline filter on the back of the motor to make sure I wasn't sucking in air, all resulting in no change.

                            I picked up an aftermarket high pressure fuel pump. I guess I have to crack it open a third time. Any other suggestions? I may play with the float a little. It seems the VST is starving for fuel or the pump is failing at full throttle.

                            Comment

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