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HELP! Yammy 300 hpdi maxing out at 4800 rpms and dropping....

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  • HELP! Yammy 300 hpdi maxing out at 4800 rpms and dropping....

    UGh! I can't tell you how frustrated I've become, and unfortunately i already took up drinking so that can't be my solution so here it goes: I have a 2005 contender 21 with a 300 hpdi with 500 hours.. I've had the usual issues over the years with filters, screens, vst tank etc, but this one has been plaguing me for a while now and has everyone stumped..

    my boat maxes out at about 4800 rpm and maybe 40mph. recently, like today it is down to 4500. it burns fuel and oil however like it is at 5800 rpm and doing 50-55mph..... so heres my run down:

    there is not water in the hull.

    changed plugs and ALL filters. hooked it up to a computer and no codes. found a bad fuel pump relay but replacing it made no improvement....fuel pressure says its good but bulb is slack and requires a lot of pumping to get it hard. bulb is soft at WOT but not sucked in.

    it is not the prop. had a 21 pitch prop originally and the boat was too scary to go full throttle. i took it to the yamaha dealer here to figure out my missing rpms and they attempted to solve the problem with new injectors. Hmmmm- what a surprise it didn't change a damn thing! you would think a couple thousand $ later i might be running better. nope. My dealers solution? put a 17 pitch aluminum
    prop on there. WTF? sure it gave me rpms but the boat didn't go any faster in fact the prop would cavitate regularly. Have a 19pitch on now but really want to put my 21 back on but then i will be down to 4000-4200 rpm if I'm lucky.

    ...related to all this or not- I have a 110gal fuel tank and could only get 60 in last time i filled. gauge said i was full. after leaving the dock fuel level eventually dropped back to half where it should have read. bottomline, could all of my performance problems be related to a venting issue? we ve looked at everything- or at least it feels like it....

    The big issue: the boat runs fine. not rough, just won't max out. missing all that high end power. burning lots of fuel and oil like it is maxed out. Suggestions?. I am losing my mind and am on the brink of re powering my boat because i cannot take the frustration any more......

  • #2
    fuel system

    Ok,,I'll try and type after a 12 hr day fixing boats...first get some clear hose the same size as your yammy hose (3/8) is close enough..don't use screw clamps..use oetikers...run a section near your first pump and "T" in a Vacum gauge..if any vacuum over 3" you have a restriction... any,any air bubbles is a air leak..if all is good then its time for more tests under the hood/cowling..most guys don't own the tools to test rail pressure...but that would be the next step if you want to procede...

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    • #3
      Originally posted by bajakeith View Post
      I'll try and type after a 12 hr day fixing boats....
      Slacker....
      Scott
      1997 Angler 204 CC, 2006 F150 TXR

      Comment


      • #4
        Originally posted by bajakeith View Post
        Ok,,I'll try and type after a 12 hr day fixing boats...first get some clear hose the same size as your yammy hose (3/8) is close enough..don't use screw clamps..use oetikers...run a section near your first pump and "T" in a Vacum gauge..if any vacuum over 3" you have a restriction... any,any air bubbles is a air leak..if all is good then its time for more tests under the hood/cowling..most guys don't own the tools to test rail pressure...but that would be the next step if you want to procede...
        Think Keith is bang on,
        Look at all the fuel connections, I am no expert but I read for hours and your problem sound like many, fuel line air leak. Keith's idea is brilliant, clear hose and a vaccum gauge...
        Cheers
        Scott

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        • #5
          thanks guys- i appreciate the advice. ill get on that and see whats happening. i will let you know what i find .....

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          • #6
            I would make sure I did not have a ruptured lift pump.
            then I would make a run with a fuel pressure guage attached to the VST and YDS to monitor HP rail pressure.
            if HP drops and the VST maintains 50 PSI its between the VST and the injectors.

            if both drop its between the liquid level in the tank and the VST.

            would also be a good time to hook up a vacuum gauge between the engine filter cup and the lower lift pump intake, less than 1" Hg and suspect an air leak.
            more than 6"Hg suspect a restriction.

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            • #7
              Thanks Rodbolt - I was hoping you would have something to say....just trying to find time to get back out there and run through these suggestions.....ill update as soon as i can.....

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              • #8
                trust me.
                this sheet is simple.
                don't focus on what may be.
                eliminate what is or is not wrong.
                life is that simple.
                it also helps if you have a few years and schooling on how the system works and troubleshooting.
                the CIWS block 1 was a nugget .
                hook up YDS,or a voltmeter to the HP.
                hook up a gauge at the medium pressure.
                hook up a vacuum guage at the lift pump inlet.

                all we be clear soon as to which part of the system fails.

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