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1999 Yamaha 150 SS OX66 EFI Bogs down

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  • 1999 Yamaha 150 SS OX66 EFI Bogs down

    i have twin 150 Saltwater Series OX66 EFI Yami's and i am having a problem with one of them bogging down at rpm's over 2500. I have replace the fuel/water seperators, both fuel pumps in this motor, the plugs and even removed and cleaned the FST. I am still having the same problem. I noticed more than normal blue smoke from exhaust from this motor as well. I dont know what else to do. Can ANYONE give me some solid advise on how to fix this problem?????

  • #2
    Re: 1999 Yamaha 150 SS OX66 EFI Bogs down

    test compression, test spark. if both pass the next step is test fuel rail pressure at the trouble RPM, having the diagnostic test lamp attacthed when testing rail pressure may give even more information.
    if the VST cannot get the fuel due to a restriction between the liquid level in the fuel tank and the lift pump the VST runs dry.
    so a fuel system vacum test is also in order.
    vacum must be less than 4"Hg at about 60 liters per hour at the lift pump intake.
    your describing a classic loss of fuel in the intake manifold.
    break the fuel system into 3 main parts, the supply side, this covers everything from the liquid level in the tank to the lift pump output.
    then the VST ,this includes everything from the inlet needle at the VST to the 6 injector screens at the injector rail.
    then the 6 injectors.
    be aware that ALL lubricating oil is injected with the fuel via the injectors.
    failure to have an adequate fuel supply in the intake manifold can and will result in a detonated piston and/or connecting rod bearing failure.
    you have mostly gone as far as you can without buying or making certain test equipments.
    if for whatever reason,the fuel rail cannot maintain 33PSI minimum the engine runs lean.
    the lower the pressure the leaner it runs.
    all of the above is assuming your not getting any alarms and the alarm system is working.
    if your twins are correctly rigged with a 704 dual control and the correct dual ign switch panal the switch panal contains a dual engine switch , this switch will bring the RPM of both motors to below 2500 RPM in the event of any alarm condition.
    no tech questions by PM, they wont get answered.
    you have to be trusted by the people that you lie to .


    • #3
      Re: 1999 Yamaha 150 SS OX66 EFI Bogs down

      some things to look at: #1 compretion and /or leak down test. (probbubly not the issue but easy to do) #2 i normaly would start here with the steps needed to trouble-shoot the fuel system-but- the fact the motor acts up at 2500rpm, is the same as the rpm limit of an over heat or no oil warning. this could happen if you do not have a yam. gauge with warning lights and a bad or disconected warrnig horn. will the r.p.ms go to 3000 in nutral?(do not "free wheel the motor more than a cuple seconds). if r.p.ms do not exeed 2500, try unplugging the temp senders and try aigin. a temp sender can trigger r.p.m reduction, but not trigger the warning horn. if motor rev's freely take a look at the fuel system. #3 "o.k", fuel system basics:
      chk. the rail pressure at the time the motor is bogging/missing. it should be about 35 p.s.i. next, chk. for exessive fuel vacum between the motor mounted filter and fuel pumps. (dont forget to "t" off after the in line chk. valve) you should not have more than 4 in hg. use a 3/8 clear hose in line with the fuel line and chk. for exssesive "air bubbles" in gas line. you said the fuel pulse pumps are new. a quick way to truble shoot them is to prime the fuel system with the primer ball, then use a presure tester pumped up to 5 to 6 psi. it should hold, if it drops a dighram is leaking.#4now the harder stuff! your motor has an oxygen sensor. this usually can affect the runnng of the motor above 4000 rpm's, not usually at 2500. next:t.p.s= there are specs. that are easy to test for in a good repair manual. sorry so wordy! good luck!


      • #4
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        Re: 1999 Yamaha 150 SS OX66 EFI Bogs down

        Thanks for your help RODBOLT. Something else i have noticed since this last thread is that the outdrive smokes REAL BAD at the problem RPM, (approx 2200 or 2300) to a point where your eyes burn. A buddy of mine thinks this could this be a problem with the O2 sensor? I havent been able to test the items you asked me check since i havent been able to pick up the test equipment. Do these motors have a timing setting that should be looked at?