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  1. #1

    Default '97 225 OX66 Yamaha WOT rpm loss... with a twist

    Ok,

    So I have been experiencing an intermittent loss of rpm @WOT (5400 rpm) on my outboard. This problem first started about 2 years ago and was a relatively rare event. I immediately took the boat to a very reputable Yamaha technician who checked the boat six-ways to Sunday in a tank. He was unable to replicate the problem, didn't charge me a dime (thankfully) and said to just wait until it became a consistent problem. Well, at the beginning of this year the problem started to happen more frequently. At this time, I get the behavior on every trip...

    The problem is very similar to those previously described... a drop from 5400 to 4800-5100, very immediate, sometimes it will jump back up to 5400 and may hold for 5-10 minutes and then jump back down. I also have noted that my fuel consumption seems higher than usual.

    Checked compression, had injectors cleaned and tested, Checked low pressure fuel, VST high pressure fuel (35.5 psi invariant under all conditions, even during the loss of rpm event), Cleaned the O2 sensor, Checked link & synch, checked TPS. observed the O2 sensor output during idle in the water (~0.47V) start to push the throttles up and watch the O2 sensor start to lean-out (down to ~0.3V), approaching WOT and the O2 sensor starts to come back up and stabilize (0.4-0.5V). When the loss of RPM occurs, I can see the O2 sensor quickly start to climb up (0.67-0.7+ Volts). when the rpm comes back online, the O2 responds by heading back to the mid-range again.

    Now here's the twist. When I was checking the TPS, I noted a nice smooth voltage output response and a constant 5.0V input. However, I also heard a buzzing sound when I advanced the throttles. It seemed odd, so I relayed this information to a trained Yamaha Mechanic friend who said that I should not be hearing any buzzing and that I must have a short in the TPS, even though I did not see any spikes in the input/output. Furthermore he had seen this before and suggested that I get a TPS. I spent the $100, dropped in the new TPS and guess what... still buzzes! So I look a little further and discover that it is not the TPS that is buzzing, it is the fuel injectors. With the key in the "on" position, motor is NOT running, when I advanced the throttles very slowly I can get to WOT without the injectors going off, but, if I move the throttles at what I would call a normal hole-shot, all 6 injectors will fire simultaneously for a half-second or so,... almost like an accelerator pump! The length of fire seems to vary and the throttle position at which they fire also seems to vary and might even be speed dependent... My mechanic buddy says that the injectors should not fire at all without the motor running and now thinks there is something wrong with the ECU... I am at a loss... any thoughts?

  2. #2
    Senior Chief Petty Officer cousinabe's Avatar
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    I had a similar problem, turned out to be my O2 sensor.
    How many hours on the sensor? Sometimes they just go. Mine lasted 350hrs with reg maint.
    I would see fuel consumption go from 12 to >20 gph, I would shut the engine off and restart and then it would be good again for 30-60min before it acted up again.
    It should rapidly fluctuate between .3 - .6v.
    A bad spark plug cap out of spec will do funny things under load too.

  3. #3
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    I wouldnt worry about the buzzing. When I adjusted my TPS I experianced the exact, and I mean exact same thing with the injectors. I think it is normal.

  4. #4

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    Cousinabe,

    Even though it is obvious that the O2 sensor is functioning between 0.29-0.73V as previously described above (during a sea-trials with a voltmeter hooked up). You still think that the O2 sensor is giving a bad signal (out of calibration)?

    Thanks for your reply... hopefully somewhere along the way something will make sense.

  5. #5
    Senior Chief Petty Officer cousinabe's Avatar
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    most likely, yes. the high fuel consumption is a symptom of a faulty O2 sensor. the voltage fluctuations should be RAPID too.
    fyi, i just did a tps adjustment and got the buzzing. that is normal.

  6. #6
    Senior Chief Petty Officer cousinabe's Avatar
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    on a side note, another important maint item is checking the oil transfer process. I kept putting it off because my oil tank was below deck and ended up paying for my laziness...$$$

    make sure the engine tank switch is working in all three positions, oil harness is good, remote tank switch and remote tank filter are all clean.
    If you have never cleaned the remote tank, make sure all the sludge is out of it and everything is working correctly.

    I love my OX66 motors!

  7. #7

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    Cousinabe,

    I recently went through the remote oil tank filter issue, I was getting low oil warning which I successfully traced back to the filter... a cheap lesson learned!

    Concerning the O2 sensor. I went ahead and pulled the sensor again based on your "rapid response" advice (which I was not getting). It turns out that, despite cleaning the sensor perhaps <10 hours ago, that the transfer tube was almost completely blocked on the pressure side. I am guessing I may have a carbon build-up issue. I plan on running a shock treatment of ring-free and re-cleaning my sensor... I may end up replacing it. In any case. What is the normal range of operation I should be looking for? I have seen it as high as 0.73V?

    Hopefully this is going in a positive direction!! I'll keep everyone updated.

  8. #8

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    Just to clarify... When you say "normal" operating range being .3-.6V are you saying that if it is outside of this range the O2 sensor is likely broken?

  9. #9
    Senior Chief Petty Officer cousinabe's Avatar
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    have you seen this doc?
    http://www.boatsetup.com/O2sensor.pdf

    disregarding the bench flame test.

    The voltage should be fluctuating very rapidly between .3-.6v.
    Voltage going higher indicates a rich condition and may cause that draw tube to clogged up.

    fyi, there was also a tech bulletin/redesign of the draw tube.

  10. #10

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    Well... I recleaned the O2 sensor and tested it, the response was still sluggish.
    I did a little research on how these guys work... Turns out that they get coated
    with combustion products over time that even a good soaking can't fix...
    I dropped the $300 for the new O2 sensor. Unbelievable! it runs like brand
    Spanking new! I can't believe how sensitive this motor is to the sensor. Anyways,
    Thanks to all for the advice... Especially Cousinabe!

  11. #11

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    I'm a new OX66 owner (1999 200hours) and have been experiencing the same exact WOT RPM loss! when you Hole-shot or jump on trottle from low RPM..it hits the 5500 top end...and then cuts down to 4500. I can then back off throtle for a few secs and then "slowly" go up to WOT and it will run at 5500 for hours. My secret was a "slow" transition to WOT. I'm very glad to hear your resolve as I have checked everything else. QUESTION - Did you notice NOW that fixed "if" at lower RPM & Idle that you engine "smokes" less...less oil burn? I have noticed that the OX66 seems to smoke a little more than from what most state it should. I've adjusted the Oil Linkage rod given most dealers fail to do so past breakin. Anyways congrats on resolve....I too love the OX66....great engine!

  12. #12
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    funtastic
    start a new thread for your issue.
    your engine is breaking,ignore it and it will puke parts.
    there were several DIFFERENT versions of the OX 66 motor avalible in 1999.
    post your full model number.
    oil link rod is ALWAYS adjusted correctly when new,yea right.
    but no the adjustment procedure is the same for a new out of the box engine or one with 3000 hours.
    no rod adjustment for breakin.
    do some forum searching on OX66 and you will see myself and some others have extensivly covered these motors but do NOTHING until you know WHAT version of OX66 you have.

  13. #13

    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    The Engine is a 61AX102114 U. I agree...and HAVE seen your and many other excellent posts on various issues/resolves. In Fact I'm replacing my 3 LP Fuel filters this spring.

    I also wanted to know if there is an easy way to find out IF my engine qualifies for the redesign of the draw tube? I sent a request to Yamaha direct...an NO response? Does anyone recommend a solid online company to deal with to buy parts? I saw many like **********?

  14. #14
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    Default Re: '97 225 OX66 Yamaha WOT rpm loss... with a twist

    can you start a new thread?
    can you post a model number?. until the 2006 production run yamaha motors for the US market were identified by the MODEL number.
    2006 and up uses BOTH the model number AND a seriel number or most likly you get the wrong part.
    by the fact yours is a 1999 and has 3 lift pumps we know its a 3.1L,which ign fuel system we still dont know cause they made a few versions.
    start a new thread with a model number and a problem and the oil question and I can answer it.

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