I have an XO66 250 that runs great at all RPMs, up to 4800. The issue arises when I go WOT, which I believe is low, at around 5100. The engine runs well and then starts to roll back slowly, from about 4500 to 4400, and then slowly rolls back up to 5100, repeating this process over and over. If I pull the throttle back to 4800 or in that range, it runs great. Until I push it back up to the 5100 rpm. I have a new O2 sensor and am considering replacing it. What are the thoughts of the engine guys on the page?
Yamaha OX66 250 WOT RPM Searching?
- Thread starter gradywhite248
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Is it "sputtering"? What happens if you push the throttle all the way...and go past 5100 RPM's? Sounds like it might be a fuel pump situation? Has it been happening for a while or is this something new? Any way you get get us a video of the situation you describe?
Good luck!
Good luck!
enfish
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I had a buddy whose boat had a similar problem. Turned out the fuel lines were beginning to deteriorate and would collapse when the motor started sucking fuel at a high enough volume to get the WOT RPMs. There was nothing wrong with his motor. Replacing all fuel lines resolved the issue.
My first guess would be a clogged VST screen.( or other screen filter if present on high pressure pump pressure regulator.
Mustang65fbk
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Agree with the above comment about the fuel lines possibly being an issue. My uncle's old boat had the same problem where there were pinhole leaks in the fuel lines and so it would start to sputter or run lean and then die on him. I'm a firm believer of starting with the basics first and then go from there, that way you aren't spending potentially thousands of dollars on something that might be less than $100 worth of parts and your own "labor".
I agree with seasick. Should put a fuel pressure gauge onto the vst while at high speed. If fuel pressure goes down while wot, then pump the ball. If pressure does not come back, the high pressure pump screen is clogged. Also could be the little filter at the pressure regulator. Could be fuel pickup from tank also. Could rule out hoses and tank issues by using a remote tank. If u are using ethanol, it is very possible that the vst inlet needle has swelled up and is restricting the vst tank from refilling fast enough to keep the engine running at high speed.
While reliable in the sense that it will always get you home, the ox66 fuel system is it's Achilles heal. Make sure all of the usual culprits are addressed 1st. Then do a thurough decarboning treatment including the sniffer tube for the O2 sensor. If all of those have been addressed, as a last resort, try the O2 sensor.
In 10 years, I ran into issues similar to yours and replaced the O2 sensor 2x. It would fix the issue, but only if the above had been completed. I'll admit it didn't make complete sense, but eventually I had one of the best running ox66's I had ever witnessed in person. To keep it running well, Techron Marine virtually eliminated all carbon throughout the engine.
In 10 years, I ran into issues similar to yours and replaced the O2 sensor 2x. It would fix the issue, but only if the above had been completed. I'll admit it didn't make complete sense, but eventually I had one of the best running ox66's I had ever witnessed in person. To keep it running well, Techron Marine virtually eliminated all carbon throughout the engine.
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Sparkdog brought up a possible cause that I didn't mention. In addition to possible a pickup issue, a partially blocked vent can cause fuel starvation under load when fuel demand is highest. I still would suspect the VST first but it isn't always the issue.
No sputtering slowly rolls back and returns to 5100, then repeats the process. Will not go over 5100 WOT—thanks for the reply.