250hp Yamaha Ox66 Problem?

[John_Madison CT]

Well, it looks like I finally have a problem I need all your help to solve.

Engine: 2002 250hp Ox66.
Hours: Approx 300.

Up until yesterday, the engine always ran perfectly.


While cruising out of the harbor I noticed that my Navman fuel flow gauge was showing higher than normal fuel burn. For example, at 4500RPM's I normally do not burn more than 15gph, but today it was reading close to 20gph. Otherwise though, the engine seemed to run fine.

When coming home from fishing, it felt as though the boat was being held back by something. I needed to advance the throttle more to get cruising RPM's. Now I noticed that the Navman was showing ALOT of fuel burn. It would fluctuate and show at least 50% higher than normal.

I was confused as to whether the Navman was bad or something with the engine.

At one point the engine RPM's kind of "broke free" and ran much better. I panicked and throttled back to idle and the engine stalled. (again this engine has always run perfectly and never ever stalled)

I sat there for 10 seconds, restarted the engine and came home. Once again, I noticed that the fuel burn rate was higher than normal.

Back at the dock I checked all the spark plugs, all were normal (no metals filing thank GOD. I replaced with new anyway). I also took out the O2 sensor and checked that. The transfer tube was clear, some gooey carbon, but the hole was fine. The sensor may need a cleaning, which I'm going to do, but this was not an obvious problem.

So, what to do next? What would cause the engine to feel sluggish and cause excessive fuel consumption??? Could it be bad low pressure fuel pumps? Clogged VST?


Thanks guys.


John

[HeadHunt]

John_Madison CT - 6/3/2007 6:17 PM

Could it be bad low pressure fuel pumps? Clogged VST?


Thanks guys.


John

Those are the next obvious things to check. rodbolt taught you and I well. Too bad he is out of touch. Sounds like your sensor is OK. VST would not cause excessive fuel burn. How old are your low pressure pumps? I changed mine last year as a preventive measure.

Alan

[rocksandblues]

sounds simple but my guess is battery is weak and o2 sensor is cold. happened to me. i could not believe it but had the same problem this winter.

[John_Madison CT]

Anyone else?

[KevinM]

Just looking at the sensor doesn't mean it's okay, you have to warm up the engine and read the output voltages when running.
You should also have the TPS checked for the correct operation.

[suncoug]

I agree with KevinM, my guess would be O2 sensor gone bad or TPS out of whack. But, to be sure, I would hook up the winkie-blinkie, o2 sensor harness, and TPS harness (w/ volt meter of course) to check them. Both O2 sensor and TPS failure would cause excessive fuel burn. Also, I would hook up the fuel pressure gauge and see what shows up. Bad low pressure pumps and faulty VST pump could be the cause as well. I would swap out the low pressure pumps if you haven't done so in a while anyhow.

[John_Madison CT]

Guy guys are great. Gonna try a cleaned O2 sensor tomorrow. If that doesn't work, I have another O2 sensor I bought off a THT'er to try. If that doesn't work, gonna do the VST filter and Low pressure pumps. If that doesn't work, time to look into the TPS.

[KevinM]

Let us know!

[John_Madison CT]

Gonna work on her tonight.

[vinnyf]

ok i'm no mechanic but had a similar problem with my 250 ox.my yard said it the ethanol in the gas.not all of us are having the problem but many are and Star tron is taking care of the problem.it seems that the ethanol when it sits in your tank for a longer period of time clean tyhe inside of the gas tank to make it free of gunk and water .the ethanol seperates the water molecules from the gasoline.thus when gunk or water get toi the engine it miss fires runs rough or stalls.the Startron adheres to the water and gunk molecules and lets them burn in the engine.now that i said all that while typing with two fingers,my boat has still not beemn run since last year so like the song says, i'm wishin and hopin..........check out startron.com

vinnyf

[John_Madison CT]

Guys: I put in a different (used) O2 sensor that I had kicking around. It seems to have fixed the problem. I don't know if this problem is intermittant or not and will only be confident after a bunch of trips, but right now, all is well.

Thanks for the help guys. I'll holler if it ends up being something else.

John

[sea draggin]

The guys that I used to know that ran those motors always kept spare O2 sensors on board. Seems they went bad often enought to keep a spare. They said it was an easy fix, even on the water, if you had the sensor.

[KevinM]

Not sure if you use ringfree, but it does seem to help the sensor from getting clogged up.

[Harpoon]

John,

If you haven't changed the low pressure pumps lately you still may want to do so. I noticed a huge difference after doing that. Same for the VST Screen.