2001 Yamaha 250 OX 66, Sporadic loss of RPMs

[JBird]

My 2002, 23' Hydra Sport is powered by a 2001 Yamaha 250 OX66. The motor is regularly tuned every 100 hours. Currently, I have 383 hours on the motor. Over the last 6 months I have had a bizarre & random problem, which happens on SOME trips. I can be cruising at 4100 RPMs for any amount of time, sometimes 35 minutes straight or 15 minutes, and then all of a sudden, my RPMs drop down by as much as 700-800 RPMs, down to 3400 RPMs. The motor sometimes stays at this lower RPMs for any given time, sometimes a minute, other times several minutes, then it may start to climb back to where the throttle was original set. (There is nothing wrong w/ the throttle cable and I’m not cavitating). Once this happens on a trip, it will continue to fluctuate until I bring it back to the dock. I’ve been offshore returning home and had to deal w/ this on more than one occasion. The first time it happened, I shut the motor off & restarted it. When it's in neutral idling, it shows no problems and idles as it’s supposed to, then I’ll accelerate to a desired cruising speed and the problem will start again. Another scenario, I’m cruising at 4300RPMS, and the RPMs drop by 500-800RPMs, if I’m impatient, I’ll give it more gas, to get it closer to the 4300RPMs, then when it starts to work through its issues, the motor will reach an even higher RPM based on where the throttle is. When I lose the RPMs, it’s almost like it’s not getting enough fuel or it’s misfiring.

I informed my mechanic and the first time he took it out he thought I was crazy. It took a few times before he had the same circumstances.

My mechanic has checked the fuel filters, there is no water or debris in them.
Most recently, he replaced all 3 fuel pumps on the motor b/c 2 of the 3 were cracked. I was very hopeful that the broken fuel pumps were the issue. Additionally, he rebuilt the VST tank, cleaned the Oxygen Sensor, did a compression test and cleaned cylinders w/ Power Tune. After I paid the bill and took it out, it ran very well, then a couple of trips later, the situation occurred only for a few minutes then rode fine. A couple trips after that, it happened again and happened for longer time closer to 30 minutes. The last 3 trips, I have had no problem. It seems to happen usually during rides offshore w/ any slight waves.

I thought about having someone clean my fuel tank (boat is a 2002) ,but my mechanic still does not believe that is the issue. We looked at the fuel filter again and no debris or signs of water. He now thinks it’s the oxygen sensor. I asked him if he takes the oxygen sensor apart can you confirm it was bad? He says you can’t. I don’t want to continue to replace parts which are not fixing the problem.

Would a bad oxygen sensor cause my symptoms? He did state that it was filled w/ a lot of carbon when he cleaned it before.

Any other suggestions on what this random problem may be?

[jb123]

you can TEST the sensor :

http://www.boatsetup.com/O2sensor.pdf

and the random rpm drops can be caused by it.
there were other posts about rough water/bouncing that caused similiar issues but i forget the cause, possibly if you research back thru the posts or another member will ring in.

to be sure the issue is on the motor side, run motor on an auxillary tank.

[JBird]

Thanks for the quick reply. I read the information on the O2 sensor. And I went through a lot of other posts.

Follow up questions. It sounds like it really could be the O2 sensor, but if it was bad, would it always run poorly, on every trip? Or is it possibly that some trips it will not function correctly and have power surges and other trips it runs flawless?

Same thing w/ if it's a fuel issue, would it always run poorly until fixed or could it possibly run fine one trip and poorly on another?

Lastly, Ring Free Fuel additive, from the other posts that sounds like it may be a good investment, what are your thoughts?

[Skeeterman]

JBIRD, sorry to read about your loss of RPM's. Some things just are difficult to locate and put a cure to them. These outboard engine manufacturers sure have put in every little minor problem that could occur for us who enjoy being on the water. Of course, I understand it's money in the bank to create such problems..

Now, reference to Yamaha "Ring Free" additive you asked about.. I own a 1986 ProV-150 that I bought new with the bass rig, and have used it since day one. I firmly believe in this Ring Free is great stuff. I can take a penlight and shine into the spark plug holes, and the tips of the cylinders are shiny and clean.. The ProV-150 has never been worked on, until this past few weeks when I installed new thermostats and water pressure value.. Now I knock on wood hoping I will not start having problems.

[jb123]

use an aux tank to rule out your tank;
try a shock treatment with seafoam or ring free- you can find the instructions on past posts;
pull each plug wire while running and listen for an rpm change;
disconnect each injector while running and listen for the same sound change.

[rwstoney]

I had a very simlar problem with my 200 Hp OX66. that I finally chased down to the very fine filter on the backpressure control valve that was clogged. I cleaned the filter Yamaha part #65L-13905-00-00 and the motor stopped the rpm loss. It seems the 10 micron fuel filter is not enough to catch all the gunk stirred up by the new gasoline. The motors are sensitive to fuel injector pressure and the backpressure valve controls fuel pressure.

[bernieb]

I have the same engine and have had the same symptoms and found a couple of leaves in the main fuel tank ......take heed here, always somebody that wants to get even,regardless how hard you try not to tee someone off. You may have a anti-siphon valve on top of your main fuel tank ,a screwed in L shaped fitting that holds a ball and spring and is the catch all for trash in the fuel system. Your sparkplug orange rubber caps unscrew from wiring and should ohm out at 5 thousand or close to it.Each injector has a tiny filter,the fuel regulator has a screen.You can block fuel line at the main tank and use a hand vaccum pump at the fuel filter supply line under the hood ,see if it holds a vaccum. You can try using a fuel pressure gauge on the scrader valve while under way and watch for any fluctuations when the problem arises,should get a reading at 35psi.