'03 OX66 250 acceleration issue possible fuel problem

[Fanboatman]

I will test it again but in the water as soon as I can get someone free to run the boat for me. I understand what you mean about testing at the failure load and rpm. I just thought it may also do it on the trailer because for one it has even more load on it, and two it also seems like it doesn't want to rev up when power loading. In the mean time, I tested the fuel pressure regulator as stated in the manual. When I applied 25kpa as the book states, the fuel pressure dropped to 33 psi. I also tested the electric fuel pump resistor's resistance. It measured 1.5 ohms on the lowest setting on both my digital and new analog meters. The book states it should be .53-.57 ohms. I'm not sure if this would have anything to do with the issue, but I figured I'd let you know. I also checked the knock sensor per the manual instructions. I set both digital and analog meters at the lowest AC voltage setting and tapped the sensor with a screwdriver lightly and fairly hard, with the sensor disconnected and connected to the harness with power on and off. I could not get any reading at all on either meter. Therefore, am I correct to assume that the knock sensor is faulty? Could this be the possibly be what is causing the acceleration issue? Hopefully I can lake test it later this week to check the fuel pressure at the proper load and rpms.

[Tucker]

What did the manual state for correct voltage going to knock sensor? And does it register when "knocking"?

[Fanboatman]

The manual did not state correct voltage. It simply stated to have one probe going to the wire and the other to the body of the knock sensor. Then tap it lightly and the meter should register several miliamps. I did this with my meter on the lowest AC voltage setting which is what it says to do in the manual, and the meter registered nothing even when tapping the sensor fairly hard. I think I may go ahead and order one.

[99yam40]

you cannot read miliamps when meter is on a voltage setting

[Fanboatman]

I was kinda wondering about that, but the Yamaha manual says to set meter to AC voltage to test the knock sensor. Didn't really make sense to me. I will do a little more research on it.

[99yam40]

Manuals are sometimes printed with wrong info.
need for Rodbolt or Boscoe or someone else with another manual to respond

maybe wrong setting or read out info

[rodbolt17]

the knock sensor is a piezio crystal device,
shake it and it shocks.
shock it and it shakes.

typically it is fed a 5vDC current, this is the detect for open or short.

when a knock is detected it feeds an AC signal back to the ECU on the 5Vdc line.
they rarely fail.

[Fanboatman]

Ok, so I replaced the orings on the pressure regulator, high pressure pump, and vst tank. Also verified that the hp pump screen was clean. I replaced the check valve in the fuel line between the filter and lp pumps with a straight thru fitting just to make sure there was not an issue with that. I went to the lake and checked the fuel pressure at the problem load and rpms. It stayed at a constant 35-36 psi, and I squeezed the return line while at the problem load and rpms and that did not make any change. I also unplugged the knock sensor and that made no change either. I am thinking at this point it may be and issue with the tps, ecm, or pulsar coil. I am waiting on the test harness for the tps as it is was out of stock. I will pull the flywheel next to check everything under there. Is there anything else to check, or any other test harness or tools I need to get? I am almost at the point to take it somewhere, but I am having a hard time finding anyone within a couple hours drive that knows these motors well enough to figure this out.

[Tucker]

Check all rodbolt's post here and Iboats. Someone has had to live this problem. I believe the Yam website has links to their master techs and where they are. Gotta be somebody in Tx that still knows these OX66's. Please post developments, this is a real beaut...

[rodbolt17]

you really really should use the diagnostic test lamp.
when you unplugged the knock sensor,or any other sensor, the ECU reacts by fixing base timing from about 7*ATDC to 7*BTDC and the engine idle speed will increase to about 1100 RPM.
depending on the sensor WOT may or may not be reached.
the O2 sensor is NOT monitored for faults.
the O2 sensor simply trims fuel.
however if it fails or is open circuited it will run lean above 4000 RPM. DO NOT run above 4000 RPM if the o2 sensor is dissconected,open or failed detecting an over rich condition.

that ECU wont store codes and will only display them at the time the code is present.

[Fanboatman]

I have searched and cannot find anything anywhere online about a issue like this. I also do understand that by unplugging a sensor will default the motor to 7 degrees timing. I thought that maybe if it was some sort of timing issue it would tell me something. I am also assuming that even though the test of the knock sensor didn't read anything that since it does change timing when unplugged that it is working. I just got the diagnostic tool and went to the lake. I have no code after warmup other than normal which is one blink. During warmup for a minute or so it gave me a code that was timing being corrected (cold start) I forget the number. I also tested the tps. Voltage was .48 at idle and when i put the throttle all the way down it jumped straight to 3.90 then slowly climbed to 4.12. The motor had not been getting on plane at all for the past 3 outings. Tonight it would slowly make it up on plane, but once it got above 3k rpms or so I could hear what almost sounded like slight backfires in the intake. I wouldn't have heard it but I had the cowl off this evening. I'm starting to lean more and more to this being a spark issue somewhere. I will check my plugs tomorrow and will check whatever else you guys suggest.

[Fanboatman]

I believe the problem has been solved, if it hasn't we found another bigger one. I took it to a dealer about 1 1/2 hours away who had two master techs. They search and tested everything electrical and fuel related and couldn't find any issues. He the decided to start from scratch and run a compression test. When he pulled the #6 plug he noticed the electrode was bent down, so he put his camera in the plug hole and sure enough the piston was messed up. He then pulled the head and the piston is missing a piece about 5/16" at the widest and 2" long. The top ring is also broken, but the second is fully intact. Somehow it managed to have good compression. When I bought the boat I ran a compression test and all 6 were 112-118 psi. I also did another compression test back in January or February and had the same results. At no point was ther even a remote idea there was something mechanically wrong with the motor as it idled and ran very well other than getting up on plane. I am now debating on fixing this motor or replacing it with a new or newer one, but I have plenty of time to decide while I save up and sell a few things in order to fix it. Thanks again for all the help.

[TownsendsFJR1300]

There have been a few posts on the "PIN" that keeps the piston rings from rotating, moving upward, allowing the ring(s) to rotate.

Once the ring end gets to a port opening it catches / breaks and your damage occurs..

That sounds like what happened.

Is the cylinder scored up at all?

Pic's?

[Fanboatman]

That could be exactly what happened. Just amazes me that it had such good compression the whole time. The cylinder is scored pretty bad. It may clean up with a bore and hone job, but it took a small piece out of the top edge of the exhaust port.
[IMG][/IMG]

[99yam40]

I wonder if that cylinder can be re-sleeved and what the cost would be