ox66 problem

[nickmo]

I have twin L225TXRX Yamahas.
This Spring, I replaced low pressure pumps, VST filters, Racor filters, cleaned o2 sensors, changed spark plugs. Port engine will not exceed 4500 RPMs at WOT. It does not surge.
I checked voltage at the o2 sensor using the Yamaha harness with motor in the water and warmed up. My DVM readout is in the following format 0.00.
With the motor running the readout fluctuates between 0.43 and 0.55. The other (non problem) motor fluctuates between 0.38 and 0.70. I'd like some of the Yamaha gurus here to confirm or deny a faulty o2 sensor before buying one for $300.00.
Thanks in advance and have a great Memorial Day weekend.

 

[cousinabe]

Re: ox66 problem

try swapping the o2 sensors and see what you get.

www.boatsetup.com/O2sensor.pdf
disregard the bench flame test.

 

[nickmo]

Re: ox66 problem

I swapped the sensors and ran the boat. Both motors ran perfectly. At WOT they ran 5500 rpms. I don't know if I should be happy. Is it possible there was a bad connection at the o2 sensor plug?

 

[rodbolt]

Re: ox66 problem

maybe.
if the O2 sensor is dissconected the ECU sets NO codes.
it will however fix fuel rich at low speeds and lean above 4000. DO NOT RUN ABOVE 4000.
the above is directly from the yamaha book.

 

[nickmo]

Re: ox66 problem

Thanks Rodbolt. Did you take a look at the DVM#s in the OP? Are they within the guidlines?

 

[rodbolt]

Re: ox66 problem

barely, but I wasnt there to check them.
did you check the O2 sensor heater voltage, did you run the engine for 10 minutes or so at a medium speed before testing the output voltage.
did you follow the tech bullitens for covering the idle air bypass hole at idle and observe the sensor output and the other test at a steady 3000 to 3500 RPM cruise and observe output?
the flame test in the manual and tech bullitens I wont use, seen to many pass a bench test and totally fail the real world test.

 

[nickmo]

Re: ox66 problem

I did not do the flame test as I read some of your posts as it being worthless.
I did warm the engine but not for 10 minutes and not at medium speed. I did cover the idle air bypass hole and the voltage increased slightly and did not decrease as the bullitin says it should, which is one of the reasons I thought I had a bad sensor. The motor is still running great so I'll try the voltage test again as per your post.
Thanks again.