2003 3.1 L 225 VMAX fouled O2 Sensor at 14 hours run time

[Amistad Tackle]

Model VX225TLRB (2003) with 590 hours on it. It has the original sensor which I have cleaned numerous times (with Yamaha combustion chamber cleaner). Last cleaned at 576 hours and installed new plugs. Compression good at 576. Tested the sensor with the test harness at this time and it was within Yamaha manual specs although the voltage at idle was a little high, but still within specs. Perhaps overoiling at idle? This time (590 hours) the engine stopped coming out of the hole like it usually does. It was definately not running on all cylinders. Looked at the plugs and they were oily. The lower one on the passenger side was especially oily. Pulled the O2 sensor and there was pooled oil in the bottom of the tube the sendor screws in. Checked the low pressure pumps and the top one had a little bit of fuel weeping from one of the bolt holes. I rebuilt it and re-installed.

My question is why does this engine have such a problem with fouling this sensor? I have adjusted my oil rod to specs which seemed to help for a while. I end up changing plugs about every 25 hours, now its like every 14. Should I just go ahead and buy a new O2 sensor? My bet is what I've done will get her running again just fine and then 14-25 hours later I'll be pulling the O2 sensor again or at least changing plugs. I don't drive the boat very fast with the price of fuel. Usually 3-4K rpm is all I run and I do quite a bit of idling while I'm graphing spots. Could this be leading to my o2 sensor fouling and going through the plugs so quick?

[Amistad Tackle]

This is an old post, but as a follow up after I put some hours on the motor, and to help others, what is described in my last post solved my issues. 668 hours and runs perfect. Haven't changed plugs once this summer, and I spent a good portion of the time idling with the side scan. Quick out of the hole and full power at the upper end.

[Northeast Bassin]

Had you ever tested the performance of your O2 sensor? Curious to know if it was reading ok, but not working ok

[Amistad Tackle]

Yes I did test the old O2 sensor with a test harness and digital volt meter while the boat was running on the water. It was reading within specs, but as I remember the voltage readings at idle where higher than they should have been, resulting in a rich mixture at idle. I believe this led to fouling of the plugs. I'll be interested to pull the new O2 sensor and see what it looks like compared to the old one. The old one was always oily and heavily carboned.