High Ox66 fuel burn after o2 sensor change

[TxCurt]

Hi guys,

looking for a little help from the brain trust here. I have a pair of 2000 Yamaha ox 66 motors recently purchased on a new to me boat. Both motors are running well but as part of maintenance I decided to replace all filters, plugs etc since I really did not know when they had been done last. My starboard motor was making a strange noise at idle and reading at tht it was noted it could be caused by the o2 sensor. Both motors were running well and fuel burn numbers on each motors were almost identical at the time. In an attempt to fix the noise and because I didn’t know when this motor had last had its sensor changed I ordered and installed a new oem o2 sensor. At the same time I also remove the “dumdbell” which was completely blocked with carbon like completely blocked. I soaked it and got the hole through the middle cleaned out and put it all back together ( now writing this I’m thinking i did not replace the gasket i left the original gasket in place). Once back together, the noise is gone and the motor is running strong but my fuel burn has increased about 35 percent on that motor( i have garmin fuel flow sensors on each motor) . Honestly I had expected the new o2 sensor to maintain or perhaps improve fuel burn certainly not decrease it. Any reason why cleaning the “dumbbell” or dogbone would have changed my fuel burn?? It seems more related to the cleaning or just the opening of the o2 sensor compartment than the sensor itself. New sensor is brand new from sim.


Suggestions on what to check??

Thanks!!

 

[GA_Boater]

(now writing this I’m thinking i did not replace the gasket i left the original gasket in place).

A leaking gasket could create a lean or sensed lean condition. Lean means add more gas to correct the fuel/air ratio.

Replace the gasket before looking for another why.

 

[GA_Boater]

And welcome aboard!

 

[TxCurt]

Gasket on order. I’ll give that a shot

 

[TxCurt]

Following up on this issue. Replaced the gasket and ran the boat yesterday about 100 miles. Still seeing the same thing. At cruise one motor is burning about 9 gallons per hour and the other is burning 13-14. Here’s a screenshot of the fuel burn at cruise ( i took it of the wrong screen this shows mpg) from this view the difference doesn’t seem as extreme as looking at gallons per hour. So I’m still looking for a cause. Prior to the o2 replacement and more specifically the cleaning of the dog one fuel burn has increased. Unless its coincidence but I run the boat frequently and that seems unlikely.

 

[GA_Boater]

Swap O[SUP]2 [/SUP]sensors between motors. Might be a bad one out of the box.

 

[TxCurt]

Swapped sensors and results are approximately the same. Put in a new dogbone and gasket and maybe I’m just expecting too much. After this my results at 3500 rpms 32 mph burning combined 17.9 gallons per hour. 8.3 from one and 9.6 from the other I’m thinking i need to just accept the difference.

 

[GA_Boater]

8.3 and 9.6 GPH is an improvement from 9 and 13-14 GPH.

Is the idle noise gone? How many hours on the twins? Maybe the port motor is running just a bit better than starboard.

Did GPH use to be closer?

 

[TxCurt]

Yes idle noise is gone. Hours are unknown but definitely over 1000 but honestly they both seem to run strong. I’m making at about 5000 rpm on both motors and topping out around 50mph. Took a look at the props and they probably need some work as well which may further improve efficiency. For now i think I’m going to call it good. Thanks for the help it’s nice having someone to bounce the ideas around and make sure I don’t just go in circles!! Thanks!!