O2 Sensor question

[MechanicMike]

Im running a 2000 yamaha 150 0X66 SWS II, SX150TXRY. From time to time idling it will get a sneeze and stall. Start right back up. I bought the shop manual and tested spark plug caps- found 2 out of spec, replaced em. Compression is 115-120 across the board. Tested low pressure pumps, 1 would not hold pressure/vacuum. Replaced both of em. Checked VST and regulator screens- o.k. Engine mounted fuel filter and water seperator are new. New spark plugs too. Even ran engine with clear hose to check air leaks- o.k.. Ran auxillary fuel tank hooked to engine fuel filter, still will sneeze at idle & stall sometimes. Pulled O2 sensor, badly carboned up with chunks of carbon, looked like coal. cleaned O2 as per yamaha bulletin found on here. Checked and cleand transfer tube and O2 block. Re-assembled and ran up to operating temp. Checked voltage at O2 plug engine harness side. At idle I was getting .010-.012 volts. Tried to force rich by covering hole in number 1 throttle plate, no change. Tried bringing idle up to 1500 and recheck got same readings. The boat is not currently in the water and all checks were done with engine running in a water tank.
Question: I've read the propane torch test the manual outlines is useless. I've also read you want the engine temp over 120 with a range of .3-.6 volts and it to toggle but cant find under what conditions to test the O2. Does the engine have to be run under load to be tested? Does it have to be above a certain RPM? Any info would be much appreciated.
Thanks Mike

[Tucker]

Had same motor Mike and had same problem. Did a shock carbon cleaning and ran Ring Free with every tank and problems went away. In fact, people said it idled as smooth and quiet as a 4-stroke. You even mentioned the 02 sensore was carbon-fouled.

[MechanicMike]

Tucker,

Thanks for the reply. I will definately try the decarb. I replaced the O2 today, and it seems to have fixed the problem. Idles around 700 rpm when fully warm, no sneeze or stall. Weather permitting this week I will get it back in the water and see how it runs.

Mike

[Tucker]

Well this begs the question. Wonder if the 02 sensor would have worked after a decarb? Or simply can a decarb fix an 02 sensor? Or, is it FUBAR once it stops working?

[MechanicMike]

I went back and forth with that question all day. Especially being this O2 wasn't cheap. In the 15 years I've been an auto tech, I never had great success cleaning O2 sensors especially when they were this contaminated. It was clear this one hasn't been cleaned in a while if ever. Plus I was afraid of hurting the motor if it ran too lean. I still plan on running a shock treatment and hope that wont contaminate my new sensor.....

Mike

[Tucker]

Can you test the 02 sensor with a resitance meter (ohm)?? I don't think my volt meter goes to .010 and 012volts good grief!

[jb123]

are you using the test harness?
as far as i know motor needs to be up to normal operating temp

[MechanicMike]

I've only checked resistance on the heater circuit. Output test i've always checked voltage.

Mike

[rodbolt17]

I use the voltage test on the heater circuit,heater circuit resistance test on the sensor.
I soak the O2 sensors in combustion chamber cleaner,overnight is better, then spray off with carb cleaner.
do not allow the sensor to sit in a liquid above the threads.
.01-.02V is very lean.
O2 sensor output on a warmed up engine in the water should vary rapidly from about .35v to about .7v.

[MechanicMike]

Thanks rodbolt.

I put the boat back in the water yesterday and took it out for a spin. The boat idled smoother and quieter than it ever has. No more spitting, sneezing or stalling. I did not retest o2 voltages as I probably should have but it seemed to solve the problem.

One more question for the experts .
I may be over thinking this system but, does the engine parameters have to be re-learned or reset now that the o2 is working properly? Is there something like a fuel trim that the comp learns overtime that needs to be reset or relearned to get maximum efficiency? If so how would I go about doing it?

This is a great site, pointed me in the right direction. Can't thank everyone enough for their input.

Mike

[rodbolt17]

nope. nothing left to learn.
that ECU is 30 years behind the automotive market.
but thats what the O2 sensor does, its for fuel trim.