OX66 O2 Sensor Check

My motor has been stalling on me at low RPM's. I talked to the local Yamaha mechanic and he said there is a yellow wire on the ECM that I could unplug and cause the ECM to ignore the sensors and run a default map. If the motor quit stalling, then I would know it was a sensor or ECM issue. I pulled the wire and wa-la, it ran without stalling.

Next, I pulled the O2. I had cleaned it about 2 months ago, so it was in good shape, but I cleaned it again anyway. I reinstalled it this AM, and the same issue. Next I decided to hook up my meter to it to see if I could get a reading. From my understanding you hook into the grey/black wires going to the ECU, correct?

The way I did it was to use two wire splices. Yamaha techs, take a look and see if you think this setup would give me a reading. I switched the ends of the meter between the wires and it never moved from 0. Either I have a bad connection or a totally dead O2 sensor. Since I am not an electrician by any means, I wanted to ask the forum to see if you guys to could something I may have done wrong, and would cause the meter to not read any voltage.





Thanks
Chris

Re: OX66 O2 Sensor Check

find a real tech.
replace the now defective O2 sensor.
NEVER cut into a sensor wire, it leads to failure.
NEVER run the engine with the O2 sensor removed.
when you place the ECU in bypass mode it fixs ign timeing at 7*BTDC,fixs fuel rich and is stupid, cant test anything that way.
takes 10-15 seconds to use the diagnostic test lamp to check for any sensor codes.
no codes no worries.
the O2 sensor has NO codes unplug it and fuel is fixed rich at low speeds and lean above 4000 RPM. DO NOT run above 4000 RPM with a defective or unplugged O2 sensor.
so you destroyed an O2 sensor and ran it in bypass mode.its just to simple to use paper clips and back probe the connector.
what did it tell you?
TPS voltage is easily tested, CPS and CTS are easily tested.
fuel rail pressure is easily tested.
lift pumps are a simple test and the 3.1L motors eat them.
atmospheric and intake air sensors are easily tested; its also why I use the 75 dollar diagnostic test lamp, no codes then I dont need to test sensors other than the O2 sensor.
in normal mode on your 3.1L the ign timing and fuel injection map is controled by the ECU based on various sensor inputs. the CDI is still the same.
the CDI is part of the ign producing function, the ECU and sensors simply modify it.
the O2 sensor simply samples the combustion gasses and alters fuel injector on time to help maintain a 14.7/1 A/F ratio.
post a full model number as the VX had a different ECU map than the SX.
radio shack sells jumper leads and back probe leads.

Re: OX66 O2 Sensor Check

Thanks for the reply Rod. I have already replaced the o2 sensor. Turns out a buddy had one in his garage from when he use to own a OX66. He also had an extra set of the low pressure fuel pumps, but I tested mine and they didn't leak.

I had a feeling splicing into the wires wasn't the best idea, so I was ready to plop down the extra. Honestly, I am trying to learn a little about this motor, and all the electronic controls/sensors are a bit new to me. No excuse though, now I can see how that was a dumb move.

By putting the ECU in bypass I was basically using it to tell me if I had some type of fuel deliver issue from the boat to the motor. I figure if I had a small air leak/bad gas/deteriorating hose/clogged filter then it would still stall out on me. By having the motor run with the ECU unplugged, I ruled those things out. I also thought it would rule out just about any mechanical problem and let me know the issue was either a bad sensor or an electrical issue. If the motor runs while the ECU is unplugged, then (and this is just my thinking) all of the pumps/filters/etc are able to function property, give that they are given the correct info from the ECU/sensors. If this is incorrect thinking, please let me know. .

I am not sure what back probing is, but I'm sure Google will fill me in on those things. I will also look into getting a test light. I did a quick search on that, and seems like it will be well worth the money to be able to check for any codes from the ECU.

The motor itself is a VX250TLRA.

Thanks again for your reply Rod. As you can tell, I am very much a novice when it comes to these motors.

Re: OX66 O2 Sensor Check

A Yamaha service manual for your motor and the proper test equipment will do great things and save you tons of time and money.

Back probing the wires is slipping the small probe/ paper clip into the back side of the connector where the wires go in, along the edge of the wire far enough to make contact to the conductor but not damaging the wire insulation.

Re: OX66 O2 Sensor Check

while back probing is a poor substitute for the correct test harness I use it on a case by case based on how often I need that harness.
incorrect baro sensor could cause stalling, could be an inrange failure.
by unplugging the ECU bypass line all you proved was the default map worked.
normal ign timing at idle on that motor is about 4* ATDC, when bypassed it gets locked at 7* BTDC, thats a total of 11* of ign timing advance.
fuel gets fixed rich and the idle speed should have increased to about 1200 RPM.
so you still have to trouble shoot everyhing all over again.
all you proved is that with timing locked at 7*BTDC,fuel fixed rich and increased idle it runs,kinda.
start with the basics, test compression, test spark .
then test the rest.
thats why the diagnostic test lamp is so invaluable, it can eliminate a lot of things rapidly.
it wont find inrange sensor failures, only way to find them on that motor is experience and the service manual.
so lets do the basics, also test the plug caps as per the manual.
electronic failures are rather rare on that ECU and the sensors.