Unusual o2 sensor waveform

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Unusual o2 sensor waveform

Postby wellspin » Sun May 28, 2017 8:36 pm

Just gotten round to scoping the O2 sensor voltage on a renault scenic 1.6 16 valve.
The voltage was behaving oddly during graphing in live data.The voltage seems ok but the trace is full of noise,not what Id expected from studying various videos etc,the engine does have a cracked exhaust manifold above the sensor, drawing oxygen in upsetting the sensor?
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Re: Unusual o2 sensor waveform

Postby steevegt » Sun May 28, 2017 9:17 pm

Hi,

Regarding the O2 sensor signal, related to the O2 sensor, everything looks ok to me...
The switching is happening consistently, and it looks like it is happening at a fair frequency.
That capture was made at a idle right (750 rpm)? If you would like to validate the sensor speed (frequency) you should do that at an higher rpm (like steady 2000 rpm).

Regarding the "noise", let me first ask you what ground do you use?
If you zoom in the "noise", you would see that it has a structure, and that is not random noise.

o2_sensor_zoom.PNG
O2 signal zoom


That looks like current ramps, because they actually represent current as volt drop. The big ramps should be caused by the coils current draw, and the small ones by the fuel injectors. It was from the timing of that ramps that I was able to deduce that the engine was at 750 rpm.

In my opinion the 110 mv volt drop max, can be normal... Do you see that volt drop in the live data? Are there any issues with the engine that could be related to the O2 signal?
You could try to scope the signal using the O2 sensor ground as your ground. That "noise" should disappear...
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Re: Unusual o2 sensor waveform

Postby wellspin » Mon May 29, 2017 3:08 pm

Hi
I was using the engine block as ground.
Live data is showing the voltage drop,and then pausing or stumbling accompanied by an engine stumble.
The engine is suffering misfire issues,that come and go,sometimes plenty of power other times the car wont get out of its own way,Everything else seems to check out,fuel pressure,injectors,coil packs compression etc
Yes the car was at idle 750 rpm,Ill run it again at 2000 rpm and use the sensor ground.A few years ago My friends father had an issue that was corrected by running a separate earth for the sensor

Many thanks Ian
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Re: Unusual o2 sensor waveform

Postby steevegt » Mon May 29, 2017 4:17 pm

wellspin wrote:...
Live data is showing the voltage drop,and then pausing or stumbling accompanied by an engine stumble.
...


I'm not really sure if I understood this.
If you could show the O2 voltage PID in graph mode that will be more clear.

wellspin wrote:...
The engine is suffering misfire issues,that come and go,sometimes plenty of power other times the car wont get out of its own way,Everything else seems to check out,fuel pressure,injectors,coil packs compression etc
...


My advice is that you look also at the fuel trim values when the issue occurs. That may point you to a different direction.
In this case, if the fuel trims are not moving to an extreme value, not likely that the O2 signal is creating the symptoms.
Also keep in mind that a direct measure of the O2 signal voltage (scope or DMM) can affect the signal value. So, in my opinion, the best (at first) is to look at fuel trims and O2 voltage, using scan data without any direct measuring device connected.
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Re: Unusual o2 sensor waveform

Postby ben.martins » Tue May 30, 2017 11:24 am

Hello Wellspin,

Thank you for the upload of the waveform. Taking a look at the capture you have steevegt is right. The 'noise' that you are referring to is structured. Using the rotation rulers you can see that it is a uniform event.
Lambda noise.jpg
This could be down to injector or ignition events. Again as steevegt has mentioned it may well be worth trying a different earthing point. I know it can be tricky as you may end up with wires all over the engine bay but you could try to get on the negative terminal of the battery which may see a reduction in the 'noise.'

the engine does have a cracked exhaust manifold above the sensor


This will of course upset the readings from the Oxygen sensor but this should be reflected by the Short Term and Long Term fuel trims. Do you have these figures from your scan tool? Either that or if you have access to a gas analyser these results will help get a better idea of what is going on.

Also see the library pages on the website for more information regarding O2 sensors. https://www.picoauto.com/library/automo ... -post-cat/

Kind Regards

Ben
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Re: Unusual o2 sensor waveform

Postby wellspin » Tue May 30, 2017 11:38 pm

I can take a video of the scan tool readings for short term trims,they are what I would describe as fluctuating wildly
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Re: Unusual o2 sensor waveform

Postby Fat Freddy » Wed May 31, 2017 12:48 am

TBH I would just move on from this whole O2 diagnosis thing regardless of scan tool, scope.

Personally I would apply a maths channel to the CAS with a sync to 1 to see what cylinder/s is/are playing up and go from there. Using battery -'ve as gnd.

HTH
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