Newb - interpreting narrowband O2

[dekes1]

Looking for some feedback on what value logging stock O2 sensors provide. I know that basically i should only see the voltage (mV) change from relative lean to relative rich and bounce back and forth, but since it's not a true AFR and only tells me rich or lean, is it really of any value? As long as I don't see the voltage show lean under load, I can't really draw any other meaningful conclusions from it, right?

For reference, i'm working on GM Turbo 2.0 with factory O2 sensors.

[dermotw]

Under non-WOT (PE) conditions they will tell you accurately if it is rich or lean, and the switching behaviour and speed tells you if all is 'normal'. Once the A/F goes away from very near 14.7:1 they are indeed not much use... it is possible to tell if for example the fueling is very rich or just rich, and vice versa, by watching the voltages, but its certainly not a way to tune under load! (imho).

See attached pic of my car in OL "testing mode". Look at the red circles, at LH side where the vertical line is, you can see that one O2 says "lean" (low) and one says "rich", thats because they are very very near the the switch point (my WB is saying 14.7, near enough, the blue line) and they flip across fast.. Then to the right where I booted the throttle, they both went hard high..

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[dekes1]

Thanks @dermotw, that helps a lot. One more question, why does my log show the O2 values as constantly jumping up and down? Is there a way to smooth or average the values?

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[dermotw]

When in CL 'mode', they switch high-low-high-low... and it is the switching that the ECU uses to decide the fueling. If they go low (lean) the ECU richens the fueling (gradually, according to some rules' it has) until they go high, then it starts leaning it again... and so on in a cycle.

They switch maybe somewhere around once a second (can be quite a lot more or less though...).

So in fact, in CL the fueling is never really EXACTLY at 14.7... it is always slightly above, or below, and heading to the opposite side, low to high or vice versa. Here is my car in CL: It is a 10s window, so the o2's are switching quite well, my WB is the blue trace, the vertical line shows how the WB says lean, the o2's also say lean... then it will cycle to rich and so on... you see that the STFT's also follow the same pattern (LTFT was off during this run...).

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