oxygen sensor readings

[Orin Eman]

> So how do I go about using the signal as a vague indicator of the A/F ratio?
> Average it?  Over how long?
> I'm not planning on doing any fueling with this, just another gadget to display
> on the LCD.  Really I'm just looking for good,bad,danger, etc.

Well, I just put the signal into one or two LM3914s to create an LED
bargraph display.  Mr GRUMPYNOMO has written a very good description
below of the O2 sensor operation below.

With the bargraph display and no filtering, you'll see
the voltage swing back and forth from somewhere around 100mV to
somewhere around 800mV.  Maybe a couple of times a second at
idle to 5 or so times at cruise.  You'd need to average over
at least a second to get some indication...  then you would
have to calibrate to your particular sensor using an exhaust
gas analyser... and then that would only work if the temperature
of the sensor was constant... and it would change over time
as the sensor aged...

> >From what you're saying, Assuming normal operating parameters with a good
> running engine, would 800-900mV indicate 14+ to 1?  Would 450mV be a good
> mixture?

Not really.  800-900mV means rich, likely between 12:1 and 14:1.

450mV is stoich, 14.7:1, but you'll never see the output stay
at 450mV.  The curve of voltage against air/fuel ratio
is so steep at 14.7:1 that you can't manage the fuel to
get 450mV.  You are always higher or lower!

> GRUMPYNOMO wrote:

> > Don't think of the O2 sensors indications as rich or lean.  Think of them as
> > "all oxygen used up" (around 900mv) and "some oxygen left over after
> > combustion" (around 100mv).
> > This may seem picky but if you see a low voltage and assume the engine is
> > running lean, you may be looking at incomplete combustion or a miss.
> > More significant yet is a jagged waveform at the high (rich?) side which might
> > look like a cylinder that is lean but could be 'puff' of EGR gas (not rich)
> > from a faulty valve.
> > Sensors vary but popular pattern is a variation from 0mv to 100mv at the low
> > side to  800mv to 900mv at the high side with an average of 450mv.
> > Oxygen sensors will never stay at the mid point.  They will flip-flop from
> > seeing no spare oxygen to seeing some oxygen remaining in exhaust stream.
> > They are incredible devices: so rapid and sensitive that, with a good scope,
> > you can identify vacuum leaks and bad injectors.  They are a very
> > underutilized diagnostic tool.