[V8] O2 Sensor Readings

d saad dsaad at icehouse.net
Tue Jul 3 18:58:08 EDT 2007

I am not sure you can deduce much at all from this Scott.

O2 sensor is not easy to read. To do it right, you really need an
A nice digital volt meter with a calibrated bar graph
would also work OK - as long as the response time was fast enough. The
problem is that the sensor is a very high impedance device - so just
hooking up your meter affects the output. And the output is not a steady
state voltage - unless you are at extreme rich or lean condition. It is a
non-linear device with the usable range being a very narrow band around a
air/fuel mixture of 14.7. For most practical purposes, the sensor can only
tell you if you are above or below this number.
If you look at the
signal with a o-scope, you see something like a square wave - with the
duty cycle of the waveform indicating the mixture. A volt meter can give
you a rough idea of the duty cycle - because it averages the readings, but
it is a very rough indication.

And - no - your O2 sensor
wiring does not sound  OEM. Mine just snakes over the axle and right
into the connector. There are metal cable holders there to keep it off the
axle too.


> Can someone interpret these
readings for me?
> To induce a lean condition, I pulled
the vacuum hose off the FPR*.  To
> induce a rich condition, I
plugged the FPR's vacuum hose and/or sprayed
> some carb &
choke cleaner into the hose (it was the only thing flammable
> I
had, besides electronic contact cleaner.. which I sprayed too).

> First run, cold engine
> Idle:  0.01 - 0.03 VDC
> 1000 rpm:   0.02 - 0. 03 VDC
> 2000 rpm:   0.23 - 0.28
> 3000 rpm:   0.29 - 0.35 VDC
> 2500 Lean:  0.22 -
0.25 VDC*
> 2500 Rich:   0.23 - 0.25 VDC
Second run, hot engine
> Idle:  0.4 - 0.5 VDC
> Idle Lean:
 Very sluggish drop from 0.5 to 0.19* (sluggish meaning it
> took
3-6 seconds)
> 2500 rpm:  0.51 - 0.52
> 2500 Lean:
> 2500 Rich: Sluggish rise to 0.66
> * -
in the end, the lean condition from the vacuum hose could have been
> offset by the excess fuel dumped by the FPR
After the first run, I thought I'd nailed it.  A "lean" signal
from the
> sensor resulting in excess fuel being dumped which
would explain my poor
> mileage (8-10 city) and flutter/miss at
idle on occasion.  But the
> second run numbers look pretty good. 
The lean condition at idle did go
> down to the correct numbers,
albeit sluggish.  The Lean 2500rpm can't be
> trusted, IMO.
> The only thing pointing to a bad sensor is the sluggish
changes.  Should
> I go ahead and replace the sensor or wait until
my second-hand laptop
> arrives for VAGCOM?  Sensor replacement
was going to be a Ford but I
> haven't decided on 3 or 4 wire.  Is
4 wire more trouble than its worth?
> One last
question:  How long is the sensor wiring?  Mine comes up from
the sensor, zip-tied to the transmission tube, then it loops to the
> shock tower (zip-tied to the brake line brackets) before going
> under the distributor and plugging in.  Is this OE?
> Thanks,
> Scott S.
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