[V8] O2 Sensor Readings

[urq]

I don't see any mention of the input impedance of the unit ... so I'd advise
caution.  As was correctly pointed out earlier, the output impedance of the
OXS is very high, and connecting a low impedance meter will change the
reading.  

Over the years I've realized that there is very little value to be gained
from monitoring the voltage output of the OXS.  If you suspect it is
malfunctioning, make sure you've eliminated all other possible culprits, and
then simply swap the OXS out for a new one.  The ECU varies the mixture all
the time to validate stoichiometric, so any sort of messing with the FPR or
adding HC to the intake just becomes a variable the ECU deals with as it
drives the mixture to stoich as best it can.  

The best tool for figuring out what is wrong with the engine is to use
VAGCOM or equivalent.  Unfortunately it appears that ProDiag has died ... so
unless you already have one it is no longer an option.  With ProDiag I can
read out and clear the ECU codes, and I can take a test drive and log the 10
parameters the ECU provides.  I suspect VAGCOM has the same capability.  You
can tell right away if the ECU has decided that it cannot control the
mixture effectively (generates a code) or if it is responding to an
erroneous input (like an incorrect coolant temp).  

Steve B
San José, CA (USA)

-----Original Message-----

This is exactly the tool you need - short of an oscilloscope.
There
are also volt meters with similar functions (bar graphs) built in, but
they will set you back a good $300 for a nice Fluke or equivalent. The
cheapo meters are not worth much in this application.

Dave



> Here's one that you can mount inside the car.  
It fits with 3 wires
> (+12v, signal wire on O2 sensor, and
Ground).
> 
> http://tinyurl.com/39bfnd
> 
> MikeL's V8Q was going through O2 sensors, the temp sensor was
the
> culprit.
> 
> He wrote to the u.washington
list:
> 
> "It's easy to test.  let car sit
overnight, then measure the resistance
> between the 2 pins on the
sensor.  Should be between 1.5 to 3 ohms.
> 
> Mine was at
4.5 and it made the car run so rich that you got a huge black
>
cloud when you stepped on it.  As well as a pile of soot under the
> tailpipe
> if you let it idle for 5 minutes.....
>

> BTW,  This is what ruined my 02 sensor :-("
> 
> 
> "d saad"
<dsaad at icehouse.net> writes:
>>
>>
>> I am not sure you can deduce much at all from this Scott.
>>
>> The
>> O2 sensor is not easy to read.
To do it right, you really need an
>> oscilloscope.
>> 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.
>>
>>
Dave
>>
>> > 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
>> VDC
>>
> 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:
>> 0.52*
>> > 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
>> back
>> > under the distributor and plugging in. 
Is this OE?
>> >
>> > Thanks,
>>
> Scott S.