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Help me understand O2 sensor readings by Alex4mula
Started on: 08-25-2000 08:51 AM
Replies: 6
Last post by: terryk on 08-25-2000 06:28 PM
Alex4mula
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Report this Post08-25-2000 08:51 AM Click Here to See the Profile for Alex4mulaSend a Private Message to Alex4mulaDirect Link to This Post
Have been reading some articles & now I'm confuse. Have heard that when reading the O2 sensor (AutoXray here) the key rule is it to read over 800mV. But at which RPM? I'm interested at WOT. Should it read over 800 from start of WOT all the way up to 6K RPM? Only over 5k? Only in the area of RPM of peak torque/HP? What about at partial throtle?
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elfiero
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Report this Post08-25-2000 11:26 AM Click Here to See the Profile for elfieroClick Here to visit elfiero's HomePageSend a Private Message to elfieroDirect Link to This Post
OK,this is a little difficult so follow
carefully-
1.the O2 sensor doesnt care about throttle
position,RPM,nothing.
2.800mv is the "static" or meidian voltage
output from the sensor when the engine is
burning a air/fuel ratio of 14.6to1.
3.what you can't see is the sensor output is
rapidly swinging from 0 to 1.0v.the computer
is "reading" this and adjusting the fuel
input to maintain 14.6to1.
4.heres is the tough part...the scanner can
only look at "glimpses" not the raw data
stream.So you can't use this info for any
critical tuning
5.NOTE*-the ECM does not use O2 sensor input
in the WOT mode!If anyone tells you different
they have their heads in a dark,smelly place.
6.reading between the lines,I assume you want
to monitor your WOT A/F ratio...the best way
to do this is w/an aftermarket gage with its
own O2 sensor.if I can be of further assistance,let me know.
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Alex4mula
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Report this Post08-25-2000 12:07 PM Click Here to See the Profile for Alex4mulaSend a Private Message to Alex4mulaDirect Link to This Post
Thanks. Very good info. I know I see at glimpses (samples) cause the Xray jumps like every second. But if all readings are on the same side then I could guess is either lean or rich depending on side and how much depending on voltage.
The ECM may not use the O2 at WOT but the O2 reading still could be usefull with the above info.
I guess the aftermarket gage will give me better resolution (more samples/sec) but still giving the same info.
I'm just trying to do some rough tuning with my FPR before taking it to a Dynojet with all the A/F analysis.
Still my question is how do you want the reading to be across the RPM range on partial load & WOT?
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elfiero
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Report this Post08-25-2000 01:04 PM Click Here to See the Profile for elfieroClick Here to visit elfiero's HomePageSend a Private Message to elfieroDirect Link to This Post
remember,the closer you are to the perfect
A/F ratio,the wider the "swing" will be.The
problem is you can't see it with the scanner
-you may only see high numbers then all of a
sudden you'll see a low one and say"where did
that come from?" when in reality,it has been
swinging back and forth all the time,but the
scanner just hasnt "caught"it.thats why this
info is useless-I'm not sure I am getting
across what I'm try'in to say,so thanks for

being patient.
P.S.I forgot if your scanner monitors
"cross counts"-that is the number of times
the O2sensor is crossing over the "median"
value of 800mv this is a "real time" reading
relative to the baud rate of your ECM(year)
the '84&85's are real slow,but'86to'88 are
OK

[This message has been edited by elfiero (edited 08-25-2000).]

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Alex4mula
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Report this Post08-25-2000 01:28 PM Click Here to See the Profile for Alex4mulaSend a Private Message to Alex4mulaDirect Link to This Post
The scanner reads cross-counts and injector pulse. Could that help me more? I appreciate your help & don't worry I'm very patient.
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Cooter
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Report this Post08-25-2000 02:19 PM Click Here to See the Profile for CooterSend a Private Message to CooterDirect Link to This Post
800 Mv is rich, maybe a little too rich for maximum power (thanks Terry!), but it is safe. A "Perfect" mixture is closer to 450 Mv. But, the system maintains an average mixture by swinging from rich to lean.
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terryk
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Report this Post08-25-2000 06:28 PM Click Here to See the Profile for terrykSend a Private Message to terrykDirect Link to This Post
800mV is the *unknown* rich. On one O2 it could be 12:1 and 12.5:1 on another, etc. etc. The O2 is only accurate at 14.7:1. With the bias voltage from the ECM, the O2 will be at .456V.

Without an EGT, 800mV is the safest O2 value for WOT. Below that, you are risking piston/valve failure. Maybe yes, maybe no. Wanna find out?

If you go with the 800mV rule, you want the O2 above 800mV at all RPM's when you are in power enrichment (which is above 3600 RPM and a change in TPS% of 9% or more).

When you are not in PE, the O2 is controlling the fuel. The key here is that the BLM and INT are around 128 at steady throttle. The O2 will then be switching equally above and below .456V (14.7:1). This is called a bang-bang controller. The ECM will go rich then lean equally. The average will be 14.7:1.

I'm not sure I made any sense there......

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