O2 sensor voltage at idle? 3ac tuned for smog check

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xirdneh
Highest Ranking Member
Posts: 1977
Joined: Thu Aug 04, 2005 11:38 am
My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by xirdneh » Tue Apr 22, 2008 8:42 am

4wdchico wrote:
xirdneh wrote:Assuming all things are functioning properly except the O2 sensor.
Will that affect gas mileage?
What kind of gas mileage are you getting now?

I posted in the "general discussion" "quest for better mileage" the various mileage of my three tercels.
basically one (white car) gets 35-40 mpg @ 125 compression, re-ringed engine, abt 250,000 on O2 sensor.
(blue car) 26-32 mpg @ 155 compression, 200,000 miles on engine and O2 sensor
(yellow car) 26-30 mpg (unkown compression) recently changed engine/sensor (abt 130,00 orig miles on both)

mileage seems to drop during winter months (abt 2-3 mpg)
Love those Tercell 4x4 wagons but they sure suffer from road noise.

4wdchico
Highest Ranking Member
Posts: 563
Joined: Sun Mar 30, 2008 6:51 pm
My tercel:: 1985 tercel 4wd
Location: Chico, Norcal

Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Tue Apr 22, 2008 8:55 am

xirdneh wrote:
4wdchico wrote:
xirdneh wrote:Assuming all things are functioning properly except the O2 sensor.
Will that affect gas mileage?
What kind of gas mileage are you getting now?

I posted in the "general discussion" "quest for better mileage" the various mileage of my three tercels.
basically one (white car) gets 35-40 mpg @ 125 compression, re-ringed engine, abt 250,000 on O2 sensor.
(blue car) 26-32 mpg @ 155 compression, 200,000 miles on engine and O2 sensor
(yellow car) 26-30 mpg (unkown compression) recently changed engine/sensor (abt 130,00 orig miles on both)

mileage seems to drop during winter months (abt 2-3 mpg)
Wow that is pretty good mileage on the white car. Is it a 4wd? What year?

Best mileage I have heard on the tercel 4wd is 34-35 mpg highway by a friend that had bought a new '84 back in the day. He is a pretty conservative driver and keeps good records.

Is the yellow car getting better mpg since you did the work?

xirdneh
Highest Ranking Member
Posts: 1977
Joined: Thu Aug 04, 2005 11:38 am
My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by xirdneh » Tue Apr 22, 2008 10:04 am

[/quote]

Wow that is pretty good mileage on the white car. Is it a 4wd? What year?

Is the yellow car getting better mpg since you did the work?[/quote]


the white car is the odd duck
1983
others on this site have posted 35 plus milage
seems to be rare but it does happen
i really feel the need to know why
i am not very good at figureing out carb/emmission/vac adv. issues
when something goes wrong i'm a parts switcher
i keep a good supply of spare parts

the yellow car gets the same mileage as it did with the last engine (which was good but rear main seal mounting plate gasket blew out, it will go back in another car)

i have tried four different carbs (i rebuilt them, also did the white car rebuild) on blue car with no mpg difference

the major mechanical differences between the white car and the blue/yellow are
white car 83 tranny , low compression, no power steering, some pinging, smallest
diameter tires of three
blue 86 tranny, mid range compression, power steering
yellow 86 tranny , mid range compression, power steering
Love those Tercell 4x4 wagons but they sure suffer from road noise.

rocketscience
Top Notch Member
Posts: 139
Joined: Wed Oct 28, 2015 12:46 pm
My tercel:: 1983 4WD SR5
Location: Tenino, WA

Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by rocketscience » Wed May 30, 2018 10:54 am

4wdchico wrote:
Tue Apr 08, 2008 1:43 pm
Mac wrote:.9V is the diatonic ratio, 14.5:1 air:fuel mixture.

lower voltage means lean, higher voltage means rich.
No, the chemically perfect air/gasoline ratio is 14.7/1 and a correctly functioning o2 sensor will read the results of it's combustion in a healthy 4 stroke gas engine as 450 millivolts (.45V). Less than 450mv is lean more than 450mv is rich. A feedback fuel system (FFS in tech speak) is designed to switch the mixture back and forth across the 450mv threshold to enable a 3 way cat converter to do it's job with max efficiency.

The 3ac terc 4wd's FFS only does the switching between rich/lean above 1300rpm. The Idle fuel mixture is set by the old idle drop method that is often called for on old pre-FFS vehicles.

I'm a California Clean Air Course (smog tech) graduate. No, I never intended to do smog for a living, just wanted to not have to rely on techs who a lot of the time really don't know what they are doing...
I know I'm bringing up a 10 year old thread but I'm doing troubleshooting on a rich condition and have a question. 4wdchico said that the feedback system only uses the o2 sensor above 1300 rpm but I wasn't aware that the ECU on these cars had any kind of tach sensor, so how could this be so? It looks more like it would cycle between open/closed mixture loop depending on coolant temp instead. Also, there was some disagreement about the voltage reading, whether it should be .45 or .9 at the diatonic ratio; I've read that the voltage of a normally operating o2 should fluctuate between .1 and .9 like an AC sine wave, so using a graphing voltmeter or a oscilloscope is the best way to monitor it, which is I think what the confusion is about. You should see it get up to .9. if it isn't oscillating or is trending more towards the high or low end of the spectrum it'd be either faulty or reporting on a rich or lean condition. Does this sound right?

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