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

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4wdchico
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O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Sun Apr 06, 2008 3:14 pm

I'm prepping my '85 4wd M/T terc for the CA smog check, yes it is a CA car. I see in the FSM that the fuel feedback system does not control the mixture below 1300 rpm. What kind of 02 sensor voltages are you folks seeing when at idle on a terc like mine when the idle drop idle mixture adjustment method has been done? Wondering as I'm only seeing around 125 millivolts. That seems pretty lean to me, especially as I'm pretty sure that I hear a slight lean misfire. The system switches rich/lean across the 450MV midline just like it should at higher rpms.

Then, when the cooling fan comes on, the system goes pretty rich at 750-780 MV. Does this seem normal? The FSM does not really address this particular issue other than cautioning not to perform carb adjustments while the cooling fan is running. Seems like I am doomed to failing the idle speed portion of the emission test if the cooling fan comes on while the exhaust is being sampled.

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

Post by takza » Mon Apr 07, 2008 4:19 am

My O2 sits at around .8v after warm...doesn't cycle...doesn't work?
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Orwell said: War is peace! Freedom is slavery. Ignorance is strength...

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

Post by Mac » Tue Apr 08, 2008 3:35 am

.9V is the diatonic ratio, 14.5:1 air:fuel mixture.

lower voltage means lean, higher voltage means rich.
Tercel 4WD "POWER WAGOON" with 4A-C
aka: "no powa steering tercel, oh oh oh!"
mods: ignition at 10 DBTDC and 90 octane gas.

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

Post by takza » Tue Apr 08, 2008 5:40 am

Mac wrote:.9V is the diatonic ratio, 14.5:1 air:fuel mixture.

lower voltage means lean, higher voltage means rich.

They are supposed to cycle...can watch this with a digital meter...from online FSM...

http://www.nonags.org/members/nijqk/O2sensortest.jpg
Give a boy a gun-give a biatch a cell phone-and pretty soon you almost got yourself a police state.

Orwell said: War is peace! Freedom is slavery. Ignorance is strength...

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4wdchico
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Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » 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...

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

Post by Mac » Thu Apr 10, 2008 12:57 am

i never said the O2 sensor was not suppose to switch, i have no idea where that came from.

the only reason i can see for a .45V reading is California cars are set up differently because every time I plug in the OBD scanner and view the O2 sensor data, or a multimeter to the O2 sensor, it was or at least close to .9V

agree with the comment on techs not knowing what they are doing. there are lots of crappy doctors, lawyers, plumbers, business execs, etc... but why is the mechanic trade thought to be any different?
Tercel 4WD "POWER WAGOON" with 4A-C
aka: "no powa steering tercel, oh oh oh!"
mods: ignition at 10 DBTDC and 90 octane gas.

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

Post by takza » Thu Apr 10, 2008 4:41 am

Mac wrote:the only reason i can see for a .45V reading is California cars are set up differently because every time I plug in the OBD scanner and view the O2 sensor data, or a multimeter to the O2 sensor, it was or at least close to .9V
I try at least to run EFIEs on cars and I set up a multimeter to watch and adjust the EFIE and watch for O2 cycling while driving....my Terc (Cal special) stays at ~.8v...no cycle. O2 maybe 2 years old.
Give a boy a gun-give a biatch a cell phone-and pretty soon you almost got yourself a police state.

Orwell said: War is peace! Freedom is slavery. Ignorance is strength...

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4wdchico
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Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Wed Apr 16, 2008 8:14 pm

Mac wrote:i never said the O2 sensor was not suppose to switch, i have no idea where that came from.

the only reason i can see for a .45V reading is California cars are set up differently because every time I plug in the OBD scanner and view the O2 sensor data, or a multimeter to the O2 sensor, it was or at least close to .9V

agree with the comment on techs not knowing what they are doing. there are lots of crappy doctors, lawyers, plumbers, business execs, etc... but why is the mechanic trade thought to be any different?

The 450mv thing is not specific to CA cars, it is actually very generic, with the exception of a few vehicles that use wideband 02 sensors. Below 450mv = lean and above 450mv = rich for a standard 02 sensor, no way around it. The standard 02 sensor cannot exceed 1000mv output except in really unusual circumstances. You can prove it to yourself by getting your hands on any late model OBD II car so that you can view the sensor data with a scanner. Drive the engine rich by introducing increasing amounts of propane into the air intake & watch the 02 voltage rise to nearly over 900mv, the engine will die before you see 1000mv. Then scan for codes. The PCM should have set a fuel control code referring to compensating for a over rich condition. Clear the code. Disconnect and pinch off the vac brake booster line. Start the car and put your thumb over the end of the hose and unpinch the hose. Slowly create an increasing vac leak, by lifting your thumb very slightly, while watching the 02 sensor volts on the scanner the voltage will drop way below 450mv and the engine should die somewhere below 100mv and the PCM should set a code referring to compensating for an overly lean condition.

The above is the best way to see what is going on with an 02 sensor and understand it's function. You are removing control from the PCM (power control module, the ECU term is badly outdated) by manually driving the mix rich/lean and observing the results. This is the only way that you can read 02 voltage with a voltmeter or scanner on a modern car as they switch rich/lean so fast that these tools are useless to see what is really happening when the PCM is in fuel control.

If you want observe and measure the 02 sensor voltage of a modern car, while it is in fuel control, you need to use a digital storage oscilloscope (DSO) so that it can store the sensor data and graph voltage vs. time in really small segments of time.

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

Post by Mac » Fri Apr 18, 2008 11:12 pm

i know exactly what is going on, I'm no stranger to OBDII scanners, O2 scanners and lean/rich running conditions. Just because my DD is from 1983, doesn't mean I've never touched or worked on anything newer than 1996.

Its also preposterous to say a O2 sensor is incapable of creating 1 volt unless its a wideband; with the correct air fuel ratio any O2 sensor will make 1 volt. I figured out what you are talking about with the voltage; 0.45V is the average voltage while 0.9 is the higher of the two numbers you see, and the one you should be paying attention to.
Tercel 4WD "POWER WAGOON" with 4A-C
aka: "no powa steering tercel, oh oh oh!"
mods: ignition at 10 DBTDC and 90 octane gas.

4wdchico
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Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Sat Apr 19, 2008 12:51 am

Mac wrote:i know exactly what is going on, I'm no stranger to OBDII scanners, O2 scanners and lean/rich running conditions. Just because my DD is from 1983, doesn't mean I've never touched or worked on anything newer than 1996.

Its also preposterous to say a O2 sensor is incapable of creating 1 volt unless its a wideband; with the correct air fuel ratio any O2 sensor will make 1 volt. I figured out what you are talking about with the voltage; 0.45V is the average voltage while 0.9 is the higher of the two numbers you see, and the one you should be paying attention to.
Not trying to make this a pissing match, but I'm confused , why would 900mv be "the one you should be paying attention to". I think that is where our miscommunication lies. I have never been able to drive an engine over rich (with propane or raw fuel)to the point of seeing over 950mv or so from the 02 without it just dying from being so far over rich. Since I have tried several times to see if I could get an engine to run while it's 02 sensor hit 1V I really don't think that it is possible in the real world.

To restate the kernel of what I'm trying to say in this thread is that: the perfect mixture is a 14.7 to 1 stoichiometric air/fuel ratio. So given a stoichiometric "perfect" air/fuel mix (14.7 to 1) a correctly functioning 02 sensor will read the results of it's combustion in a healthy 4 stroke gas engine by producing 450 millivolts (.45V). That means that 450mv is not significant because it is an average, it is a significant point because that is where a theoretically perfect air/fuel mix will cause an 02 sensor to produce that voltage. I was simply trying to help you see that the statement: .9V is the diatonic ratio, 14.5:1 air:fuel mixture. lower voltage means lean, higher voltage means rich will lead you really far astray in diagnosing engines if you continue thinking that less than 900mv is lean as any voltage over 450mv is actually rich. Please see this for more details: http://www.aa1car.com/library/o2sensor.htm Pretty good stuff.

BTW thanks for the sweet new word: diatonic. Really cool stuff about ratios in music and crop circles. Just not sure how it applies to the issue in this thread.

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

Post by Mac » Sun Apr 20, 2008 5:28 pm

your the one who turned it into a pissing match.

my girlfriends escort was showing 1.1 volts before i had to ajust the air flow meter to compensate for a weak spring inside.
Tercel 4WD "POWER WAGOON" with 4A-C
aka: "no powa steering tercel, oh oh oh!"
mods: ignition at 10 DBTDC and 90 octane gas.

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

Post by xirdneh » Mon Apr 21, 2008 8:43 am

Does anyone have an idea as to how long these O2 sensors last?
Does one need to be concerned once they reach a certain mileage age?
Can they last indefinatly?
Love those Tercell 4x4 wagons but they sure suffer from road noise.

4wdchico
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Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Mon Apr 21, 2008 10:06 am

The unheated 02 sensors used on the tercs and many of the first FFS vehicles don't last really long. The vast majority of older cars that fail a smog check need a new 02 sensor. Bosch says 30k miles lifespan on the unheated units. They may be right, given the cost of a smog check these days, I always have new plugs, 02 sensor and oil and filter minimum before taking mine down for the inspection.

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

Post by xirdneh » Mon Apr 21, 2008 11:40 am

Assuming all things are functioning properly except the O2 sensor.
Will that affect gas mileage?

All of my tercels have mucho mucho miles on the O2 sensors.
I do not have to pass any smog tests
Love those Tercell 4x4 wagons but they sure suffer from road noise.

4wdchico
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Re: O2 sensor voltage at idle? 3ac tuned for smog check

Post by 4wdchico » Mon Apr 21, 2008 4:20 pm

xirdneh wrote:Assuming all things are functioning properly except the O2 sensor.
Will that affect gas mileage?
Yes, the FFS can't switch rich/lean across the 450mv threshold where max mileage/ min emissions will occur if your 02 sensor is not functioning correctly. All feedback fuel systems are designed to go rich if something malfunctions to prevent the engine damage that a long term lean condition would cause. The problems associated with running rich, while not great, are not nearly as deadly to an engine's health. So in your case, since odds are your engine(s) are loaded up a bit with carbon from running rich, I'd suggest doing your decarbon treatment of choice followed with a valve adjustment, new plugs (NGK V power are my choice) and fresh oil and air & oil filters. After doing that, a new 02 sensor should give you a solid boost in mileage. Also, an engine running correctly will last longer.

If your car smokes if left at idle for a while, changing the valve stem seals is really cheap & easy, not much more involved than adjusting the valves on the 3a engine. Made a huge difference in mine, no smoke at all, ever.

What kind of gas mileage are you getting now?

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