BlueRidgeMark

'97, AW4.

I've been searching for a while, and no success. Lots of threads about melted wires and blown blue fuses, but no solid data on what voltage readings to expect from the 02 sensor while the engine is running.

I got my new installed today, and ran it with http://www.torque-bhp.com while on my commute home. Trying to figure out a random misfire.

The O2 sensor voltage, as displayed on my phone, was bouncing like crazy the whole time (with one exception), from 0 to about .75 volts.

The only time it stayed steady was while going down a steep grade, 3rd gear, no throttle at all. It went to zero and stayed there until the grade flattened out and I started using the throttle again.

Well, that's very interesting, but without knowing whats normal, it doesn't really tell me anything useful. I have a feeling it should be fairly steady, but I really don't know.

Anybody?

cruiser54

Normal.

CCKen

The upstream O2S Short Term Fuel Trim (STFT) should toggle about every 2 seconds.


See what the STFT is at idle then at 2,500 RPM.

Muddz

If your scanner software is able to display Mode $06 data (non-continuous monitoring) you can find some more information about how your o2 sensors are behaving. Mode $06 data is from non-continuous tests that's stored in the PCM. Some PCMs store more Mode $06 data than others depending on the vehicle and year.


Chrysler Mode $06

http://www.aa1car.com/library/2005/us010516.htm

BlueRidgeMark

Wow! Great information! Thanks all of you!

It was running... decently today, then I stopped to get the mail (about a mile from my house) and it started chugging pretty badly. Again, like flipping a switch. I didn't have the scanner running, but when I pulled into my driveway and got stopped, I got it going, and the O2 sensor was very stable at ~.18 volts @ idle. Ken, I'll try it 2 2500 rpm and report back.

DFlintstone

Here's more from Eco Mike...He gets into OBD/HO a few posts in. > http://www.naxja.org/forum/showthrea...ht=sensor+test)

BlueRidgeMark

Excellent stuff.

Yesterday was a down day for vehicles - I made a dollhouse for my daughter instead. Gotta keep the priorities straight.

Anyway, I fired it up today and it was running VERY badly. Fired up the Torque app and found that the O2 sensor was pegged at zero at idle. I watched it for maybe a minute or two and it didn't budge.

Had an errand to run, and when I got back I checked again. About .16 at idle and the same at 2000-2500 rpm. Pretty steady, no bouncing.

What should I be seeing?

BlueRidgeMark

Good info, Muddz. What software do you run on the laptop?

CCKen

I'm not sure what you are looking at but you should be able to see O2S1 (upstream O2S) and O2S2 (downstream O2S) voltages and STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) activity. STFT and LTFT are expressed in %. STFT is the PCM's immediate response to adjust the Air/Fuel ratio based on the voltage outputs of the O2S. The ideal STFT is zero.


You should also see the Open/Closed Loop announcement, i.e., You should see "OPEN" when you first start the engine and when the PCM time out it should announce "CLOSED"


When the PCM is in Open Loop the Oxygen Sensors are ignored by the PCM and will not show any activity on your scanner, except that the voltage readings should be 1.00 volts. When the PCM goes in to Closed Loop the Oxygen Sensors are brought in to play and you should be able to see O2S1 voltages toggle back and forth about very 2 seconds above and below the rich/lean crossover - approximately .045 volts (.001-.900). You may see the O2S1 voltages toggle between .120 and .760.


O2S 2 voltages will be a little bit slower but should never show the same exact voltage of the O2S1, or stay near .900 .


If the O2S voltage stays at 1.00 it means the O2S is dead.

Muddz

I once had an o2 sensor that would occasionally stick on 1 volt, causing the engine to run rough. Didn't produce any error codes. Took a while to discover, as it would stick only under certain conditions while driving and I didn't have a scanner with live readout at the time to observe o2 voltages.
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_ excerpt from online " When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor’s reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm)."
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Under certain conditions your o2 sensor may be possibly sticking at a zero 0 voltage and/or there is an intermittent open somewhere. I might try a jiggle test of the wiring harness to help diagnose a problematic wire or connection.

One method I've used to test o2 sensor(s) is to unplug a PCM sensor such as for air intake temperature. This will effect the PCM fuel map tuning and cause the O2 sensors to react to new exhaust conditions. A sticking o2 sensor sometimes will stick on one volt when another sensor is disconnected. Near zero voltage ~.1 is usually when there is similar amount of oxygen in the exhaust as the outside air being fed to the sensor.

Another test may be to disconnect both oxygen sensors. The PCM should revert to it's default fuel trim, similar to an open loop. If the engine starts to run smooth again, indicates a faulty o2 sensor and/or wiring.

The sensors only work correctly when heated within a certain temperature range. Possibly something wrong with the heater and/or wires. Once heated the sensor element (Zirconia) produces it's own voltage based on a stoic 14.7:1 air fuel ratio.

Good find, you found the fuse to be the problem.

OXYGEN SENSOR HEATER MONITOR

Once the o2 sensor reaches operating temperature 300° to 350°C (572 ° to 662°F), the Zirconia element generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.

ASD relay -------- Fuse 23 -------- upstream ------------ downstream (heater circuit)
G-117 ----------GND---------------------|-------(S107)--------|

PCM ---------------------------------- upstream (pcm sensor circuit)
PCM ---------------------------------- downstream
SENSOR GND ------------------------------|

Later models XJ, 4.0L - Lambda Sensor; 4 wire rectangular plug, Length: 1140mm, Diameter: 18mm, Thread: 1.5, 4 ohm, Zirconia

Muddz

It's been a while since I've used the laptop and elm module. In the past I have used some of the freeware & public domain type applications under Dos, Windows and Linux. The apps with more advanced functions are normally commercially available software, you usually need to pay for additional scanner features and diagnostics.

Elm compatible software:http://elmelectronics.com/obdsoftware.html

http://scantool.imechatronics.com/downloads.htm

You can also access an Elm by issuing commands from a terminal under Dos, Windows, Mac and Linux. Plus other operating systems designed for hand-held devices.

http://elmelectronics.com/obdic.html#ELM327

http://elmelectronics.com/ELM327/AT_Commands.pdf

http://elmelectronics.com/DSheets/ELM327DS.pdf

BlueRidgeMark

Okay, this is interesting!

It's been very busy this week - long days at work and no time to look at the beast. But!

On Monday before leaving work I remembered something, kicked myself a few times, and popped the hood to check the O2 heater fuse. Such a simple thing should have been done FIRST!

I didn't remember which fuse supplies the O2 heater, but I remembered it's a 15 amp, so I just pulled all the 15s one by one and gave a visual inspection. They all looked fine, but the contacts were a little bit grungy, so I put them in and back out a a few times to clean things off.

It hasn't run badly since. I wouldn't say it's running perfectly, but there's no hesitation or obvious misfiring. Idle is still a tad high, but not like it was.

I filled up that evening, and again last night, so I've run one full tank since cleaning off the fuse contacts. I got 19.6 mpg on that tank!

I don't think it's running rich anymore!

This really doesn't make sense to me. I don't understand why the heater would make such a big difference once the engine is up to temperature, but obviously, this problem has been there at all operating conditions from cold to fully up to temp.

cruiser54

R*C*B*R*C

BlueRidgeMark

Ehhhh.... what?

DFlintstone

Refresh Contacts Before Replacing Components?