oxygen sensor idle voltage

cashman · 01-28-2012, 01:47 PM

My car runs perfectly fine with no idle or acceleration issues but I was looking for a new project to do so I thought I would check the condition of the O2 sensor and adjust the CO. This is what I found:

After the car was warmed up, I disconnected the O2 sensor and hooked up my Multimeter. Voltage was .3. I tried to adjust the AFM screw to raise the voltage to .4-.6 and found I could not get a stable reading. At times, I would be at .5. Then all of the sudden I would drop to .1. Basically, I can't get a stable .4-.6 and it fluctuates often.

I hooked up the O2 sensor and I have a constant .5-.55 condition during idle and high RPM. Therefor, I am assuming my O2 sensor is fine.

Is there a reason I can't adjust the AFM to read a constant setting with the O2 sensor disconnected? Is this indicative of a vacuum leak? If so, Any ideas where? If I remove the oil cap I do get a slight drop in RPM so at the moment I don't think I have any leaks.

Again, no problems with the car. Just trying to check the O2 sensor and adjust the CO.

cashman · 01-31-2012, 08:58 AM

Bump
C'Mon, don't let me down. I know someone has something valuable to add. You always do.

Jesse16 · Jesse16's Garage

Looking for a new project eh. Should've been here lately when mine wouldn't start after putting the engine back in with the new clutch. (big dummy move by me that finally got corrected) Saw your note and since I was just in checking lots of sensors, etc., I cracked the bentley and looked up your info., which I'm sure you already know. Did the voltage drop when you took off the oil cap ? All it says in the book is that the voltage will "fluctuate", .1 to .9 volts. Be interested to hear if anyone says that the system should "settle" and not fluctuate within some reduced range. Did you take your AFM apart and do a rebuild ? I read that there is some maintenance that can be done if you're looking for an easy project.

T77911S · 02-01-2012, 04:09 AM

i dont k now if this has anyhting to do with it, but i know when checking the mixture with an LM2, my readings are only good for about 15-20 seconds of idling. IE, you cant let it idle for long periods.
just a speculation.

schumicat · 05:04 AM

I think it is normal to fluctuate like that. if you have a car with a narrowband a/f gauge (which is basically just reading the voltage), you can see it dances constantly. the led ones are basically a light show at idle and only will give a steady reading when given lots of steady throttle.

hcoles · 02-01-2012, 05:01 AM

Steve Wong will know but ...
On a Motronic when on the non-full-throttle part of the map the O2 signal will continually swing back and forth. I've read that as the O2 gets old or is wearing out the swing frequency gets lower.

wwest · wwest's Garage

Since an oxygen sensor is a binary device, one level of voltage for oxygen being present, another for not, the engine ECU "dithers" the mixture up and down continuously to keep an average mixture ratio around the optimal. Disconnect the sensor output and the ECU will probably drive the mixture in accordance with a factory default parameter.

Normally connected and operating correctly the ECU expects to see a voltage output from the oxygen sensor that moves up and down, triangular waveform, in synchronization with the dithering of the A/F mixture.

rusnak · 02-01-2012, 04:02 PM

I don't want to give erroneous information, but I recall one of our resident ECU guys writing last year that the 3.2 DME lacks the ability to confirm whether the O2 sensor is present, and therefore there is no "limp home" mode once the engine is warm. If the O2 sensor is disconnected, it would therefore seem that the DME continues to operate as if the O2 sensor is working, it just does so with erroneous information or no information. In other words, the DME is adjusting injector pulses as if in closed loop when in fact it's operating open loop.

What I usually do is run an LM-1 in a second bung, with the O2 sensor installed and working, in order to check in on the status of closed loop running.

cashman · 02-01-2012, 04:24 PM

Thanks for the responses. I knew you (Pelicans) would come through.

Let me clarify my observations noted above.
During Idle - The Voltage would bounce slightly but within a limited range. It started at .3-.4 range with slow movement. I tried to adjust the AFM to get it up to the .4-.6 range. I was successful doing this but then all of the sudden I would read .1-.2 range and the AFM adjustments appeared to be doing nothing. Turning to either extreme.

Once the sensor was plugged in, It would hold a .5-.55 slow bouncing consistent reading. This was true at idle and under higher RPM.

Once disconnected, I would experience the same symptoms as noted above with the AFM adjustment appearing to have no permanent hold. I would fluctuate in the .1-3 range for a while, then it would climb to the .5-.6 range for a while, then back down.

My understand is that .5 would be an ideal average. Therefore I should have a fast fluctuating voltage of (.1-.9) or (.4-.6) etc.

I don't have the LM1 so I was trying to check things out using the O2 sensor as noted in the Bentley.

rusnak · 02-01-2012, 04:39 PM

Normal operation shows the afr oscillate from .97 to 1.03 lambda, probably every second or two at steady throttle with light load. This is normal. As far as testing at idle with no load, probably not very useful.

T77911S · 02-02-2012, 04:08 AM

how many miles on the O2?

cashman · 02-02-2012, 08:40 AM

50,000 on the car. Not sure about the sensor but it doesn't look 22 years old. Maybe it is, I have had the car almost 5 years.

wwest · wwest's Garage

Since the sensor voltage is more triangular, quite possibly also with a DC component, a voltmeter will not, can not, read the true average.

An O'Scope would be my recommendation.

But I strongly suspect that your are headed down a dead-ended road.

When you hook up the sensor the control ECU will seek, and "find", the A/F mixture ratio that satisfies "its" factory stored/defined control parameter for O2 sensor output voltage.