I've just installed a new single wire O2 sensor into my car as I suspected the old one was past its use by date. I then hooked up a DMM to watch the output voltage of the new sensor.

I looked on howthingswork.com to see what the various voltages meant but I'm not sure I understand it, or how applicable they are to a 3rd gen, but maybe someone here can tell me what my car is doing.

With the car warm, at idle I get a reading of .22V. When cruising at 60MPH I'm getting a reading of about .88V. While cruising at 60MPH, if I take my foot right off the accelerator the voltage reading drops to almost 0V. If I cruise at a lower speed of about, say 40MPH, I get a higher reading of .98V.

What are these readings actually telling me and are they normal?

In addition to the car details in my sig I have also removed the air system from the exhaust manafold and also the charcoal cannister. The carb has a bung over where the control valve etc plugged into it. PCV and EGR are still on the car.

 

Your ECM has several operating modes.

When you lift off the pedal all the way, the ECM shuts down the fuel injectors, so you should get a near zero reading from the O2 sensor.

During cruising around stuff, less than 70% or 80% throotle, the ECM is in closed loop mode and is trying to use the O2 readings in a feed back loop. The ECM is trying to keep the AFR at about 14.7:1... Now, since these O2 sensor are the switching type, you should see the O2 readings bounce around wildly above and below 450mV which is the perfect 14.7:1 AFR value.

In power enrichment mode (PE), the ECM ignores what the O2 sensor is seeing and the ECM applies the extra fuel which is commanded from various tables. 880mV from the O2 sensor is about 12.8:1 to 13.0:1 AFR. However, these O2 sensors are very inaccurate away from 450mV, so you can not count on yours seeing 880mV and be at 12.8 AFR.

Well, did I help, or confuse things????

 

Originally posted by doc
Your ECM has several operating modes.

When you lift off the pedal all the way, the ECM shuts down the fuel injectors, so you should get a near zero reading from the O2 sensor.

I have a Quadrajet, I'm not fuel injected. But yes, I do get a near zero reading.

Now, since these O2 sensor are the switching type, you should see the O2 readings bounce around wildly above and below 450mV which is the perfect 14.7:1 AFR value.

I figured I should be seeing a figure in the middle of 0V and 1V while cruising, but I don't.

Well, did I help, or confuse things????

Kinda helped thanks. I get the basic idea now, but I'm still wondering about my car. Is the O2 sensor telling me I'm running too rich at 60MPH with a reading of .88V? The value I see doesn't bounce around a lot at all, it's fairly stable.

 

ttt

 

Not sure that's a valid test. If you do not have the O2 hooked up to the ECM it will throw a code and go into "limp home" mode- altering the signal sent to the carb's mixture control solenoid. If you DO have it hooked up to the ECM and are tapping into the wire you might be getting odd readings having both ECM and voltmeter both reading off the same wire.

Get a scan tool and see it how the ECM is seeing it.

 

Originally posted by Damon
Not sure that's a valid test. If you do not have the O2 hooked up to the ECM it will throw a code and go into "limp home" mode- altering the signal sent to the carb's mixture control solenoid. If you DO have it hooked up to the ECM and are tapping into the wire you might be getting odd readings having both ECM and voltmeter both reading off the same wire.

I still have the ECM hooked up, my reading is by taking a parallel reading with the DMM. The internal resistance of the meter is enough not to effect the reading of the sensor.

Get a scan tool and see it how the ECM is seeing it.

Unfortunately I don't have access to one of those in this country.

Any other thoughts?

 

Yes, a 2Mohm/V digital meter shouldn't drag down the signal enough to affect it significantly. Even at 0.3V, the meter's internal resistance should be at least 650K ohms.

In a theoretical nutshell, 450mV is a "neutral" sensor output at a stoichiometric (14.7:1 air/fuel) ratio exhaust gas oxygen content. Lower output voltage is a leaner ratio EGO, higher output voltage is a richer ratio EGO.

Your mV readings at the sensor should give you some indications of what the ECM is reading, and you can surmise how it might be reacting to that.

 

Originally posted by Vader
In a theoretical nutshell, 450mV is a "neutral" sensor output at a stoichiometric (14.7:1 air/fuel) ratio exhaust gas oxygen content. Lower output voltage is a leaner ratio EGO, higher output voltage is a richer ratio EGO.

Yeah that's what I figured.

Your mV readings at the sensor should give you some indications of what the ECM is reading, and you can surmise how it might be reacting to that.

Cool, this is what I wanted to hear. So basically my car is running too rich. I'm sitting on round 880mV on highway cruising and it's richer for round town. So obviously my carb needs tweaking. Fair enough statement you think?

 

Well worth the read,

Carburetor Tuning the Scientific Way and Other Fuelish Ramblings

its says holly carb at the top of the page but it can help anyone. Especially understand how to tune with an O2 sensor on any car with any fuel system.

http://www.bob2000.com/carb.htm

 

Well, at least your oxygen sensor thinks your engine is running rich. That may or may not be the case. False rich indications are more rare than false lean indications, so the sensor is probably close to right.

As for the second part, carburetors always need tweaking.