Contour Enthusiasts Group Archives Forums Modifications Duratec Performance O2 sensor voltage to A/F ratio conversion?

OK, I will be doing a before and after with the ecotek valve this weekend, so I'd like to know my way around O2 sensor voltage. AFIK, 0.86 is lean, and 0.88 is rich...

Anyway I can convert these voltages to A/F ratios? What is a safe A/F or voltage on a steady 80mph cruise? What about WOT?

TIA

Not as simple as the voltage from the Lambda sensor, Its actually a complex alternating signal and you need an osciliscope to check it. The Computer counts the pulses per second as the shape, the residual dc voltage you see is not the whole story

4.5 volts is approxamately lambda, but I have seen duff sensors give this when well off

Actually, the latest version of OBD II software gives the O2 reading and plots it as an osciliscope reading, indicating rich and lean. If I recall correctly, the O2 sensor is a square wave sensor.

0.88 is by no means rich! That is very lean and 0.86V at WOT would be seriously causing some damage.

Ideally for a Duratec engine. O2 voltages of ~0.935V yield the best and safest results.

You might find this useful:

http://robrobinette.com/autometer_af.htm

Not sure how consistent the temp is on our O2 sensors, but I wouldn't be surprised to see a fair difference between normal driving and hard acceleration. This will change the 'ideal' O2 sensor voltage that you're shooting for.

Therefore I don't think it's as simple as saying "you need this exact voltage". Given that normal (stock) Contours seem to run >0.9V I would say that is probably the safest route.

Note that the GN guys evidently look for voltages in the 0.75-0.8 range. And they're running turbod cars. This is probably caused by the higher exhaust gas temps that they see thanks to turbocharging [notice in the web link that the O2 voltage goes down for a given A/F ratio as the exhaust temp goes up].

-Lance

My OBD system was reading 0.84-0.86 volts at WOT in fourth gear at the time I checked it in January. When I went to the dyno with NO changes to the car, the wideband O2 sensor read 12.25:1 in the same gear and the same points. This means that for me, my O2 sensors are running 0.86 when I have an optimum AF ratio. I wouldn't expect the voltages to be the same on every car.

warmonger

Standard HEGOs aren't really oxygen sensors--they are more like oxygen switches. (Technically, this isn't really true either since they are actually voltage generators, but this isn't really important for this discussion.) Thus, they will have one voltage when they detect more oxygen than their switching threshold and another voltage when they detect less oxygen that their switching threshold (but it isn't usually quite a square wave function). Unfortunately, these two voltage values may vary depending on the HEGO part number (and how much contamination and aging has affected them). The RMS voltage values for rich versus lean (but still switching) will vary according to this variance. You would be better off to scope the signal (or do approximately the equivalent with a scan tool).

If you really want to do some tuning, you should replace your HEGOs with UEGOs. They cost the OEMs about 10 times as much, so they probably cost 2-3 times as much on the open market. :rolleyes: UEGOs have a continous voltage range where the voltage output is actually proportional (more or less) to the difference between oxygen concentration in the exhaust and in the atmosphere (yes, altitude affects your O2 readings).

The last post hit it right on the head. Let me put this in non-tech speak:

The O2 sensors are highly accurate right at stoich, which is approx. 0.45 volts for our OEM sensors. Anything away from there (leaner or richer than stoich 14.7:1) and your accuracy goes down, and is influenced in a greater degree by the temperature of the tip.

What the PCM does is richen when below 0.45 and lean when above 0.45 (using short term fuel trim). In doing this, it keeps the "average" A/F ratio at stoich during closed loop operation. Always bounces back and forth.

You can't say that .86 volts is xxx A/F ratio for all cars, since there are too many variables. (BTW, 0.94 is about 12:1 on my car.) What I have done is recorded my O2's during a dyno run at a known A/F ratio and use that to tune my S-AFC. Best that I can get w/o using a wideband O2 sensor (like on the dyno).

Bottom line, if the number is going lower, you are probably running leaner, but there is a pretty good precision error on these things away from 0.45.

Do you have converters on your car? (1-3?)

If so then wideband readings are irrelevant (for the most part) since the cat's scrub the A/F ratio before the tailpipes.

Not picking on you per se; I've brought this fact up before...

My 02's read around .86v now. (I run at .90v with the S-AFC until I have it dyno-tuned) Before the last 2 mods it was around .87-.88v and last spring before the exhaust it was .90v
So that denotes it is getting leaner and leaner.
For the record the my dyno was around .90v range. It showed high rpm flutter for (as of yet) some undetermined reason. (KKM, Advance, fuel???)

I didn't have them run a wideband O2 at the dyno because the shop owner himself said it was worthless with cats. He could have easily just hooked it up and charged me for it.
It will always read about the same on any car with cats even though it's not truelly the same before the cats.

Werd Demon. My dyno guy went through the trouble of sticking one where the UPSTREAM sensor is on Bank 2. Doesn't even have one of the "up the tailpipe" style. Was going to try the downstream location until I told him about the pre-cats.