aggravation

I've read this somewhere and think I understand the basics of it. the presence of unburned fuel in the exhaust gas replaces or is read as oxygen? so the sensor see's the ratio as lean...
Is this correct and if so help me understand how much unburned fuel would have to be present in the mix in my '88 928 before the O2 sensor goes from telling the truth to telling the opposite.

I'm guessing in order for the O2 sensor to start reporting lean from excess fuel it would have to be "dumping fuel", as in clouds of black smoke.
And is the same true of a wide band O2 sensor as is with a stock narrowband sensor?

mark kibort

I had the alternator failure and my mixtures went way lean.
Power also went flat
Good have been way rich due to unburned fuel in the ehaust or way lean due to low fuel presure from low voltage on the pump
??????

WICruiser

I think what you are seeing is rich to the point of misfire - when there is no combustion the oxygen coming into the cylinder is not burned so the oxygen sensor in the exhaust (or air-fuel meter) sees what it thinks is lean - excess oxygen. It is not really the excess fuel that is being evaluated but the oxygen content.

In the case of the alternator failure I suspect the voltage drop was enough to change the air-fuel ratio control and ignition timing so it probably was actually running lean.

123quattro

A wideband will generally read A/F ratio correctly so long as their isn't misfire or extreme temperature. If the engine is continuously firing, you can trust the sensor output. My AEM reliably reads from 10:1 - 18:1. That pretty much covers the entire range anyone would want to control an engine in. If you are outside of those bounds, you have other bigger issues.

A misfire while being correctly fueled will show as lean because you have a whole cylinders worth of unburnt air coming down the exhaust.

hacker-pschorr

+1 what Roger said

Unburnt fuel is not measured by an OXYGEN sensor (they do not measure fuel).

It doesn’t necessarily need to be a miss caused by being too rich. If you car suddenly develops a misfire for any reason, the air / fuel will lean out on the sensor.

My car and more recently Todd’s car proved this theory (that was heavily debated in a past thread). When one of Todd’s cylinders would miss, that cylinder would lean out compared to the others (remember, he has eight O2 sensors on his car).

The exhaust is reading as lean because there is less burnt fuel – it’s that simple.
The raw fuel flying down the tailpipe is ignored by the O2 sensor.

aggravation

Ah ha! That makes more sense!
The reason I ask is I had a mechanic suggest my idle was too high because the mix was running too rich yet my A/F gauge reads average of 15.5 while at idle.
Unfortunately he saw the car when the ISV supply hose was restricted so his observation is tainted by the artificial conditions I created. When he first started it up after it sat in his shop overnight and took it out for a drive it was extremely rich, pouring black smoke. When he called me and told me that I thought he must be wrong since it hadn't been doing that when I dropped it off to him. When I got there he started it up and sure enough it was so bad it would barely run!
I don't know why it did that, he may have tried something and was afraid to tell me but we cleaned the plugs and it's back to normal now other than the idle.
I took it home, removed the intake to get to the ISV, my temporary restriction in the supply hose has now been removed, of course now the idle is high again, averaging 900 with the engine just warmed up, around 90 degrees centigrade, not hot, A/F still averages 15.5 at idle.

I wondered if he could be right though. I'm getting enough fuel unburned to make my A/F gauge read lean when in fact it's a bit rich? Could the ignition be missing just enough to do that? Could that somehow also cause the idle to be high? Maybe the LH ECU trying to compensate for the false reading?

The car seems to run pretty strong but lately when I get down on it it has that low power from rich conditions feel to it...it drops to low 12's A/F pretty quickly if you step on the throttle anywhere near approaching WOT mark. I'm not seeing any black smoke though except after a battery reset when it's cold. Once it warms up the exhaust appears to be correct but the A/F gauge reads lean and still getting some wild jumping to 18 - 20 A/F under light throttle...tweaking how far the intercooler outlet goes into the MAF boot helps some but the lurching is still there accompanied by the A/F gauge going crazy by seeming to want to stay around 15 but spiking up to 20 at times.

O2 sensor adaptation in SharkTuner is always +20, the o2 sensor adjust sits closer to 0 plus or minus 5%....

This is with a rebuilt MAF, rebuilt LH ECU, new wideband and new narrow band O2 sensors and minimal air leaking only at lower flappy bearing shaft and throttle plate shaft, everything else is air tight up to 30 psi...not sure how to quantify how much is too much in that air leakage department nor how to completely stop it!

mark kibort

interesting! well, that goes back to the charts i referenced about lean above stoich for best fuel economy and moderate power just before misfiring (near 17:1 A/F ratio) . So, what you are saying is what i was worried about and didnt catch it and wonder about it until i was 2 laps into the restart of the race. (the alternator went bad during the caution laps) when the green dropped, my AFR was WAY in the red and it was running strong 'til about 5000rpm and then fell flat on its face. it was red anytime i was at full throttle. way way red. probably in the 18:1 range. the, after the power loss , where it wasnt missign, it started to miss and cough and buck. then, all i could do would be to cruise around at partial pedal unitl i lost all volts and it just died.

I suspect you are right, and i was REAL lean, but so lean that no extra heat was present. maybe lucky that i didnt have any danger zone lean WOT time, due to that caution 3 laps where the battery slowly died. before hand, it was full Blue lights which is normal rich condition. maybe i was slightly leaner and thats why my car was running so good on the first lap on power! (11.5:1 normal, going to a more desired 12:5 :1 )
anyway, the plugs were really black and wet and looked like it was just unburned fuel or rich mixture. Ill never know because i put a battery in it to get it running on the trailer, and an didnt touch it again until i fixed the alternator.

so, you dont think that the low voltage could have caused low spark and didnt combust all the way so that some raw fuel was present and mistaken by the o2 sensor. or was it fuel pump and pressure going down due to lack of voltage and it just leaned itself out! either way. Thats not going to happen again.
mk