Readings right with bad o2 sensor?

[JohnCIV_Talon]

I know my o2 sensor is bad. I dont need to go over that. On my logger, when reading stft its either at 17 when idling and driving or 0 when at full throttle. Are these false readings because of a bad sensor or are these right? If they are right then that would make my stft exactly where I want it. This is probably a dumb question, but I just want to make sure. It it were the correct readings, there would really be no need for a o2 sensor.

 

[dsm-onster]

The ecu uses the feedback from the o2 sensor to determine the trims. So if your o2 sensor putting out a false low or high signal, the ecu will adjust the trim high or low respectively.

I wouldn't try to analyse or negotiate w/ fuel trims until you replace you rbad o2 sensor and drive it for while. Then look at your trims and see where you're at.

When repacing your o2 sensor, I'd disconnect your battery positive for 15 minutes ot insure that the ecu resets the trims to 0 (or 100 if you're a 1G).

Regardless, reading 17% on your stft (which would suggest that it is shooting out a low signal voltage at where the fuel map in the ecu says the fuel should be at stoich), is w/ in the range of control for the ecu. If your o2 sensor were bad, it would always put out a low or high voltage signal. So does the o2 reading cycle during normal driving?

 

[JohnCIV_Talon]

I havent really checked that but while doing a few logs, it was only at .02 and stuff. Not up at .92 where I would like it, and my timing looked decent, so im pretty sure its bad.

 

[dsm-onster]

That would make your o2 sensor being bad a POSSIBLITY. It may just load up w/ gunk at wot or you may be running terribly lean . The sure test is to see if it is cycling... Definately replace it before buggering w/ fuel trims. They will read wrong and 17% is the ecu telling you, "hey, based on the o2 sensor, I'm not getting enough fuel for the pulse width i'm giving the injectors." Check for cycle before replacing an $80 part.

 

[JohnCIV_Talon]

What do you mean load up with gunk at wot?

 

[dsm-onster]

It's not likely but I've at times been sooo bloody rich that the wideband will read very lean (I mean like readding a percent o2 number insteand of a lambda number or a/f nnumber on my LM-1). I figured the cause was the black partially burnt fuel was just filling the "holes" in my wideband sensor. . . I leaned it out a bit and started to read really rich then leaned it out some more and it started to read a little leaner.

 

[JohnCIV_Talon]

Ok i see. Jeez I have learned a lot just from this conversation, thank you. Anyways, back to the point. I went for a cruise and It would pretty much stay at .02 the whole time, once or twice in first or second it would go to .04, and when I let off the gas it would go to .00. what should it be reading while cruising, up around .92? Also why does it not throw a cel?

 

[dsm-onster]

it should bounce from .90 v or more to .04 v or less at a constant low flow throttle.

If you cruised long enough to get to operating temp and checked the o2 cycle at constant low flow throttle and you've double checked for boostleaks then it's time for a new o2 sensor. . .

 

[JohnCIV_Talon]

Wait.. Could a boost leak cause this to be reading like it is?

 

[spyderturbo007]

Sounds like your front O2 needs to be replaced. As Matt said, you should see it bouncing up and down at light cruise / idle. When mine died, it was bouncing around from 0.00v to 0.04v. You can get a cheap OEM O2 sensor here:

www.sparkplugs.com

Denso OEM O2 sensor (front) $47.59

 

[JohnCIV_Talon]

It would be bouncing around like you said even with a boost leak, correct? I know I have aboost leak, I have to wait 3 weeks when I get out of school to fix it though. Its also not a horrible leak. Im pretty positive the leak is what caused my o2 sensor to go out also, so I need to fix that before I put a new one on.

 

[zippyshoe]

See how your front O2 voltage responds when you go WOT - it should jump to around 0.9-1.0 volts and remain there as long as you remain at WOT. If it stays at close to 0 volts instead, then this is a pretty good indication that your front O2 sensor is dead.

 

[dsm-onster]

A significant leak will cause your engine to injest more air than is metered by the maf when not in boost. When this happens the fuel injector pulse width goes down (less fuel) to match the lower maf reading. The o2 sensor will read lean (low volts). And the ecu will then compensate.

This looks like where the 17% positive on the stft came from. It can *possibly* lead to an o2 sensor failure. Why can't you get the boost leak fixed? Is a pipe cracked?

I'd just replace the sensor at this point. Too many symptoms point to a bad sensor. If the leak isn't bad enough to cause gobs of black smoke when going WOT, then it likely won't cause a good sensor to go bad. Perhaps your sensor was on it's way out anyway and the failure after the leak developed was just a coincidence. I've never seen boost leak leading to a fuel trim deviance under 20% cause an o2 sensor failure.

 

[dsm-onster]

I agree w/ zippyshoe. Pretty much since you said the o2 sensor is staying at low during a WOT run your sensor is bad. How many miles are on the sensor?

 

[JohnCIV_Talon]

78k, the original.

 

[dsm-onster]

I've seen them last less. Replace it.

 

[JohnCIV_Talon]

Yep, I ordered one already. My dads 2002 silverado already had one go out, just last year.