mistermike

I'm seeing .030-.040 volt difference between banks in the average front O2 sensor readings as reported by EFIlive. The longer the car's been running, the bigger the difference. I have not yet swapped the sensors to see if it follows the sensor. (I suspect it will). This issue existed before and after the header/cat/exhaust install, and I don't appear to have any obvious leaks.
Questions:
1. Is this within the normal tolerance spread for GM 02's?
2. Is this anything to be concerned with?
3. If so, how do I determine which O2 is accurate?
4. Can the average O2 voltages be reliably translated into A/F?
5. Does the ECM control each bank independently?

crainholio

iTrader: (2)

Swapping the sensors around is a good first step in troubleshooting. That the condition varies predictably as a function of car run time likely rules out wiring (ground) as a cause.

Now let's model out the next steps:

Result 1: problem follows sensor, or does not.

Follows sensor:
-rule out sensors; replace one w/ new sensor, retest, then move new sensor to other bank, retest

Doesn't follow sensor:
-rule out injectors; swap bank to bank and re-test

We can collectively build out the next round of results and rule-outs as we go...

1fastWS6

iTrader: (3)

I noticed a similar split when I installed my SVO injectors. Not 100% sure if that's it yet or not but crainholio's post is a pretty good start.

BTW, the answer to questions 4 & 5 are no.

mistermike

Thanks guys. I've got some work to do, and I'll come back with the swap results.

GuitsBoy

iTrader: (4)

I have a smaller discrepency, but I passed it off as being due to the fact that one o2 is located further down on the passenger side header as opposed to the drivers side. That means that the exhaust has more time to cool, which means denser air, which means more oxygen concentration, which means the sensor reads low, and fuel is added to that bank. Mine is only off by 20 mV though.

Another_User

Mine are always off by 20mv. I have also swapped them, no change. It definitely has something to do with either location in the exhaust, or I was thinking it may even be resistance in the wire. You have to figure...that really isn't that far off in the grand scheme of things. It would put the AF on both sides close enough for OE purposes. I also don't understand why there are two different part numbers for the O2s on mine. They appear to be the same to me...same length wiring and everything. Those mysterious O2s...

HALLZ

iTrader: (1)

Exaust leak somewere upstream from the one O2 can cause the diffrence also. Even a small one.

Another_User

Yeah, but mine was off by the same amount before and after my shorty install, and I used new gaskets and cranked 'em down good.

mistermike

Here's an interesting development. I did a PCM reset, which initially did nothing. If anything, it was running richer than before. (Pig-rich after header install) After about 600 or so miles, the difference between O2's disappeared! They're now within 1-2 millivolt average. As usual, this has generated more questions than answers.

Does a PCM reset clear all LTFT's immediately, or do they slowly get re-written?

Why the difference in the first place?
My pet theory is because GTO uses 2 different types of mufflers, one being less restrictive, and no H pipe, causing the PCM to set up a different mixture for each bank. With LT's, X-pipe, and equal mufflers the PCM is happier? But why did it take so long after the reset for things to balance out? This would also tend to indicate that each bank IS independently controlled. Can you tell I'm an EFI newbie?

Why do headers make an engine run so flipping rich anyhow? Can't the 02's bring it around? The GTO cam is reported to be 200*/203*@.050, .500/.500, 116* LSA
I ask this question all the time, and the variety of different, and often contradictory answers I get is mind numbing, so I'm still collecting them. My average O2 went from below 400 mV (lean?) to just under 600 after the headers under light cruise. WOT on a dyno, A/F is in the 11's. I'm being robbed of power, that's for sure.

jimmyblue

iTrader: (11)

Headers make the O2s run cooler and that's the root
of the problem - a cold hibernating O2 sensor looks
just like one that's way lean, so the PCM trims it
rich, falsely. You learn rich at idle and then learn
back proper at cruise, most likely, but your trims
drift all over the place based on driving history.

HALLZ

iTrader: (1)

Great read!!

Should help clear a lot of things up regarding the PCM and how things are effected.

http://carprogrammer.com/Z28/PCM/LS1/Holden_LS1.pdf

It sounds to me that the ST & LT FT's did their job.

my LTFT average +.1/-.2 as long as the 02's are holding heat and sw properly.

from my understanding if the FT's are reset than YES the LT's should get corrected by the STFT's and get close to 0 if they are not way off.

mistermike

OK. Here's my confusion. Rather than generate another long thread on all the theories about why LT's result in a net rich condition. Let's cut to the chase. I'll even assume one of the theories (entirely plausible) that headers actually cause a lean condition that the PCM overcompensates for. Why is it that the system is apparently unable to trim the mixture back to the commanded value? Whatever condition initially caused the computer to trim rich, the fact remains that my O2's are averaging nearly 600 mV at a part throttle cruise. That's rich in anybody's book. Why doesn't the PCM see this as overcompensated and pull back? Isn't that the point of Lambda sensors in the first place?

Another_User

If the O2 sensors don't get hot they don't work right. On longtube headers the O2 sensor is moved too far back to get a solid reading back to the PCM. So the PCM overcompensates because it thinks that the vehicle is running lean.

mistermike

How can it possibly intrepret 600 millivolts as a lean condition?

Another_User

I doubt it is (anymore). If it was still thinking you were running lean it would keep adding fuel until you went past 600 mv. Do you have COT disabled (not that I have any idea why it would show up now)?

mistermike

COT is not disabled. Until I upgrade my EFILive to the new tuner version, all I can do is dink around with commanded AFR, and the change doesn't "stick". Interestingly, I bumped the A/F up to 15.7 and the instantaneous mileage indicator went up about 1.7 MPG and the average O2 voltage dropped about 20-25 millivolts at a 65 MPH cruise. I'm pretty sure I'm all the way through a complete drive cycle since the reset, except for the turn on all the electrical appliances thing.
Unfortunately, my O2's are still high. This is what's irking me. I can certainly accept that the calibration of the 02's may be off due to exhaust temps, overscavenging, or whatever. But I'm baffled as to why it's letting the O2's stay at such a high level.

HALLZ

iTrader: (1)

They are "averaging" 600 mV? you mean they are switching fine and when you highligh or average them they are in the 600 range. Why is this a problem, I maybe missing some thing here but mine average in the .550 V range @ cruise. After all this is the average from low-high per cycle.

mistermike

O2's are switching fine. The "average" voltage is reported by EFILive on the data page. Before the header install, the average O2 voltage was 380-410 millivolt. Afterwards it is just under 600 at a light cruise. I'm not trying to say that the average voltage indicates a specific A/F ratio, but that the difference before and after indicates a definite rich trend. This trend was corroborated by a rich WBO2 on a recent dyno run at WOT, and also by a 2-3 MPG drop in fuel economy.

HALLZ

iTrader: (1)

Got ya!