Wild O2 sensor swings?

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03-20-2019
Daveo91

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Wild O2 sensor swings?

I'm getting some wild O2 swings in my data files, what does it mean when output is less than 100 mV? I'm thinking these swings may have something to do with the low speed and decel surging I'm experiencing - they seem to match up time wise in the log file. Would anyone be willing to take a look? It's a long file, but some of my worst surging was around 1400 seconds in, if you want to go straight there? Anything else amiss in this file that could be causing surging, especially at decel?

I read in another post that this problem could be caused by a small exhaust leak, allowing fresh air to be pulled in on decel only. Well, I had a custom y-pipe made some time ago and frankly I've had a lot of exhaust leaks since then. Unfortunately I have been back to him about four times now due to pinhole leaks in welds. And I know there is yet another one - I'm hoping to get it taken care of this weekend - but it is downstream from the O2 sensor. But perhaps there is still another leak upstream that I'm missing. I've duct-taped my shop vac to the tail pipe to help find leaks once before, I plan to try that again.

Well any advice anyone can give based on log file or anything else would be much appreciated. Thanks.
03-20-2019
johnny_b

O2s are supposed to oscillate in closed loop that is normal

450mV is “stoic”.

If you had an exhaust leak before or near your O2s they would likely stay in voltages consistently below stoich
03-20-2019
Daveo91

Quote:

Originally Posted by johnny_b

O2s are supposed to oscillate in closed loop that is normal

450mV is “stoic”.

If you had an exhaust leak before or near your O2s they would likely stay in voltages consistently below stoich

Understood, but from what I’ve read on them they should be oscillating from a lean range of 100-300 and then 600-900 on the rich side. Mine is spending considerable time below 100 and it seems that time is corresponding with my decel surge issue.
03-20-2019
johnny_b

In that case yeah odds are very likely you have an exhaust leak causing corrupted o2 readings.

So it’s adding fuel because it thinks it’s lean due to the fresh air entering the exhaust

It’s also very likely that is causing your surge

most pcms have some type of defco (deceleration fuel cut off). So if you’re seeing it only on decel that could be it. Might need to tune some defco settings
03-21-2019
steveo

Quote:

Understood, but from what I’ve read on them they should be oscillating from a lean range of 100-300 and then 600-900 on the rich side.

i'd say they should be oscillating from a lean range of 0-400mv and a rich range of 600-1000+ mv.

somewhere in that 400-600mv range is where stoich is

millivolts are really small. you can probably get a couple millivolts of voltage by holding your multimeter probes to your finger.

it's more the rate of switching you should be concerned with. what's worrysome is if it's 'hanging' at low voltage then 'hanging' at high voltage (switching slowly) which either means your o2 parameters are screwed or your sensor is dying. or worse, its just stuck at high or low voltage which means your ECM isn't correcting for some reason
03-21-2019
Daveo91

Quote:

Originally Posted by steveo

i'd say they should be oscillating from a lean range of 0-400mv and a rich range of 600-1000+ mv.

somewhere in that 400-600mv range is where stoich is

millivolts are really small. you can probably get a couple millivolts of voltage by holding your multimeter probes to your finger.

it's more the rate of switching you should be concerned with. what's worrysome is if it's 'hanging' at low voltage then 'hanging' at high voltage (switching slowly) which either means your o2 parameters are screwed or your sensor is dying. or worse, its just stuck at high or low voltage which means your ECM isn't correcting for some reason

OK, thanks. Good to know that below 100 mV is not abnormal. Because it's definitely going below, but it doesn't spend a lot of time there or appear to be getting hung up. If anything it may be spending too much time at the high side, but when it goes lean it seems to correct very quickly and then swings back high.

I burned a new chip tonight and only thing I did was turn off DFCO by setting the high RPM limit to be lower than the low limit. I will drive it tomorrow or Friday to test. Other thing I did tonight was set up my "exhaust leak diagnostic lab" (shop vac hose duct taped to tail pipe) again and did some more diagnosis. Could only find one joint leaking and it is downstream from O2, albeit only slightly downstream. Not sure if that could cause O2 issues or not. Going to take it back to the exhaust shop one more time regardless though if only for the fact that I hate that noise.

Thanks for the help!
03-25-2019
Daveo91

I finally found (and resolved) my exhaust leak! Not sure what happened but the exhaust manifold gasket at No. 8 cylinder was broken in two and some of it was missing. I can only think that it was misaligned from the beginning. I use Mahle gaskets that are separate for each cylinder except the middle two - the upside with them is no need to remove the manifold to replace, downside as I saw was you have to make sure they're positioned correctly. Did that today and short drive later, voila, no exhaust leak. Back to tuning.

I did not have time to do much today, but I datalogged on my short drive and my IAC counts are at 38 at idle fully warmed up. Too high? Demanded idle is 650, but it would only come down to about 725. I've done the minimum air/IAC reset adjustment a few times in the past, but going to try it again now that exhaust is airtight. Set minimum air by the book (diagnostic mode, IAC parked and unplugged, adjust RPM with throttle stop screw to 100 rpm less than desired) or by IAC count? If IAC count, what should I be shooting for?