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Downstream fuel control experiments

  • Tyler
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11 Feb 2019 16:08 #26973 by Tyler
The subject of downstream fuel control has come up several times in SD material, most recently in the Nissan P2A00 series:

www.scannerdanner.com/scannerdanner-prem...sensor-1-part-1.html

I've always felt like I had a good handle on these systems, and which cars use them. I can also count on one hand the number of fuel control issues I've seen related to a downstream issue. It's way, WAY down on my checklist when addressing fuel trim problems.

Then I saw an iATN post that... Kinda pissed me off? :silly:

Blair from Missouri wrote: The post-cat O2s "IS" a fuel control input. Toyota and SAAB have used the post-cat O2 to control fuel trim since 1988; before OBD-2. This sensor is used to fine tune the air-fuel ratio to maximize catalyst effeciency. It can also adjust the air-fuel ratio to compensate for a degraded catalyst. If you were to measure the actual amount of time the post-cat O2s is used during the running of the catalyst monitor (maybe once per trip) and compare that to the amount of time the post-cat O2s is used to fine tune the air-fuel ratio (almost always in closed loop), you will see that this sensors major function is fuel control. Every vehicle manufactured today uses the rear O2s for fuel correction; even if it is undocumented.


members.iatn.net/forums/search/search.as...orum1&ft=1&fv=4&qv=1

Oh really? :blink: The fact that the OE's may not document it doesn't surprise me at all. But every vehicle uses it? I can't say my experience in the field agrees.

But hey, that's what experiments are for! So I rig up the good ol' Scion xA guinea pig with a backprobe in the downstream O2 signal wire, scanner carefully watching fuel trims, and a test light at the ready. After getting everything warmed up with a good long test drive, I pull the downstream O2 signal low with the test light and watch the data. The arrows denote when the test light is connected:



Yeah, that didn't change the trims at all. :unsure: No codes were set while testing, so I don't think it went into any kind of default strategy. Even if the amount of control is limited to a few percentage points, I'd think we'd have seen it here.

Viewed another way:



Noooooope, no change. :( Maybe the Scion is too cheap a car to include downstream control? Or maybe it doesn't like the 0V reading. Might be a job for a variable resistor.

Side note, while I was there, I decided to see if I could create a false P0420 code by pulsing the downstream O2 low while driving. Turns out, yes! :lol:

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11 Feb 2019 20:28 #26983 by Noah
Replied by Noah on topic Downstream fuel control experiments
I applaud your tenacity sir :)
And at highway speed according to the data, your insurance company must love you, lol.
Thanks for sharing the data captures.

I'm a little suprised there was no effect I guess? Considering the Scion is a Toyota product and (I'm assuming) is most likely fitted with an upstream AFR sensor.
Maybe it was just ignoring full ground, or maybe it's effect on trim is not immediate?

I'm still examining both sides of the theory I guess. It does seem to be widely accepted as fact in the iATN community. That doesn't necessarily mean that it is though.

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12 Feb 2019 06:40 #26993 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Downstream fuel control experiments
Following with great interest.

" Welcome to the 21st"


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12 Feb 2019 09:47 #27000 by Tyler
Replied by Tyler on topic Downstream fuel control experiments
Yeah you'd think it would have an A/F sensor, but nope, just a garden variety O2. I wish it was a wideband - more wacky test opportunities. :silly:

I may set up a variable resistor next time, just in case it doesn't like the 0V reading.

I feel like it's safe to say if it has a wideband sensor, it has downstream control. If it has an O2, it may or may not have downstream control. The older the car, the less likely. Fair?

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12 Feb 2019 12:51 #27002 by Noah
Replied by Noah on topic Downstream fuel control experiments
That's always been my assumption, sounds fair enough to me.

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22 Mar 2019 19:35 #28014 by Tutti57
Replied by Tutti57 on topic Downstream fuel control experiments
I wonder if you unplugged or tampered with the a/f sensor it would default to using the rear one as a back up strategy?

How well would the rear sensor work in a case like this since it has that delay before its voltage changes when the mixture is altered?




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22 Mar 2019 22:31 #28017 by ScannerDanner
Replied by ScannerDanner on topic Downstream fuel control experiments
Awesome Tyler! That iATN post had a lot of "always" and "never" comments in it, at least he was confident.
But I'm with you, if they've been using downstream trims since 88? (They didn't even have downstream sensors then), you'd think we would have seen some issues along the way.
Thanks so much!
Hope you guys don't mind having me around more lately :-)

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23 Mar 2019 10:30 #28035 by Tyler
Replied by Tyler on topic Downstream fuel control experiments

ScannerDanner wrote: But I'm with you, if they've been using downstream trims since 88? (They didn't even have downstream sensors then), you'd think we would have seen some issues along the way.


Agreed, something doesn't match up here. :unsure:

I wonder if you unplugged or tampered with the a/f sensor it would default to using the rear one as a back up strategy?


There's an idea! :cheer: I'll try that. But, from previous experience, I expect it'll just go open loop.

www.scannerdanner.com/forum/off-topic-se...blem-with-video.html

The longer I use the Scion as a test mule, the less impressed I am with it's diagnostic capabilities. :silly: It doesn't even have injector circuit codes.
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24 Mar 2019 22:42 #28064 by Noah
Replied by Noah on topic Downstream fuel control experiments
I think it would be interesting to see the results of similar testing on a dual bank wideband fuel control system.

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