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O2 voltages are never ever stored on/in the ECU - they are the result of the heated O2 active element responding to the environment its mounted in - iow - since there is no exhaust flow with 2 identical sensors they absolutely MUST produce near identical signals for the ECU to react to. The about 0.4V with ignition on and engine not running is correct and expected.

O2 voltage measurements are used to drive the STFT .................... and the STFT (when consistent) is driving the LTFT
 

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Discussion Starter #63
I've moved the Denso [good one] to the upstream position and moved the Renault [bad one] to the downstream position. I've ordered two new Denso's, so will replace the downstream one when they arrive.

The Renault sensor looked very sooty when I removed it. That may have been because the car was just started and run for just a couple of minutes as I moved it from the street to the driveway prior to removal - i.e. may have been 'on choke' - I'm not sure. I've enclosed a photo of it here.

In other news, I got hold of a fuel pressure gauge and tested the fuel pressure from the tank. It's reading a steady 3.5 bar.

After I've fitted the new downstream sensor, I'll do take the car for a drive again and log all the parameters so we can check out what it's doing... then take it from there.
 

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Fuel pressure on the rail after engine switch off must remain around 2.5 to 3 Bar for a long time (many 10s of seconds) ................ this is critical too.

That O2 sensor is fugly!

So is the upstream now reading 0.4 something Volt with ignition on?
 

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Discussion Starter #65
Fuel pressure on the rail after engine switch off must remain around 2.5 to 3 Bar for a long time (many 10s of seconds) ................ this is critical too.

Ah... didn't check to see whether the fuel pressure stayed at that level after the engine was switched off :( I had to get a friend to pop around with the pressure kit, he's gone now :( I guess I can check that again after


That O2 sensor is fugly!
Isn't it just. Should it ever be that dirty... even after just a couple of minutes of running ?


So is the upstream now reading 0.4 something Volt with ignition on?
Yes it is, and the downstream [Renault] one is reading what it had been when positioned upstream, 1.275v
 

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Result! :p

O2 should never ever look like that no - at worst a sorta tan or white burnt look to it if all is well in the motor


Lets hope the O2 looks like that because of mixture issues then ..................

Pity you are not in SA - I would have loved to get my grubby little paws on those old sensors of your to have a play around and see what's gone awry
 

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Discussion Starter #67
So I did quite a bit of internet browsing on 'reading O2 sensors' and the black soot is indicative of an over-rich mixture [see link below although I think they mean high 'fuel:air' ratio rather than the other way around. That may also be the reason why the STFT is constantly hunting negative PCTs ?

I guess I'm worried that if I continue driving the car as is, it's likely to kill any new sensor I fit upstream. I really need to try and figure out what's causing this. The consensus seems to be :

o Something to do with the heater functionality of the O2 sensor. I can't remember now whether I previously verified that this was working or not - I'm fairly sure I did. I don't have a multi-meter to go back and do it now... something else to add to the shopping list
o Something related to the engine fuelling - leaking injectors is all I can think of, maybe the next option is to replace the injectors. I have known them to fail in the F4R engine - but they usually shut down completely and issue a fault code.

Reading O2 sensors
 

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So I did quite a bit of internet browsing on 'reading O2 sensors' and the black soot is indicative of an over-rich mixture [see link below although I think they mean high 'fuel:air' ratio rather than the other way around. That may also be the reason why the STFT is constantly hunting negative PCTs ?
Indeed if the mixture is way rich the O2 can look like that ........................... but its totally meaningless as a conclusion/indicator as long as the O2 isn't working - the STFT doesn't have a clue what to do or report if the O2 is screwing the pooch - STFT needs consistency from the O2 to make sense of the mixture - if you know already the O2 is screwed any conversation about STFT or LTFT after that is completely irrelevant............. as is the O2's visual condition

As I said right in the beginning - if the fuel pressure on the rail is misbehaving then sure you can end up with raw unurnt fuel on the O2 and possibkly yet another buggered O2 (you do need a lot for a long time though).................. its up to you to do the absolute basics first and see if you actually have a fuel pressure/injector problem before you start spending money on that expensive route.

As it stands the working upstream O2 you swapped in should give you a perfect indication of the ECU's ability to manage fuel mixture if you look at O2 voltage starting from cold and the changes in the LTFT wrt the STFT ..................... you cannot get away from the fact that you need a working O2 to manage the fuel system and the combination of the LTFT, STFT and O2 sensor voltage behavior can tell you in very short order whether you even need to look any further. One drive cycle from cold to hot will already be able to confirm the direction things are heading)
 

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Discussion Starter #69
You're right. A buggered O2 sensor will feed bad/no data to the ECU which will be unable to instruct the fuel system. I suppose I was jumping ahead of myself a little. Fingers crossed the Denso will stay alive long enough to give us some sensible readings from a working O2 sensor.

Let's see what tomorrow's full cycle produces. I'll start from cold and do the idle test and then drive the car to work - all with logging switched on - so we'll take a look at the data after that.

As always, massive thanks for all your help.
 

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Something to do with the heater functionality of the O2 sensor. I can't remember now whether I previously verified that this was working or not - I'm fairly sure I did. I don't have a multi-meter to go back and do it now... something else to add to the shopping list
I don't think you are fully understanding the reason for me on insisting on getting a upstream O2 voltage reading before the engine starts - if you have about 0.4something V reported on the scanner then the heater getting power, and is working and the O2 SHOULD be working normally and you don't need a multimeter to actually measure the voltage.

If like in the past you get 1.2something V without the engine running its time to start yanking out the multimeter and/or the gun because THAT reading is already telling you the O2 is possibly buggered
 

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Discussion Starter #71
OK. But is the heater not a separate circuit to the data ? Or is it a case that if the O2 heater is being powered, it registers a potential drop of 0.4v across the data terminals when no exhaust gases are passing over the sensor ?
 

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OK. But is the heater not a separate circuit to the data ? Or is it a case that if the O2 heater is being powered, it registers a potential drop of 0.4v across the data terminals when no exhaust gases are passing over the sensor ?
Exactly.

If the heater was NOT powered then the O2 sensor will not produce any voltage at all without the engine running
 

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Discussion Starter #76
Whoops... think I did the links wrong here on the forum. It would be nice to be able to upload the file itself in case somebody wanted to look at the data we're talking about here. Unfortunately Sendthisfile only holds them for a few days :(

Morning file

Afternoon file
 

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So it seems the upstream O2 you swapped in there is actually doing its job properly .

You can clearly see how the STFT is now doing its thing around 0 from very early on after the engine was started already - afternoon stft is also showing a "generally better" hugging of 0

Looking at the only working O2 atm only - I cannot see anything immediately jumping out as a reason for concern (I would still do the rail fuel pressure test thing properly for reference though)

LTFT seems to be real close to what it should be by the looks of it (STFT is doing its thing around 0)

Interesting to see the downstream O2 that is undoubtedly funky and in need of replacement started to wake up near the end of your trip this afternoon.

See no need for immediate mechanical concerns - once you get the 2 new O2s and fitted them I would reset the fuel adaptations in the ECU just to help things settle down as fast as possible.
afternoon stft.png
morninf stft.png
afternoon downstream o2.png




Earlier I mentioned that the upstream O2 should be producing sensible voltages within about 30 seconds after first start causing the ECU to enter closed loop - here you can clearly see that happening
upstream o2 from first start.png


The car accelerates as normal up to about 3,000 rpm and then dies before picking up again. More than just a flat spot, a dead spot.
This will be the crux of the matter ................... is that now better/improved? - I cannot get a feel for it in the logs but it would seem its either gone or way better
 

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Discussion Starter #78
Thank you sooo much for everything here... I really do appreciate everything you've done.

I had the tablet on the passenger seat and was watching the STFT PCT moving around... it was hard to see the pattern above from the numbers but I could see occasional positive values, so it did seem as if there was some degree of oscillation around the x-axis.

Ya that was interesting alright about the bad sensor [fitted downstream] starting to register voltages. I guess there are a multitude of reasons why that might be the case.

You mentioned before that the LTFT is influenced by the downstream O2 sensor... should we expect no movement in that until/unless the downstream sensor is working properly ?

When you mention reset the fuel adaptations, can I do that through Torque Pro ? By the way, I still have the P0130 fault code, is it OK to clear that ?

This will be the crux of the matter ................... is that now better/improved? - I cannot get a feel for it in the logs but it would seem its either gone or way better
Yes of course. It does seem to idle slightly better than it did before. I'm quite nervous about hard acceleration until it's been stable for a few days. I'll try it tomorrow morning again [if traffic permits] to see if it's better.

I've been racking through my mind on possible causes for these O2 sensor failures and there's one thing I thought about after you'd mentioned the heater operation. We know when the heater comes on [the 0.4v is registered across the data terminals of the O2 sensor]. How do we knew when it switches off ?

Again, massive thanks for all your help here.
 

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The downstream O2 has a negligible effect at this stage imo even though it funky - this fact is supported by an effective constant LTFT and a STFT swinging around 0 exactly as expected. The LTFT will not necessarily start to change unless there is a need for that and the need is reflected by the STFT oscillating around an average of some value other than 0.

Even with a brand new downstream O2 there will be a negligible effect on fuel trim as long as the upstream O2 is working as expected.

Cannot reset fuel adaptations with Torque - switch on headlights, remove battery negative terminal, go have a beer or 2 and reconnect negative terminal, switch off head lights, switch on ignition but don't start, leave for about a minute, ignition off for about 5 minutes. Now start engine without touching throttle and allow to idle for about a minute .................. should now have reset adaptations

P0130 can be cleared - atm don't expect it back but do expect a moan about the downstream O2 or cat now.

O2 heater supply is managed by the ECU - ignition on then it will power the heater - ignition off then after a while it will automatically kill the supply
 

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Discussion Starter #80
Cannot reset fuel adaptations with Torque - switch on headlights, remove battery negative terminal, go have a beer or 2 and reconnect negative terminal, switch off head lights, switch on ignition but don't start, leave for about a minute, ignition off for about 5 minutes. Now start engine without touching throttle and allow to idle for about a minute .................. should now have reset adaptations
Wow... I never knew the fuel adaptations were reset when the battery was disconnected. Really good to know.

O2 heater supply is managed by the ECU - ignition on then it will power the heater - ignition off then after a while it will automatically kill the supply
So the heater is powered on at all times when the engine is running ? I guess I had it in my mind that the heater would be switched off when the O2 sensor itself reached temperature, or that it would come on/off to keep the O2 sensor at a constant temperature.
Either way, you don't think the ECU is mismanaging the heater function ? I was just trying to eliminate an electrical source as the culprit for killing the O2 sensors.
 
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