P1126 and low rpm hesitation - all troubleshooting listed- Help greatly appreciated!

[blue62]

Originally Posted by porschefan76

Hey, thanks for the quick response. I've got to admit, I am beginning to be tempted toward just replacing a few things (especially the low-dollar stuff) and see what happens.

I think if I can't get specifics on where the O2 output signals should be or what the STFT PID is in Durametric, this limits my ability to continue troubleshooting with any reliable effectiveness. But I cannot find any confirmation that maintaining a full-sweep sine wave of 0.04mV to 0.85 mV is even a failure mode for narrow-band O2 sensors, meaning logically, they're working and it's something else. That said, after I test the cats with a back-pressure gauge, replace the spark plug tubes, test crankcase vacuum with my manometer again and smoke the intake once more, my next step may be replacing the O2 sensors. Fortunately, I enjoy working on the car and learning more about it, so I'm just going to keep at it.

Thanks for the time and the help!

From what I see in your O2 signal sine waves that you have posted in this thread in post 13 your O2 sensors are fine they show a typical sine wave.
The issue I see is your post Cat O2 signals are showing that your Cats are not working properly. So the Catalytic converter are suspect the sensors are fine.

Sorry I just went through the thread again and see in post 17 that you ran the O2 sensor data again at a more steady rpm.
But to me your bank 2 post O2 cat sensor still shows that your cat on bank 2 is suspect. maybe not failed but boarderline.

[ike84]

Originally Posted by blue62

From what I see in your O2 signal sine waves that you have posted in this thread in post 13 your O2 sensors are fine they show a typical sine wave.
The issue I see is your post Cat O2 signals are showing that your Cats are not working properly. So the Catalytic converter are suspect the sensors are fine.

Sorry I just went through the thread again and see in post 17 that you ran the O2 sensor data again at a more steady rpm.
But to me your bank 2 post O2 cat sensor still shows that your cat on bank 2 is suspect. maybe not failed but boarderline.

Hey blue do you think a failing cat would cause his problems though? I could see that if a cat were clogged and not flowing well that would cause loss of power because of choking down flow. Would out cars attempt to lean out the mixture in order to reduce emissions if it senses a bad cat? I'm not asking to disagree, it's purely from ignorance I've not ever dealt with a bad cat before and I'm trying to piece together how that would play into what he's dealing with.

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[blue62]

Originally Posted by ike84

Hey blue do you think a failing cat would cause his problems though? I could see that if a cat were clogged and not flowing well that would cause loss of power because of choking down flow. Would out cars attempt to lean out the mixture in order to reduce emissions if it senses a bad cat? I'm not asking to disagree, it's purely from ignorance I've not ever dealt with a bad cat before and I'm trying to piece together how that would play into what he's dealing with.

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1. I do not see a failing Cat causing his problems.
2.Our cars do not attempt to lean out the mixture in order to reduce emissions if they sense a bad Cat. Post Cat sensors on cars of that era do one thing = monitor Cat health via reading oxygen levels.
The only thing a narrow band O2 sensor of that era can do is sense oxygen and only in a very narrow band.
3. The reason I pointed what I see as possible failing or bad Cats is that he was talking about replacing his 02 sensors. I was pointing out that based on what I see in the O2 sensor Sine Wave, that the Sensors are fine. But the post Cat signal is suggesting a failing Cat.. So no need to replace the O2 sensors.

Ummm I think that's it Hope I answered your questions.
And there is nothing wrong with disagreeing how else could one learn he was wrong

[ike84]

Originally Posted by blue62

1. I do not see a failing Cat causing his problems.

2.Our cars do not attempt to lean out the mixture in order to reduce emissions if they sense a bad Cat. Post Cat sensors on cars of that era do one thing = monitor Cat health via reading oxygen levels.

The only thing a narrow band O2 sensor of that era can do is sense oxygen and only in a very narrow band.

3. The reason I pointed what I see as possible failing or bad Cats is that he was talking about replacing his 02 sensors. I was pointing out that based on what I see in the O2 sensor Sine Wave, that the Sensors are fine. But the post Cat signal is suggesting a failing Cat.. So no need to replace the O2 sensors.



Ummm I think that's it Hope I answered your questions.

And there is nothing wrong with disagreeing how else could one learn he was wrong

So I need to go and watch scanner danners videos about cats because my knowledge of them is still very much comic book version. When they fail what is the main symptom? Do they choke down the exhaust and kill power through reduction of flow? Or do they just stop cleaning up the exhaust and result in no other difference? I remember newarts video of how he toasted his cats when he blew his engine but that's the only other time I've ever heard of them going bad.

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[porschefan76]

Originally Posted by ike84

So I need to go and watch scanner danners videos about cats because my knowledge of them is still very much comic book version. When they fail what is the main symptom? Do they choke down the exhaust and kill power through reduction of flow? Or do they just stop cleaning up the exhaust and result in no other difference? I remember newarts video of how he toasted his cats when he blew his engine but that's the only other time I've ever heard of them going bad.

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Yes, ScannerDanner and also Schrodingers Box on youtube have great videos on fuel trims, O2 sensors, failing cats etc. ScannerDanner is a professional and Schrodingers Box is a DIY guy, so the videos are different, but both focus on the mathematics and science of engine controls.

From what I've learned watching their videos it appears that your former statement is what occurs. The failed cats typically impede the flow and result in very poor engine response and a lot of hot start issues. That said, I see exactly what blue62 was referring to with respect to my downstream O2 sensors. With the car at a steady rpm, a properly functioning cat should result in a downstream O2 sensor with a very flat output. My downstream O2 sensors do not look like they are doing that. Hence, a back-pressure test, to see if the cats are clogged or failing ... or at least that's my understanding from watching ScannerDanner and asking questions of my mechanic buddy.

[blue62]

Cats fail in a number of ways.
1. Physical damage such as hitting speed bumps. Can lead to a rupture,hole, leak in the case.
2. "Plugging up" usually caused from things like too rich of mixture over time. rings not seating causing oil blow by, leaking head gasket. Antifreeze gets into the exhaust.
3."Burning out" caused by to lean of mixture, excessive idle time, things that create to much heat.
A burned up cat has no back pressure issues or the like, it acts just like a good cat except it doesn't do it's thing.
Those are the things I am aware of.

They are designed to last the life time of the car. But they function in a very narrow range. So fuel air ratio needs to be very close to 14.7 to 1. I think somewhere around one half of one percent + or - of 14.7 to 1. Outside that range for extended periods then they burn up or plug up.

So something to try and keep in mind when dealing with OBDII and the P-Codes.
Its not about engine performance or power or gas mileage. The primary function of the OBDII system including the sensors is Catalytic Converter Performance and longjevity. Everything else is secondary.

[porschefan76]

Originally Posted by blue62

Cats fail in a number of ways.
1. Physical damage such as hitting speed bumps. Can lead to a rupture,hole, leak in the case.
2. "Plugging up" usually caused from things like too rich of mixture over time. rings not seating causing oil blow by, leaking head gasket. Antifreeze gets into the exhaust.
3."Burning out" caused by to lean of mixture, excessive idle time, things that create to much heat.
A burned up cat has no back pressure issues or the like, it acts just like a good cat except it doesn't do it's thing.
Those are the things I am aware of.

They are designed to last the life time of the car. But they function in a very narrow range. So fuel air ratio needs to be very close to 14.7 to 1. I think somewhere around one half of one percent + or - of 14.7 to 1. Outside that range for extended periods then they burn up or plug up.

So something to try and keep in mind when dealing with OBDII and the P-Codes.
Its not about engine performance or power or gas mileage. The primary function of the OBDII system including the sensors is Catalytic Converter Performance and longjevity. Everything else is secondary.

Hey again! I'm not concerned about failure mode 1 as this hasn't occurred. I have a troubleshooting methodology for 2 (back-pressure test) and the fact that for about 75% of the time at constant RPM the downstream O2 do exhibit a flat response, I'm fairly confident that the cats are not burnt-out. Combine that with the temperature testing I have done which showed marked increase in flange temperature downstream of the cat compared to the cat inlet, I'm "fairly" confident they're not fully gone, but cannot say with confidence they're not currently compromised. I will research the tests for troubleshooting and verifying a "burned out" cat, but so far, haven't found anything. Are you aware of a way to verify "burned out" that doesn't rely on the output of an O2 sensor of unknown functionality (i.e. downstream O2 sensor may have it's own problems being 20 years old)?

Thank you for the continued help and ideas!

[porschefan76]

Originally Posted by blue62

From what I see in your O2 signal sine waves that you have posted in this thread in post 13 your O2 sensors are fine they show a typical sine wave.
The issue I see is your post Cat O2 signals are showing that your Cats are not working properly. So the Catalytic converter are suspect the sensors are fine.

Sorry I just went through the thread again and see in post 17 that you ran the O2 sensor data again at a more steady rpm.
But to me your bank 2 post O2 cat sensor still shows that your cat on bank 2 is suspect. maybe not failed but boarderline.

Originally Posted by ike84

Hey blue do you think a failing cat would cause his problems though? I could see that if a cat were clogged and not flowing well that would cause loss of power because of choking down flow. Would out cars attempt to lean out the mixture in order to reduce emissions if it senses a bad cat? I'm not asking to disagree, it's purely from ignorance I've not ever dealt with a bad cat before and I'm trying to piece together how that would play into what he's dealing with.

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Hello and thanks for the follow-ups. Yes, I intend to back pressure test for failing cats when the weather cooperates, probably early this upcoming week. As far as I can tell however, the post-cat O2 sensor does not play with the ECU, so I don't think that could be causing the P1126, but maybe if the cat is failing, that could cause it?

I'll add results as soon as I perform the test. Thanks for the continued interest and inputs!

[blue62]

Originally Posted by porschefan76

Hello and thanks for the follow-ups. Yes, I intend to back pressure test for failing cats when the weather cooperates, probably early this upcoming week. As far as I can tell however, the post-cat O2 sensor does not play with the ECU, so I don't think that could be causing the P1126, but maybe if the cat is failing, that could cause it?

I'll add results as soon as I perform the test. Thanks for the continued interest and inputs!

Agree your P1126 is another issue.
Perhaps some form of vacuum leak.
Or MAF sensor but there are ways to test it.

[porschefan76]

Originally Posted by blue62

Agree your P1126 is another issue.
Perhaps some form of vacuum leak.
Or MAF sensor but there are ways to test it.

Yes, I agree. As you pointed out though, the downstream O2 sensors do look like something is up, so I won't ignore them.

The MAF appears to be operating properly, 15 kg/hr at idle and the Voltages looked about right last time I pulled data. I pulled some data on my test drive after changing the fuel injectors but I was in a rush and I'll need to pull more. Will pull MAF, O2, Fuel Trim and Engine Load data next test drive after I test cats, smoke the intake and replace the spark plug tubes.

Thanks again!

[blue62]

Originally Posted by porschefan76

Yes, I agree. As you pointed out though, the downstream O2 sensors do look like something is up, so I won't ignore them.

The MAF appears to be operating properly, 15 kg/hr at idle and the Voltages looked about right last time I pulled data. I pulled some data on my test drive after changing the fuel injectors but I was in a rush and I'll need to pull more. Will pull MAF, O2, Fuel Trim and Engine Load data next test drive after I test cats, smoke the intake and replace the spark plug tubes.

Thanks again!

A test I do is compare MAF signal to Throttle position sensor signal.
There is a direct correlation between the two.
I hook up my Durametric and put the MAF signal and the Throttle position sensor signal on the same Graph. Then I take the car for a good long drive.
I set the laptop on the passenger seat so I can watch the two signals.

What you should see is the two signals paralleling each other. The TPS signal is slightly smoother then the MAF but by very little. That just the difference between air movement generating one signal and mechanical movement generating the other signal.
So you should see Both signals following your foot so to speak. If your very smooth on the throttle both signals should increase with the same smoothness. If you floor it both signals should react the same way. If your smooth on the throttle or holding a steady RPM both signals should follow each other. If you see something erratic in the MAF signal when your throttling smooth or holding a steady RPM then there is an issue with the MAF.

I do this with my car over a 52 mile loop. Gives me every driving condition one would ever encounter. City stop and go, to country flat out, hills, curves the works.
Actually a fun test to do.

[porschefan76]

Originally Posted by blue62

A test I do is compare MAF signal to Throttle position sensor signal.
There is a direct correlation between the two.
I hook up my Durametric and put the MAF signal and the Throttle position sensor signal on the same Graph. Then I take the car for a good long drive.
I set the laptop on the passenger seat so I can watch the two signals.

What you should see is the two signals paralleling each other. The TPS signal is slightly smoother then the MAF but by very little. That just the difference between air movement generating one signal and mechanical movement generating the other signal.
So you should see Both signals following your foot so to speak. If your very smooth on the throttle both signals should increase with the same smoothness. If you floor it both signals should react the same way. If your smooth on the throttle or holding a steady RPM both signals should follow each other. If you see something erratic in the MAF signal when your throttling smooth or holding a steady RPM then there is an issue with the MAF.

I do this with my car over a 52 mile loop. Gives me every driving condition one would ever encounter. City stop and go, to country flat out, hills, curves the works.
Actually a fun test to do.

This sounds great. I'll include throttle position sensor output in my data pull the next time I am performing a test drive.

So, I've been doing further reading on the pre-cat O2 sensor because now I'm stuck on the fact that the output of those sensors does not appear to change while the fuel trims are being modified (ie. in Post #14, the Bank 1 and 2 pre-cat (FORE) sensors display almost the same sine wave while the RKAT is at 0 as it does when RKAT moves to -0.25 to -0.75 on all the way to -2.5). If the ECU is adjusting the fuel trims negative as a result of O2 sensor feedback loop reporting rich (my understanding of the function), shouldn't the O2 sensor display a sine wave that looks like it's seeing rich? I'm considering inducing a LARGE vacuum leak at idle while pulling data and see if the O2 sensors respond to this by indicating lean, at least for a couple of seconds. ScannerDanners videos show lots of response from the O2 sensors while he is changing various inputs (pressing the gas, inducing vacuum leaks, plugging vacuum leaks, etc) and my sensors don't seem to display any of that. Is this specific to the operation of these sensors? Maybe they're just bad and 200 bucks will fix these issues? I really need to see STFT to better understand this I think.

Again, any input is greatly appreciated. Thanks!

[porschefan76]

Hi all. Just a brief update, following the advice of ike84, I checked Amazon for the Bosch oxygen sensors. I'm not sure why, but I haven't purchased parts for this car from Amazon before. Anyyyyyyway, the sensors have full refundability and were at a lower price point.

Plan now will be to swap in new O2 sensors when I pull them to back-pressure test the cats. I will perform an overnight "hard-reset" (poor-mans handover) of the ECM the night before and, provided cats pass the back-pressure test, go for a test drive and collect a good deal of clean data with (hopefully) known-good cats and new upstream O2 sensors. I'll report back following these efforts.

As usual, thanks all!