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

[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!

[blue62]

Originally Posted by porschefan76

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!

The only way I know of to reliably test a Cat to see if it is burned out. (other then what the O2 sensor is showing). Is with something like a five gas analyzer. Some shops still have them. Or your local smog test station. That is one of the things smog test stations are testing when they put the probe up the exhaust pipe. They are looking at the gases from the exhaust to see if the Cat is working properly.

[porschefan76]

Originally Posted by blue62

The only way I know of to reliably test a Cat to see if it is burned out. (other then what the O2 sensor is showing). Is with something like a five gas analyzer. Some shops still have them. Or your local smog test station. That is one of the things smog test stations are testing when they put the probe up the exhaust pipe. They are looking at the gases from the exhaust to see if the Cat is working properly.

Ok, that ones going to have to wait until last then. It unrelated to P1126 so I'll get that fixed first and see where I stand. I suppose I can just run to the inspection station and run the car through at that point.

[porschefan76]

Hello all. Time for another update.

Things I've learned:
1. STFT is called Oxygen Sensor Integrator on my 986 01S (I think).
2. I reached out to Durametric regarding the discrepancy on P1126 between what Durametric was reporting "Porsche Fault Code 356 - Multiplic. mixture adapt. lower load range B.1:" and Bentley "Porsche DTC 35 Oxygen Sensing Area 1 Cylinders 4-6 - rich mixture threshold". Durametric doubled down on bank 1, indicating their data was such:
a. P1126: "Oxygen sensing adaption, lower load range, bank 1."
b. P1126: "Fuel system multiplicative Bank 1, fuel trim limits exceeded."
c. P1126: "Fuel system multiplicative Bank 1, Range 2 (multipl. fault) load (rl) > threshold and air mass > threshold."
VERDICT: This response from Durametric combined with the change in engine feel after installing new Bank 1 fuel injectors, I'm calling this a Bank 1 code. Whether it's being tripped for range 1 rich or range 2 rich or lean, I'm not sure.

OK, on to some of the work I've completed since last check-in:
I removed the upstream O2 sensors one at a time and performed a back-pressure test on each bank. Both banks had nearly zero back pressure at idle and not much more at 2500 RPM (maybe .5-1 psi). I inspected upstream view of each cat with a borescope and verified they do not appear plugged, nor are they burned through or anything like that.
I left the new upstream O2 sensors in place and collected a lot of data. P1126 did return after about 50 miles of driving. Fuel trims are nearly identical for Bank 1 and Bank 2, RKAT is now roughly -3.5 and FRA is roughly 1.3, which causes me to question why I'm not receiving a code for Bank 2 as well?

Comparing a Normalized MAF signal (divided MAF signal by idle value of 15) to the Throttle Position Sensor 1, I got the following graph:



Not sure if this tracks similar to how blue62 described or not? Also during data collection, kept an eye on MAF outputs. Is the MAF supposed to reach up over 582 kg/h (& 3.58 V) at 5260 RPM? I'm not sure on what the correct values are but this one appears to be functioning to me (output responses to increase air flow).

Next warm day, the spark plug tubes go in. I'll post any updates after that.

Any input/thoughts/new troubleshooting is appreciated. Aside from investigating the EVAP component under Bank 2 intake manifold and attempting to evaluate the 2 electronic change-over valves (one SAI and the other for the resonance flap) I'm kind of out of ideas.

Edit: Forgot to mention, observed fuel economy has largely improved and seems back to normal.