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.

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.

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:

https://i.imgur.com/UZXjZBA.jpg

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.

Your MAF signal compared to the TPS signal looks normal to me.
So I believe you MAF is fine.
You say your fuel mileage is back up???
Improvements are good. Any idea what your MPG is currently???
Do you think the fuel mileage improvement is from changing the injectors???

Hi blue62! I can't thank you enough for sticking through this with me.

Yes, fuel economy seems to have largely returned to earlier numbers. I do not have an absolute number, but I'd estimate it at about 20mpg (~75 miles per quarter tank). This is varied driving conditions while test driving. I can only attribute this change to the replacement of Bank 1 fuel injectors, because Bank 2 fuel injectors didn't seem to make much of a difference to engine performance or fuel economy. Regarding engine performance, most, if-not-all, of the cold-start idle weirdness has gone away.

I have no black smoke (rich condition) at idle, nor any soot on the tailpipes, and the exhaust does not smell of gasoline. The old O2 sensors had a mostly white-tan light coating, which seems normal. Also, my RKAT numbers for both banks have gone further negative to -3.5 since changing Bank 1 fuel injectors. Yet, I'm still only throwing code P1126.

I got a little fed up trying to decipher Durametric data, so I bought a bluetooth obd and android app to watch the new O2 sensors and the post-cat sensors (as well as STFT, this is before I figured out it's PID in Durametric). All four O2 sensors and the STFT values looked very much closer to the data from ScannerDanner's videos (flat line for the downstream O2 sensors and adaptation behavior from the upstream sensors) than anything I was able to plot from Durametric data. No conclusions drawn here, it's just the way it is. As I learn to use the Android app better, I will try to output data from there as well if you want a look.

This thread has gotten long, so I will post a roll-up (TLDR style) summary shortly to get a snapshot of the current situation and hopefully, something might jump out, because right now, I'm not sure where to go next. Thanks again!

Troubleshooting code P1126 which seems to be Bank 1 range 1 rich limit exceeded. Initially the car also began idling poorly and backfiring through the intake. Most of this has cleared but P1126 reoccurs after clearing.

TLDR-style roll-up of troubleshooting performed thus far for clarity:

1. MAF - cleaned twice, outputs in Durametric seem good, 15 kg/h and 1.28-1.33 Volts at idle - both increase with engine speed
2. Throttle body, plenum and resonator removed and cleaned - not very dirty and very little oil present, all vacuum hoses inspected and verified connections
3. Fuel filter replaced
4. Fuel pump volume flow tested at fuel filter - 1.25L of fuel collected in 30 seconds (is this a potential problem above specified 850mL??)
5. Fuel pressure tested at fuel rail - 3.8 bar with engine off and fuel pump by-passed, 3.3 bar at idle - meets Porsche spec
6. Spark plugs replaced - old ones were not overly fouled, but electrodes had wear
7. Intake Smoke Test #1 - no leaking smoke observed
8. Crankcase Pressure Manometer - -4.95 to -5.05 inches of water column, re-checked yesterday, no change - AOS appears functional
9. Bank 2 fuel injectors replaced with new Bosch fuel injectors and fuel pressure regulator replaced (while fuel rail was out) - no perceived change or Durametric data changes
10. Bank 1 fuel injectors replaced with new Bosch fuel injectors - engine idles and performs much better, fuel economy returned; Banks 1&2 RKAT (range 1 LTFT) further decreased from -2.15 to -3.5, supporting rich at idle CEL
11. Upstream O2 sensors replaced - Originals still appear functional
12. Exhaust Bank 1 & 2 Back-pressure tested - ~0 psi at idle and 0.5-1.0 psi at 2500 RPM - no obstruction
13. Visual inspection of Cats at upstream O2 sensor port - seem largley intact, a few fractures at outer edges but otherwise intact (further investigations?)
14. Spark plug tubes - small leaks at cylinder 1 and cylinder 5 - will replace soon
15. Power Brake Booster line - Went on deep discount, so purchased - will replace soon
16. Intake Vacuum Test - just learned of Porsche suggested test - At fuel pressure regulator, attach vacuum gauge and look for 5.8 to 8.7 psi of vacuum - will perform soon
17. Intake Smoke Test #2 - will perform soon

Worth noting: Bank 1 camshaft deviation is -8.2 degrees and Bank 2 camshaft deviation is -4.26 degrees. Bank 1 deviation is outside +/- 6 spec by Porsche but not enough to throw a CEL. Don't know if this could be causing the issues?

Still getting P1126 code after 30-50 miles of driving. Thanks for looking, any help is appreciated!

Hey fan, glad to hear you're making progress. I could be wrong but I think that p1126 is a lean condition? Your negative fuel trim would support that. If you've got new injectors and good pressure then I would suspect a small leak. I know you said you smoked it but as we all know small leaks can be a pita for even the best to find.

Somebody else chime in on this, I don't wanna steer you wrong.

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Porschefan: Have you looked at the freeze frame info when the p1126 code triggers???
Could be a clue in that info.
I know you have smoke tested it and that the code is for just the one bank but the p1126 code is most often associated with some form of vacuum leak!!

Yes a vacuum leak of some sort has been my thought from the beginning.
Faulty injectors, Faulty O2 sensors, Or a Faulty MAF sensor can all cause a lean situation.
Porschefan has replaced or proven all those things good so I am still betting on a vacuum leak of some kind.

Hey Ike! Thanks for following up! Yes, I've had some progress, and it's driving much better, thankfully. I finally experienced those "smiles per gallon" again yesterday. I did notice a SLIGHT hesitation coming back on the gas in third gear yesterday while being slightly aggressive on an empty stretch and corner, but otherwise much improved.

I am honestly not sure one way or the other if it a lean or rich code. I have this excerpt from the Porsche/Bosch DME code list:

In above, being "below limit" can go one of two ways in my mind. If it's referring to the fuel trim being below limit, that would indicate a rich condition to me, because fuel trim would be subtracting from the injector time. If it's an O2 sensor that is below limit, to me that means lean because below .45mV is lean and above .45 mV is rich, unless I've got that reversed. The fact that Banks 1&2 RKAT are negative seems to support rich at idle to me as well, as RKAT is "range 1" so far as I can tell, which includes idle.

Other news and updates, I just had some warm temps in the garage, so I vacuum tested the power brake booster line and it held vacuum. I also started the car, pulled the vacuum hose from the fuel pressure regulator (rpms surged) and pulled about 10 psi of vacuum (over Porsche specification, I wonder if this means anything?). So the intake vacuum appears intact, if not high. That said, once outdoor temps are above 55F, I'll be smoking the intake again to look for leaks.

Anyway, if anything jumps out at anyone, please let me know. I need some next troubleshooting steps.

I am truly hoping this turns out to be vacuum! I will get that smoke machine back on the intake ASAP (weather/plastic window ugg). Thanks again!

just curious, could a Dry fuel injector Seal could cause a vacuum leak..?
.

Hi Gilles. Having just replaced all of my fuel injectors, on the fuel rail side, I think you'd have a fuel leak (pressurized fuel rail). On the intake manifold side, I suppose you would have an vacuum leak, yes.

If you have to replace your injectors or o-rings, I used a small amount of the recommended white lithium grease on all of the o-rings and it really made everything go back together nicely and prevents the o-rings from twisting upon insertion. Thanks!

Before you bust out that smoke machine I would look at some vacuum diagrams so you're familiar with the system. The vacuum houses, changeover valves, vacuum resivoir, etc are kinda complicated - not so much in theory, just in the layout. Hairline cracks in the vacuum resivoir, for example, have driven people to the brink of insanity. If you go over everything with the smoke machine and get nowhere I would bust out the brake cleaner to double check. Good luck!

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Hey! I've had the plenum, throttlebody and resonator tube off quite a bit and I'm actually pretty familiar with the routing of the vacuum lines. It appeared to be messy to me the first time or two I looked at it, but it's actually pretty straightforward! That said, I take your point, especially with respect to the smoke machine.

I'm fairly certain it's not the reservoir, because if I pull the vacuum line between it and the one-way valve after running the car, I can hear it suck in air. Perhaps I'll get in there and just test lines and y-fittings with my hand pump because it's easy to do and doesn't stink like the smoke machine. I also wouldn't mind some information on how to test the change-over valves. I have a 12V power supply and the ability to regulate voltage, so I'd like to actually test them for operation. I'll keep looking.

All this said, I just pulled 10 psi of vacuum at the fuel pressure regulator which is pretty healthy intake vacuum, if I am not mistaken. Porsche troubleshooting manual for checking fuel pressure regulator operation specified 6-9 psi of vacuum on this hose. I HOPE it's a vacuum leak, but at this point, with RKAT at -3.5, it appears this motor is running rich at idle, not lean like I'd expect for a vacuum leak.

To further muddy the waters though, when I used my cheap bluetooth OBD-II with Torque Pro, the LTFT (not divided into RKAT and FRA in Torque app) showed that they were pegged POSITIVE for both banks, which would support a vacuum leak.

I just realized that I said something that doesn't make sense and I am now far more confused. 1126 is lean, but negative fuel trim is the car pulling fuel out. I typed that earlier in a rush and didn't realize what I had said.

But that was the original reason I recommended changing the o2 sensors in the first place - things are pointing to a lean condition but the sensors are reading rich and hence the ecu is pulling fuel.

I'm not sure I'm helping at this point lol. This seems really screwy to me though and I'm not sure I can explain it any further at this point. Sorry dude :(. I would still check for leaks if you've had every off recently but the continued 1126 when sensors are detecting rich and pulling fuel back has got me thoroughly confused.

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Thanks for the tip!

No problem! Good luck! Bank 1 injectors are worlds easier to replace than Bank 2 due to tight working quarters with respect to the fuel return line nut.

Quick update - all information I am able to find states 17 to 23 inches of HG (8.35 psi to 11.3 psi) for proper intake vacuum. This contradicts the Porsche information I had (5.8psi to 8.7 psi) but I imagine being within a +/- 1 psi might be acceptable.

Only posting this because if I have proper intake vacuum, it seems less likely I have a vacuum leak, so I'm hoping those numbers jump out. If not, I think this leaves me with the EVAP system (electric valve stuck open? but no CEL or code?) and then it's on to mechanical problems with the engine (timing, valve seat issues, compression).

Thanks for your time!