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

[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.

[blue62]

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???

[porschefan76]

Originally Posted by blue62

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!

[porschefan76]

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!