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

[Zzorro]

Originally Posted by blue62

Following your issue with interest.
Please post results once you have changed out the injectors.

Likewise.

My codes are clear after changing out all 4 O2 sensors, but still get lingering short periods of bogging, or lack of power. Have never stalled.

[Brian Minson]

I had a 2001 986 as well with the same issue and ran thru all those fixes as well. Have you REPLACED the MAF?? Just an idea.

Here is the 986 DIY playlist of videos I did on my car's issues.

https://www.youtube.com/playlist?list=PLbBMHPz04qDW2lbqUZ4p2yWK9joftSOEO

Brian
HTTP://986Brotherhood.com

[porschefan76]

Wow, this community is awesome. Thank you all for continuing to follow along and helping. I've currently got my heater running in my garage trying to break it into the 50's so I can get some work done today.

Originally Posted by Zzorro

Likewise.

My codes are clear after changing out all 4 O2 sensors, but still get lingering short periods of bogging, or lack of power. Have never stalled.

I've got my Durametric data and it appears that my pre-cat O2 sensors are working as intended. I'm fairly certain my post-cat are working properly as well, as they maintain a steady mV reading at idle, but they do change as the revs change. So, I haven't considered changing those yet, but might be something I'll do if it's recommended. Again, I've got my data and plots if anyone wants to dig in, but this is not my profession, so I only know what I've read about this data/plots.

Originally Posted by Brian Minson

I had a 2001 986 as well with the same issue and ran thru all those fixes as well. Have you REPLACED the MAF?? Just an idea.

Here is the 986 DIY playlist of videos I did on my car's issues.

https://www.youtube.com/playlist?list=PLbBMHPz04qDW2lbqUZ4p2yWK9joftSOEO

Brian
HTTP://986Brotherhood.com

I actually make reference to your video in my OP. They were very helpful and helped clarify things when I checked for fuel pump volume and fuel regulator pressure checks, so thank you! I was really hoping my problem was fuel pressure as it has a tendency to backfire into the intake and lope similar to your car in the video when it's cold, but not when warmed up.

I'm really hoping replacing the fuel injectors gives some results, because after that, I'm not even sure which animal to chase next. As I mentioned above, my bank 1 camshaft deviation is out of spec, but not enough to throw a code, so I am not sure if that is contributing to the issue or not.

[porschefan76]

Hello all, just an update. I was able to get the temp up in the garage and got to work. The P1126 CEL that started all of this was for bank 2, so I started there. It took a little longer than I anticipated, so I only got bank 2 fuel injectors replaced. I'll do bank 1 when I get some warm weather again.

Unfortunately, there was no real change to the idle/throttle condition. Upon first start the engine still responds very poorly to throttle input. Backing down the driveway, response improves slightly, but not much. I left the engine top cover off for the first 5 minute drive so I could monitor for leaking fuel at stop signs. With that cover off, I can clearly hear swooshing noises when I crack the throttle, especially on blips. At one point, when I was first pulling away from the driveway, I could even hear a whistling noise, so I'm convinced this must be a vacuum leak. Another run at the intake smoker when the weather gets warmer.

Anyway, in frustration, I took a quick (and annoyingly shaky) video with my phone to listen to in an attempt to locate the noise. It appears to me to be coming from the vicinity of the throttle body. I've decided to upload the video and post a link in this thread in case anyone wanted to take a listen and see if it sparks any thoughts on further troubleshooting.

(truly sorry for how bad this video is. I might make a more patient attempt soon)

I think I may get a unidirectional microphone I can move around the engine and listen while a buddy hits the throttle as well, to see if i can really narrow it down. I have avoided using the "spray carb cleaner" step the whole time, but I might do that tomorrow in the area where the infamous J-tube connects to the plenum.

So, I guess right now, my question to you folks is, "Do you think the sounds in the video I linked indicate a vacuum leak?" Please feel free to let me know what it could be if it does not sound like a vacuum leak as well. As usual, thank you for your time!

[blue62]

you have a interesting situation there.
Backfiring into the intake would indicate a vacuum leak
The sound in your video sounds somewhat like a vacuum leak.
But on the other hand your low RPM fuel trims (RKAT) are negative values indicating a rich condition at idle. Vacuum leaks cause lean conditions at idle. Your mid to upper range fuel trims look to be positive values which you would expect under load at mid to upper RPM range. Adding fuel under increased engine load.
Porsche and or Durametric expresses fuel trims in a manner that I am not very familiar with so I could be off on what I think I see.

What are your pre cat o2 sensors doing at idle and again at say 3000 RPM???
If fuel trims are going negative at low RPM I would expect the O2 sensors to be showing a rich condition. Voltage above .500 mil volts then dropping as RPM increases and fuel trims go positive.
The sound in your video also (to me) sounds a little like a possible exhaust restriction.
Any chance the Cats are plugging up??

Don't know if I have helped or added confusion
But those are my thoughts

[porschefan76]

Originally Posted by blue62

you have a interesting situation there.
Backfiring into the intake would indicate a vacuum leak
The sound in your video sounds somewhat like a vacuum leak.
But on the other hand your low RPM fuel trims (RKAT) are negative values indicating a rich condition at idle. Vacuum leaks cause lean conditions at idle. Your mid to upper range fuel trims look to be positive values which you would expect under load at mid to upper RPM range. Adding fuel under increased engine load.
Porsche and or Durametric expresses fuel trims in a manner that I am not very familiar with so I could be off on what I think I see.

What are your pre cat o2 sensors doing at idle and again at say 3000 RPM???
If fuel trims are going negative at low RPM I would expect the O2 sensors to be showing a rich condition. Voltage above .500 mil volts then dropping as RPM increases and fuel trims go positive.
The sound in your video also (to me) sounds a little like a possible exhaust restriction.
Any chance the Cats are plugging up??

Don't know if I have helped or added confusion
But those are my thoughts

Hi again! You're absolutely helping and I appreciate it. OK, re: the exhaust, I have not done any troubleshooting there. What's the best way to know if my cats are plugged? I will do some searching and see if I can find some posts. Would the engine operate smoothly (more smoothly, still down on power) at higher RPM and experience the bulk of the issues at idle if the cats were plugged? Would I get a CEL if the cats were plugged? I'm interested in performing any suggested troubleshooting.

Regarding the RKAT and FRA data, your thoughts echo my own, which is why I was hoping I had a non-functioning fuel injector(s) (open at idle more than it should be going rich, but not delivering enough fuel at WOT resulting in lean at higher RPMS). I have some data plotted, I'm not sure how to get the paste in from excel, so I'll try posting some image captures of the data.





If anyone would prefer, I can take a shorter time snaps, to open up the plot a little more if the data isn't clear enough. Unfortunately, I don't think I have/plotted MAF voltage, but iirc, it was within Porsche specifications, thus I haven't replaced the MAF.

Thanks again for all of your time!

[blue62]

Looking at the post Cat O2 signal I would say your cats are not working.
Post Cat signal is following RPM and Pre Cat signal very closely. Not as it should be.
Post Cat signal will have some rises and falls with RPM and engine load but should be biased towards a flatish signal voltage in the .650-.750 mil volt range.

There are some good youtube videos on testing for exhaust restrictions.
Also if you look up ScannerDanner on youtube he has some very good videos on reading fuel trims and O2 signals to diagnose performance problems.
He is a professional and very good at what he does.
You can view his videos and have a visual of what your Post Cat O2 signal should look like (flatish) under different running conditions.
Perhaps some of his videos will give you some insight on your issues.

[porschefan76]

Originally Posted by blue62

Looking at the post Cat O2 signal I would say your cats are not working.
Post Cat signal is following RPM and Pre Cat signal very closely. Not as it should be.
Post Cat signal will have some rises and falls with RPM and engine load but should be biased towards a flatish signal voltage in the .650-.750 mil volt range.

There are some good youtube videos on testing for exhaust restrictions.
Also if you look up ScannerDanner on youtube he has some very good videos on reading fuel trims and O2 signals to diagnose performance problems.
He is a professional and very good at what he does.
You can view his videos and have a visual of what your Post Cat O2 signal should look like (flatish) under different running conditions.
Perhaps some of his videos will give you some insight on your issues.

OK, I'm going to follow this and see where it leads. I've watched Schrodingers Box on youtube and he's where I got my primer on fuel trims, but ScannerDanner seems great as well. In fact, I'm watching this video now:
https://www.youtube.com/watch?v=9TlygJMxTps
I may go purchase a back pressure gauge and check each of the cats. The only hesitation I have with this right now, is that in ScannerDanner's video, he said that because the pre-cat O2 sensor indicates rich at wide-open throttle, he'll focus on the exhaust and assume it's not a fuel issue. It does not appear that I am running rich (likely lean) above idle. I also do not have a situation where the car will not exceed a certain speed. It still "almost" drives like it did before above 3000 RPM. Additionally, my water temperature gauge has never indicated that the engine was running hot.
I may start by checking the temperature of each cat at the inlets and outlets and ensure the outlet temperatures are higher than the inlet temperatures, indicating the cats are heating the gases. I will also try to capture a stream of good Durametric data from all 4 O2 sensors while maintaining 2000-2500 RPM for a minute or two. I will post results once I get a chance to run these tests, hopefully this afternoon. Thanks!