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

[porschefan76]

Hello all. I'm hoping to get some help solving some issues with my "new-to-me" 2001 Porsche Boxster S 6-spd manual. I purchased the car in December of 2019 with 52,500 miles on the original engine and it ran perfectly until early November 2020. The car now has 56,980 miles on it. The issues began with a momentary perceived lack of power while merging onto a major highway shifting into 3rd gear and a check engine light later that same day. I purchased the Durametric tool and read a P1126 code, no others. The car wasn't acting up significantly until I cleared the P1126 code. Once I cleared the code, the engine began to idle poorly while cold and would backfire into the intake manifold when pressing the gas pedal, especially a sharp rev.

For reference, when cold, the engine has a tendency to perform much like the engine in this video around the 8 minute and 55second to 9 minute mark: https://www.youtube.com/watch?v=hddx5Ux1v4Y

It has not stalled on me, even when backfiring into intake etc. Once warmed up, the engine still hesitates going back on the throttle following shifts, but idles well and runs smoothly above 3,000 rpms. Fuel economy is terrible, I'd estimate at 12-15 mpg. Previous to these issues, fuel economy was over 20 mpg.


I got to work:
1. Removed and cleaned the MAF sensor with CRC MAF sensor cleaner. Cleaned/dried thoroughly twice - NO CHANGE
2. Replaced the air filter and removed the connector to the MAF and drove the car - NO CHANGE
-assuming this means good MAF, also when warmed up MAF data from Durametric matches Spec (15-16 kg/h at idle) with MAF connector installed
3. Dug into intake manifold, removing the throttle body, plenum and resonance tube and cleaned thoroughly and reassembled - NO CHANGE
- very little to no oil in intake. AOS tubes in good shape. Oil fill tube outside coated in oil and dust but no visible cracks. Cleaned the tube for reinspection at a later date.
4. Smoked the intake using a smoke machine, finding no smoke emitting from the intake - NO CHANGE
-smoked long enough for smoke to stream from the oil fill port when I removed the cap
5. Ran a full bottle of Techron fuel system cleaner in a full tank of gas. - NO CHANGE
6. Removed fuel filter and tested the fuel pump flow rate using the steps in the Bentley manual. Pump flowed 1.25 L in 30 seconds (exceeding spec 850mL in 30 sec) - NO CHANGE
7. Replaced what looked like the original fuel filter and tested the fuel pressure at the fuel rail. 3.8 bar with engine off and 3.3 bar with the engine at idle, meeting specifications in the Bentley manual - NO CHANGE
8. Tested vacuum at the oil filler with a manometer. With engine fully heat soaked, read between 4.95 and 5.05 inches of water column, meeting specifications I've found online. - NO CHANGE
9. I've purchased new fuel injectors and fuel pressure regulator and will replace the original fuel injectors (along with pressure regulator while the fuel rail is out) when the weather improves so I do not crack the plastic window (I do all work in unconditioned garage or outside).

The car leaks no fluids. I replaced the water pump, thermostat and coolant in May 2020. Spark plugs were replaced in June 2020, coils in excellent shape, not replaced. Spark plug electrodes were worn, but the plugs were not fouled. I changed the oil & filter at the 6 month mark with 2,900 miles on the oil.

Only knowns worth mentioning:
1. Original clutch, original IMS (at least no records indicating otherwise). Planning clutch/IMS when the weather breaks.
2. Camshaft deviation on bank 1 is -8.2 degrees (out of spec: +/-6deg) Still doing research but considering an engine out when I do the clutch to replace cam wear pads as well as chain tensioner. Bank 2 in spec at -4deg. Dropped oil pan when I bought the car, a few specks (maybe 15, size of ball point) of dark brown plastic in there. Have monitored since and no additional plastic in the oil or filter. Have 6 oil changes (back to 43,000 mi) worth of UOA's and all are good; 2 from me and 4 from P.O. (yes, I know this doesn't mean anything other than "so far, lucky")

I have driven the car 250 miles total throughout all of the steps listed above. I have captured data from durametric while driving on 3 separate occassions. I captured RPM, Banks 1&2 FRA & RKAT, banks 1&2 pre-cat O2 sensor & post-cat O2 sensor voltage, MAF hot film and voltage and intake air temperature if anyone is willing to dig into it. The only data I'd call "suspect" that I captured would be the post-cat O2 sensors, which appear to change while driving (voltage moves up and down) but they remain steady at ~0.74 mV on both banks at idle. The RKAT values shift from 0 to -2.4 on both banks and FRA shifts from 0 to 1.3 on both banks throughout the data acquisition period (I reset the ECU to clear the values and watch them change as I drove the car over time).

I've also actuated both bank Variocam at idle using the Durametric and I can verify that they are both functioning properly, causing the sound/feel of engine to change noticeably when activated.

I'm hoping for some experiences as I know this code and these symptoms are not rare. I've read every forum post I could find and I've tried to follow the troubleshooting steps in those posts, but I'm not getting any success. I've attempted to be succint and demonstrate my version of a methodical approach to troubleshooting the issues. I'm willing to try additional troubleshooting steps and procedures and answer questions as time permits. Thank you for your time!

Best Regards,

Rich

P.S. - should I post to other forums as well and if so, which?

[blue62]

Do you still get the P1126 code?
Is it the only code you get?
Any pending codes?

[JFP in PA]

I'd start pulling the fuel injectors, looking at the tips for carbon fouling.

RKAT is the adaptation value near idle.
FRA is the adaptation value under load and further divided into FRAU (lower load range ) and FRAO (higher load range) ranges (perhaps the durametric doesn't display these divisions).

The RKAT values near idle are used to change the length of the injector pulse to account for deviations from ideality (ie intake leaks) to maintain the proper O2 sensor reading. The RKAT value is an additive factor and can deviate by +/- 4.5%. The FRA value is a multiplicative factor and can deviate by 1.32 to 0.70. The positive values or values greater than 1 increase the injector pulse length to account for a lean mixture, and negative values and values less than 1 are to compensate for a rich mixture.

[porschefan76]

First, thank you both for responding so soon.

blue62
Do you still get the P1126 code?
Is it the only code you get?
Any pending codes?

P1126 is the only code I get legitimately. When I ran with MAF disconnected I got some codes for the MAF, but not since reconnecting. Also have not gotten P1126 back since first time. No additional pending codes.

JFP in PA
I'd start pulling the fuel injectors, looking at the tips for carbon fouling.

RKAT is the adaptation value near idle.
FRA is the adaptation value under load and further divided into FRAU (lower load range ) and FRAO (higher load range) ranges (perhaps the durametric doesn't display these divisions).

The RKAT values near idle are used to change the length of the injector pulse to account for deviations from ideality (ie intake leaks) to maintain the proper O2 sensor reading. The RKAT value is an additive factor and can deviate by +/- 4.5%. The FRA value is a multiplicative factor and can deviate by 1.32 to 0.70. The positive values or values greater than 1 increase the injector pulse length to account for a lean mixture, and negative values and values less than 1 are to compensate for a rich mixture.

Great, as soon as it gets warm enough to mess with the plastic window, I'll pull the fuel rail and look at the injectors as I change them.

I guess Durametric tool does not divide further because I do not see FRAU or FRA0.

RKAT values of -2.4 indicates rich at idle and FRA 1.3 (positive) is lean above idle then correct? I was considering smoking the intake again but if it's over-rich at idle, likely unwarranted?

Anything else I should be checking? Thank you for your time!!

[JFP in PA]

Originally Posted by porschefan76

First, thank you both for responding so soon.



P1126 is the only code I get legitimately. When I ran with MAF disconnected I got some codes for the MAF, but not since reconnecting. Also have not gotten P1126 back since first time. No additional pending codes.



Great, as soon as it gets warm enough to mess with the plastic window, I'll pull the fuel rail and look at the injectors as I change them.

I guess Durametric tool does not divide further because I do not see FRAU or FRA0.

RKAT values of -2.4 indicates rich at idle and FRA 1.3 (positive) is lean above idle then correct? I was considering smoking the intake again but if it's over-rich at idle, likely unwarranted?

Anything else I should be checking? Thank you for your time!!

You may be seeing a lean situation because the one or more injectors are carboned up and not delivering a full pulse fuel volume.

[porschefan76]

Originally Posted by JFP in PA

You may be seeing a lean situation because the one or more injectors are carboned up and not delivering a full pulse fuel volume.

OK, this is kind of where I was going when I ordered the replacement fuel injectors, so hopefully, I get some progress there.

One more question, the -8.2 bank 1 camshaft deviation alone should not be enough to cause these hesitation issues etc, should they?

Thank you again.

[blue62]

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

[porschefan76]

Originally Posted by blue62

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

Will do. Thank you for your interest and help!

It looks like I might be able to get the top in to service mode this weekend, so hoping to get to the fuel injectors.

[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!

[porschefan76]

OK, quick follow-up. I tested the temperatures of the "pre-cat" (cat on the header) and the temperatures were higher at the outlet end of the cats than at the inlet side, which is promising. I collected Durametric data for the O2 sensors while trying to hold the rpms at ~2000 RPMs for 2 minutes, followed by ~2500 RPM for 1 minute. The data is here:





The post catalytic converter O2 sensors appear to be responding properly, only showing big movements on throttle changes (to me at least. Please tell me if I am incorrect).
I went ahead and acquired a back pressure test gauge and O2 sensor socket and will attempt to check back pressure as soon as I am able. The weather is not going to cooperate this upcoming week.

As usual, thank you all for your time!

[Zzorro]

Following with interest.

[blue62]

Originally Posted by porschefan76

OK, quick follow-up. I tested the temperatures of the "pre-cat" (cat on the header) and the temperatures were higher at the outlet end of the cats than at the inlet side, which is promising. I collected Durametric data for the O2 sensors while trying to hold the rpms at ~2000 RPMs for 2 minutes, followed by ~2500 RPM for 1 minute. The data is here:





The post catalytic converter O2 sensors appear to be responding properly, only showing big movements on throttle changes (to me at least. Please tell me if I am incorrect).
I went ahead and acquired a back pressure test gauge and O2 sensor socket and will attempt to check back pressure as soon as I am able. The weather is not going to cooperate this upcoming week.

As usual, thank you all for your time!

Post cat sensor response may be ok. Not really sure.
Very very difficult to diagnose problems over the internet.
Another issue in diagnosing is that the Durametric expresses data over units of time (bottom axis on the graphs) as compared to frames when you watch something like ScannerDanner's video's. Makes comparing data a little more difficult.

You may want to look at ScannerDanners vids on Catalytic converter efficiency or his vids related to the P0420 or P0430 codes. In most of his vids there are examples of bad and good wave forms on the graphs. May give you better insight then I am giving you
I know you don't have the P0420 or P0430 code but your post Cat Sensor signals still seem a little suspect.
Hope I am not leading you down a wrong path.

[porschefan76]

Originally Posted by Zzorro

Following with interest.

Thanks! Weather just got bad for about the next week, so updates may be slow.

Originally Posted by blue62

Post cat sensor response may be ok. Not really sure.
Very very difficult to diagnose problems over the internet.
Another issue in diagnosing is that the Durametric expresses data over units of time (bottom axis on the graphs) as compared to frames when you watch something like ScannerDanner's video's. Makes comparing data a little more difficult.

You may want to look at ScannerDanners vids on Catalytic converter efficiency or his vids related to the P0420 or P0430 codes. In most of his vids there are examples of bad and good wave forms on the graphs. May give you better insight then I am giving you
I know you don't have the P0420 or P0430 code but your post Cat Sensor signals still seem a little suspect.
Hope I am not leading you down a wrong path.

I definitely agree that the fidelity of the data from the durametric is troubling at times. The fact that the length of the time steps are inconsistent really gets to me. I will take in some more of ScannerDanner's videos and see if causes any ah-hah! moments. As soon as I can get those O2 sensors off and check the back pressure, that'll be a big data point on whether I need to focus on the exhaust to solve this issue or not.

Thanks again for following and helping!