Engine stumble/hesitation around 2500 RPM?
#677
05-15-2017, 09:07 AM
I've been thinking about getting a used 991, however, this stumbling issue is a concern (I can imagine someone trading in a relatively new car due to this issue).
Is it easy to reliably detect the stumbling issue on a testing drive? How consistently does it occur?
Thanks!
-dbk123
Is it easy to reliably detect the stumbling issue on a testing drive? How consistently does it occur?
Thanks!
-dbk123
#678
05-15-2017, 09:09 AM
[QUOTE=ettenw;4635251]
Now this is fascinating! I've had an OBDII reader on my wish list for a while - looks like I may pull the trigger on that.
My car is still at the dealer. It's been a week now. Still no updates on what they have or have not discovered. But I did get an update that Porsche engineers are now involved.
...
"Check over car, test drive to experience customer complaint. Found rough idle and stumble with light throttle at 2800 RPM. 3rd gear. Check car with scanner read out fault memory (all OK). Check live data, found fuel trim at idle off bank to bank by 1% (should be closer). Recommend check of intake system sealing. Possible vacuum leak."
...
Interesting. My car has similar symptoms, stumble, no codes, etc. I was able to collect some data via the OBDII connection and looked at the fuel trim data over a 13 minute drive. There are both long term and short term fuel trims on each bank. Here's the differences (Bank 1 - Bank 2, %) that I found:
____ Long Short
Min. -3.13 -10.16
Ave. -0.84 -0.85
Max. +0.78 +9.38
The same data only at idle:
____ Long Short
Min. -2.34 -4.69
Ave. 0.00 -1.11
Max. +0.78 +1.56
The Min and Max values may be of more interest than the Average since the car does run smoothly some of the time. I'd love to see the same parameters on a car that DOESN'T have our problem.
"Check over car, test drive to experience customer complaint. Found rough idle and stumble with light throttle at 2800 RPM. 3rd gear. Check car with scanner read out fault memory (all OK). Check live data, found fuel trim at idle off bank to bank by 1% (should be closer). Recommend check of intake system sealing. Possible vacuum leak."
...
Interesting. My car has similar symptoms, stumble, no codes, etc. I was able to collect some data via the OBDII connection and looked at the fuel trim data over a 13 minute drive. There are both long term and short term fuel trims on each bank. Here's the differences (Bank 1 - Bank 2, %) that I found:
____ Long Short
Min. -3.13 -10.16
Ave. -0.84 -0.85
Max. +0.78 +9.38
The same data only at idle:
____ Long Short
Min. -2.34 -4.69
Ave. 0.00 -1.11
Max. +0.78 +1.56
The Min and Max values may be of more interest than the Average since the car does run smoothly some of the time. I'd love to see the same parameters on a car that DOESN'T have our problem.
My car is still at the dealer. It's been a week now. Still no updates on what they have or have not discovered. But I did get an update that Porsche engineers are now involved.
#679
05-16-2017, 01:28 PM
Registered User
Join Date: Mar 2017
Location: Louisville Ky
Age: 64
Posts: 23
Rep Power: 0 
My car is FIXED, I say again, frigging FIXED! I can only hope that this is the same problem for everyone else effected by this very frustrating issue. Will Porsche do a recall or admit it? I don't know, but I'm now sharing what my service technician discovered. I had taken my car in 3 times for this issue with no remedy (or even serious acknowledgment that it was not right) the first two times. After a HPDE track session in the rain Apr 27-30 where I had to run in Sport mode instead of Sport Plus and the stumble was just awful (not an issue in Sport plus). I brought the car into the dealer May 1st. I got it back yesterday, May 15. They changed several things to include the PDK valve body, made me take off my Sharkwerks Race exhaust (per the regional reps demand) and were looking at the mass air flow sensor when my technician "discovered" that the "electrical connection for the Sport exhaust control valve was switched with the connection for the engine coolant shut off control valve"! This must have happened during my X51 install as I didn't have the stumble before hand. And I don't have it anymore either. Note, I do not have PSE! Could the other stumble cars above have come from the factory with this connection swap? Sounds crazy but I am so happy my car is "whole" again. Please ask any questions and I will also post a pic of the service receipt next. Alleluia!
Last edited by RobC4sX51; 05-16-2017 at 01:34 PM.
#683
05-16-2017, 06:59 PM
No, it's not solved yet! This is a glimmer of hope, for sure. My car is still at the dealer. 9 days now.
For me, I do not have X51 nor PSE. So I do not know this would be my issue, but as Rob mentioned he also doesn't have PSE. This is very fascinating and I will be sending this to my service rep immediately!
Congrats Rob. Now drive the hell out of your car and enjoy it.
Any chance you could get another pic of that invoice that is a little more clear? Thanks!
For me, I do not have X51 nor PSE. So I do not know this would be my issue, but as Rob mentioned he also doesn't have PSE. This is very fascinating and I will be sending this to my service rep immediately!
Congrats Rob. Now drive the hell out of your car and enjoy it.
Any chance you could get another pic of that invoice that is a little more clear? Thanks!
#684
05-16-2017, 07:02 PM
That's good news. However, I wonder if this is a general solution since my car doesn't have the sport exhaust. It's possible that the connector is there anyway. Anybody know? Anybody know exactly where those connectors are?
#686
05-17-2017, 08:16 AM
Registered User
Join Date: Mar 2017
Location: Louisville Ky
Age: 64
Posts: 23
Rep Power: 0 
In service date was May 31, 2013. C4s w PDK, SC, but no PSE. Has 26130 miles now. Had 23500 when the X51 was installed. Discovered the stumble at 24k miles. Good luck, hope this helps. My car is truly stumble/hesitation free! Loving it again!
#687
05-17-2017, 12:00 PM
Got the dreaded call from my service advisor. As I feared they are telling me that they can't find anything wrong and the car drives "normally".
Here we go...
Here we go...
#689
05-18-2017, 02:28 AM
And the saga continues
#690
05-18-2017, 07:30 AM
Picked up my car this morning. The official word is that it is driving the way the engine is designed and it is normal. They claim they drove a few other 991.1s and said it was similar. Talked with the tech and he said there is nothing they can do. So it seems that they acknowledge there is a stumble, but claim everything is working within some parameters they define, so it is "normal". He did at least admit that everything is well documented just in case there is something I find in the future even outside of the warranty period so it will be covered. This was my seventh service appointment specifically dealing with this issue. I have a paper trail a mile long.
On to plan B... I will be taking it to my independent mechanic and will have them perform a smoke test of the intake system to look for leaks.
On to plan B... I will be taking it to my independent mechanic and will have them perform a smoke test of the intake system to look for leaks.