I'm sick...think my car is developing "The Stumble"
#61
Stealthboy, I doubt it's the fuel. I know this has been a nagging issue for you, for many years. Unfortunate.
You can add as much 100 octane (make sure lead free and not over 10% ethanol) as you want and your billfold allows. Your car can accommodate it, but can't utilize and optimize the added octane without a tune. It will be mostly placebic. You may notice a very-very-very slight "improvement" up to about the average 96 level. Anything above that would be a complete waste. So, if you do a 1/2 tank 93 and a 1/2 tank 100 you will average out at about 96.5. Good luck with the experiment.
You can add as much 100 octane (make sure lead free and not over 10% ethanol) as you want and your billfold allows. Your car can accommodate it, but can't utilize and optimize the added octane without a tune. It will be mostly placebic. You may notice a very-very-very slight "improvement" up to about the average 96 level. Anything above that would be a complete waste. So, if you do a 1/2 tank 93 and a 1/2 tank 100 you will average out at about 96.5. Good luck with the experiment.
#62
I have preliminary evidence of a fix for the stumble. The suggestion that I am researching has already been mentioned in one of the stumble forums. I only have a couple hours of data right now so I don't want to say that this is the end all be all of the stumble. My current results are as such, I can still feel that cam changeovers at the appropriate rpm's but the stumble has almost completely gone. If I didn't feel it myself I would not believe it. I don't want to come out and say exactly what it is until I have some more evidence. I will report back later this week
#63
I have preliminary evidence of a fix for the stumble. The suggestion that I am researching has already been mentioned in one of the stumble forums. I only have a couple hours of data right now so I don't want to say that this is the end all be all of the stumble. My current results are as such, I can still feel that cam changeovers at the appropriate rpm's but the stumble has almost completely gone. If I didn't feel it myself I would not believe it. I don't want to come out and say exactly what it is until I have some more evidence. I will report back later this week
But seriously, keep up the good work in whatever investigation you're doing. I'm looking forward to your findings. The only solace I have in this stumble fiasco is at least knowing I'm not alone. I am finally getting to some data logging with my ODBII scanner to monitor fuel trims. Something screwy is going on there...
#64
What a tease!!!
But seriously, keep up the good work in whatever investigation you're doing. I'm looking forward to your findings. The only solace I have in this stumble fiasco is at least knowing I'm not alone. I am finally getting to some data logging with my ODBII scanner to monitor fuel trims. Something screwy is going on there...
But seriously, keep up the good work in whatever investigation you're doing. I'm looking forward to your findings. The only solace I have in this stumble fiasco is at least knowing I'm not alone. I am finally getting to some data logging with my ODBII scanner to monitor fuel trims. Something screwy is going on there...
#65
So if anybody wants to test out my theory (and was mentioned before in this thread), here is my thought process and ultimate story of how I got to this point.
I purchased a car with a bad O2 sensor, it was throwing stuck lean codes. So the O2 sensor was telling the computer it was lean and the computer was richening up the fuel mixture to compensate to get the O2 sensor to move off lean. Porsche replaced the O2 sensor. My car since purchase has always had the stumble a little but has gotten worse recently. This is where I believe something is changing in the car. As you can see by the title of this thread "I think my car is DEVELOPING the stumble". Something is changing in the car to create the stumble. The stumble feels like the car is coughing, like bad gas or a misfire even. Mostly in 2nd gear, part throttle or low throttle input at about 2500 rpm.
So I have one new O2 sensor. As I was researching the stumble because I felt it was getting worse, I checked out this thread and another stumble thread and read every post! In the middle of the one of the threads someone mentioned that they changed their O2 sensors and it fixed the stumble. That got me thinking, could it be that simple?
My theory is that O2 sensors drift especially when exposed to atmospheres and barometric pressure changes, etc. depending upon the type of O2 sensor technology used. My bank 2 O2 sensor was new, but my bank 1 had been submitted to the same rich mixture as the other potentially, as I have a x-pipe. They also had the same amount of miles on them, so if one drifted, the other may not be accurate as it needs to be for a sensitive DFI engine.
I decided to order another O2 sensor, from Advance auto just to try out my theory. Set me back a whopping $120. I did the change myself. Note: do this when your engine is cool!!! The tight spaces for routing the connector can have very hot surfaces that you can't pull your hand out of because your fit-butt is jamming your arm against it!! Not me... a "friend" had that happen to them. HA!
My preliminary data now that I have two new O2 sensors is very promising. I have put on about 30 miles and what a difference! But it may be premature, but I don't think so. All I did was change the one O2 sensor and an immediate difference (after a couple miles so the computer can adjust to the new inputs and update the fuel trims - hence my previous post about fuel trims being a part of it).
If you want to try the experiment, replace both PRE-CAT O2 sensors, don't worry about the after cat ones.
I purchased a car with a bad O2 sensor, it was throwing stuck lean codes. So the O2 sensor was telling the computer it was lean and the computer was richening up the fuel mixture to compensate to get the O2 sensor to move off lean. Porsche replaced the O2 sensor. My car since purchase has always had the stumble a little but has gotten worse recently. This is where I believe something is changing in the car. As you can see by the title of this thread "I think my car is DEVELOPING the stumble". Something is changing in the car to create the stumble. The stumble feels like the car is coughing, like bad gas or a misfire even. Mostly in 2nd gear, part throttle or low throttle input at about 2500 rpm.
So I have one new O2 sensor. As I was researching the stumble because I felt it was getting worse, I checked out this thread and another stumble thread and read every post! In the middle of the one of the threads someone mentioned that they changed their O2 sensors and it fixed the stumble. That got me thinking, could it be that simple?
My theory is that O2 sensors drift especially when exposed to atmospheres and barometric pressure changes, etc. depending upon the type of O2 sensor technology used. My bank 2 O2 sensor was new, but my bank 1 had been submitted to the same rich mixture as the other potentially, as I have a x-pipe. They also had the same amount of miles on them, so if one drifted, the other may not be accurate as it needs to be for a sensitive DFI engine.
I decided to order another O2 sensor, from Advance auto just to try out my theory. Set me back a whopping $120. I did the change myself. Note: do this when your engine is cool!!! The tight spaces for routing the connector can have very hot surfaces that you can't pull your hand out of because your fit-butt is jamming your arm against it!! Not me... a "friend" had that happen to them. HA!
My preliminary data now that I have two new O2 sensors is very promising. I have put on about 30 miles and what a difference! But it may be premature, but I don't think so. All I did was change the one O2 sensor and an immediate difference (after a couple miles so the computer can adjust to the new inputs and update the fuel trims - hence my previous post about fuel trims being a part of it).
If you want to try the experiment, replace both PRE-CAT O2 sensors, don't worry about the after cat ones.
#66
Excellent experiment. Please report back in a week. There have been cases (myself included) where you go into a dealer for something, they reset the engine control software, and it seems to be fine for a little while until the computer falls back into stumble mode, as if it's learning incorrectly. This is why I believe this to be sensor-related.
EDIT: The pre-cat sensor for the 991 is $142.50! Whoa.
EDIT: The pre-cat sensor for the 991 is $142.50! Whoa.
#67
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Following along for further data. It seems plausible.
FWIW, since I created this thread, I have been being as hyper sensitive as only a forum-going carpercondriach can be. And it's interesting that what I am feeling is definitely not consistent. Happens worst when the car is cold, in gears one and especially two. Almost completely gone when car is warmed up.
Odd.
FWIW, since I created this thread, I have been being as hyper sensitive as only a forum-going carpercondriach can be. And it's interesting that what I am feeling is definitely not consistent. Happens worst when the car is cold, in gears one and especially two. Almost completely gone when car is warmed up.
Odd.
#69
Excellent experiment. Please report back in a week. There have been cases (myself included) where you go into a dealer for something, they reset the engine control software, and it seems to be fine for a little while until the computer falls back into stumble mode, as if it's learning incorrectly. This is why I believe this to be sensor-related.
EDIT: The pre-cat sensor for the 991 is $142.50! Whoa.
EDIT: The pre-cat sensor for the 991 is $142.50! Whoa.
and to clarify that this was done in my garage and there was no resetting of the computer or disconnection of the battery.
#70
#71
The stumble is certainly an odd duck. On a drive last week, my car was virtually stumble free. Noticeably better than it has been in the past. Two days later, however, the stumble was back with a vengeance. Possibly the worst performance over the last 4 years. (No significant changes in the atmospheric condition).
Needless to say, I am EAGERLY awaiting the (hopefully promising) outcome of this experiment.
The saddest part of this journey is that PORSCHE is the one who should be attempting to resolve this issue. If they were proud of their product, they wouldn't sit back and see loyal customers suffering.
Needless to say, I am EAGERLY awaiting the (hopefully promising) outcome of this experiment.
The saddest part of this journey is that PORSCHE is the one who should be attempting to resolve this issue. If they were proud of their product, they wouldn't sit back and see loyal customers suffering.
Last edited by pfan; 07-14-2017 at 06:32 PM. Reason: Addition
#72
#73
Some of the issues you may have even if you are CPO is that Porsche will only replace parts if they are broken or result in a code. This did not result in a code for me. So I imagine it will be a slight challenge to get a Porsche dealer to change them out for you if you are under CPO. This experiment will require either a do it yourself or take it to an indie to get the best price
#74
Report for day two. No stumble at all.
just for frame of reference my stumble before was extremely noticeable especially in second gear. The stumble felt like a misfire or a cough where it's severely affected the performance of the vehicle at 2500 RPM. It did go away sometimes or at certain throttle angles, but I could reproduce it pretty much anytime I wanted to at very slight throttle angle. Some days were worse than others. For what seems to be no rhyme or reason.
just for frame of reference my stumble before was extremely noticeable especially in second gear. The stumble felt like a misfire or a cough where it's severely affected the performance of the vehicle at 2500 RPM. It did go away sometimes or at certain throttle angles, but I could reproduce it pretty much anytime I wanted to at very slight throttle angle. Some days were worse than others. For what seems to be no rhyme or reason.
#75
Hasn't Porsche said this is "normal behavior"
Good luck guys. This subject has been talked about for years. Someone needs to get to the bottom of it! It's been toooooo long. I mean, can't a competent race shop figure this out? Can't be that complicated.
If a shop is able to figure out putting in an R transmission in a GT3 RS, they surely can figure out a stumble issue
Good luck guys. This subject has been talked about for years. Someone needs to get to the bottom of it! It's been toooooo long. I mean, can't a competent race shop figure this out? Can't be that complicated.
If a shop is able to figure out putting in an R transmission in a GT3 RS, they surely can figure out a stumble issue
Last edited by STG; 07-14-2017 at 09:33 PM.