stealthboy

Thank you very much. And yes, I agree it's most likely not the fuel, but I just want to rule it out completely.

ChicagoSpeed996

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

stealthboy

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...

ChicagoSpeed996

I would be interested in your findings as well as I believe the stumble has to do with fuel trims and sensors. Specifically the O2 sensors. More data this morning on the way to work. The stumble is non-existent! Full write up coming soon, though feel it still may be premature.

ChicagoSpeed996

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.

stealthboy

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.

Needsdecaf

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.

ChicagoSpeed996

Somebody pony up for $300 and change out their pre-cat 02 sensors. Try the experiment with me!

ChicagoSpeed996

and to clarify that this was done in my garage and there was no resetting of the computer or disconnection of the battery.

stealthboy

Just for clarification, this is on your 997, right?

I found them a bit cheaper, but still I'm looking at $250 for both. I'm close to pulling the trigger on this...

pfan

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.

ChicagoSpeed996

Yep I have a 997.2 so the 9A1 engine most of you have as well.

ChicagoSpeed996

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

ChicagoSpeed996

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.

STG

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