I could swear my 3.4L 981S runs smoother in the sub-3K RPM range on ethanol-free gas. Could just as easily be a placebo effect, though. Maybe check pure-gas.org to see if there's any available locally, and try a tankful or two.

If something changes about the way your car behaves, it's usually the car's fault, but not always. Mainstream retailers are always messing around with oxygenation, reformulated additive packages, and who knows what else, both on a seasonal basis and otherwise.

 

I've been running ethanol-free for most of the time I've owned mine. This year, I've been running Chevron 94 with 0% ethanol. Still no difference on the stumble issue for me.

 

Did you change the coil packs out at the same time as the spark plugs? I changed the plugs without replacing the coils. Car felt a bit sluggish for a few weeks. Went back in and changed the coils and car felt strong again. Car still ran okay with the old coils but seemed much smoother and stronger with the new coils. Easy to do and not costly when purchased from FCP Euro. Wished I would have changed them when I changed my plugs.My car is a 2014 C2 with PDK.

 

LOL!

 

I didn't think about resetting the PDK. I might try that. What does that do, cause the PDK to "unlearn" my throttle style so it starts to learn again? Is that a big deal for the dealer to do? Or is it simply a matter of hitting a key on the PIWIS tester?

 

I did the same thing you did - changed only the plugs without the coils. Unlike your experience, though, after the plugs were changed I felt the car was more responsive to throttle inputs and it felt better than ever to drive. Of course, that could have been a false sensation created by my brain subconsciously wanting to know why the heck I paid so much money to change plugs that had only 18.5k miles on them. But whether real or imagined, the car did seem to drive better with the new plugs until I started driving it more aggressively, which is when I first noticed a stumble when hitting the throttle quickly from idle. If the engine is not under load, blipping the throttle when the car is idling causes no stumble at all. It's only when the engine is loaded by the drive train.

 

Great idea. Thanks. I tend to fill up at the same Shell station most of the time, because it's the least expensive station in town by a long shot (usually 20-30 cents cheaper for premium 93-octane than other local stations). But I never thought of checking if that gas has ethanol in it. My state requires a station to label its gas if it contains more than 1% ethanol, so I'll check the next time I fill up. Unfortunately, looking at the pure-gas.org website, there are no ethanol-free stations listed anywhere close to where I live. That doesn't mean there aren't any, only that finding one will be trial-and-error.

 

It's not a big deal for the dealer to do and actually the Cobb accessport has a setting to do it as well, if you have that or have a way to get it. I'm not sure what all is involved as I've only read about it, but from what I remember, the car shakes etc quite a bit as it's being reprogrammed.
No idea what the dealer would charge for the service. Apparently it's a good thing to do every once in a while for the life of the PDK though.

 

Hello folks.
I too had a stumble that would reliably go away for 24 hrs after the DME was reset. You can try this yourselves by disconnecting the battery for a day.
I also fought with Porsche to have them install a data recorder to find the source.
Their "findings" were that I was causing the stumble by taking my foot off the accelerator. (Yes, you read that right - my paw let off the pedal at the exact same rpm every single time).

I informed them that I could reliably re-create the stumble by using cruise control (without touching the accelerator pedal). This led to crickets followed by "data don't lie"..
Needless to say, you cannot reason with a POS company that will simply not acknowledge an issue with any of their products for fear of opening Pandora's box. (Diesel-gate.. Nothing to see here folks)

In summary;
It's not only a 991 issue. Happens with some 997's (same engine) and with other P models too. Hardly surprising given the sharing of parts
It can appear at any point in time.
Do not waste you time with high octane fuel. It makes no difference.
It's not your plugs / coils etc
It's not your throttle body either

If your car cuts out as described by C7toM2toGT4? , check the spacer on your variocam (That falls under the hesitation criteria, not stumble)
I suspect the stumble is either a flakey vacuum system or a combination of your high pressure fuel pump and solenoid. (An abnormality not big enough to generate an error code).
I think the DME is trying to compensate, which is why a hard reset at the dealership clears the stumble (temporarily) without fail.

My $0.02..

 

Never had a stumble or hesitation....maybe the Italian Tuneups I do must help this issue.

 

Good info here.

I do feel when the incident happened with my car, it felt more computer related than anything. Like the throttle input just didn't get to the brain of the car. I romped it, the car just sat there like I didn't ask it anything. Very strange. And again, I've romped it over a dozen times at this point. Never has happened again, thank goodness.

 

Just another data point..

I have a 2012.5 C2S with the exact same miles, well maintained like yours. I've started noticing the stumble over the last year after driving it in both Ohio and California. I thought I was imagining it until I saw this thread.

I'll start excellerating from a stop in Sport + and see if there's a difference.

 

I believe this is not the oft-discussed stumble, I believe that this is a vario cam solenoid issue instead. You described the exact problem my 2012.5 C2 recently encountered.
There is a thread this.

 

My new to me 15 C2S was a bit stumbly and was not that impressive when romping on it when I took delivery. CPO car so just had the major service. As Ive driven the car harder, its came to life. I know the dealer had it for 38 days for sale, so its probably sat awhile. Maintained at Porsche Ft Myers its whole life as well. Makes me realize just how much these cars beg to be driven and driven hard. Coming from M cars, they're the exact same way. Never had a hesitation though, just rather lazy. The good thing is I have a VP Racing gas station next to my house that has no ethanol 93 as well as racing fuel of all levels. Ran their 97 on my tuned M6 GC with great results. Not sure how these motors respond to that octane but Ill be using their 93 for sure

 

It helps - I run Chevron 94 zero ethanol and it's quite a nice difference from 91 zero ethanol and an especially large difference from standard 91.

I find it is even a larger impact on tuned/turbo cars. My Golf R loves the 94, but that car is putting down more wheel HP than my 911