Could the stumble be caused by coking the intake valves? DFI engines are extremely soot heavy. Is it possible that the variocam kick, where air intake fluid dynamics are altered for more air is somehow constricted by these affected cars by coked up intake valves? The argument could be made that all cars should behave the same, but that's ignoring many driving factors that aren't congruent to everyone. I know of three Porsche DFI engines that needed to have their intakes cleaned; albeit for a general loss of power over 4000 RPM. These were GT3 engines.

 

It depends on the mileage. I don't see carbon buildup happening at such low mileage as reported on some of these threads.

 

A definite possibility.

My question remains; why is Porsche not the one searching for the cause, instead of frustrated owners and forum participants?

Are you listening Porsche?

 

One small observation: I've noticed when the weather is 50-60 deg F and low humidity my stumble issue seems much less apparent. Just yesterday, for example, my ride home was actually enjoyable. I'm wondering if this indicates air flow / air mass sensor issues.

 

I still think it's because they know what is causing it, they know its software related, they programmed the car this way for a reason, and they know they can't 'fix' it, per se.

Maybe I'm jaded from riding so many motorcycles that had crappy fueling down low, but I see this as something they probably knew would happen to some of the cars due to the programming tradeoffs and they decided it was minor enough to be acceptable. I'm guessing their view is, "yes, if you drive slow to moderately in that rev range you're going to get a stumble. Don't do that."

BTW, has anybody posted video of this stumble? I'd be curious to see/hear it.

 

No video, but here's an audio clip recorded with an external mic and PSE on. I think you'll be able to hear the distinct pitch change at the point of the stumble. I still tend to believe it's caused by a "messy" VarioCam changeover.

https://dl.dropboxusercontent.com/u/...45/991/gp1.mp3

 

Are the 2016's clear of the stumbling problem?

 

To be perfectly clear, there are actually two stumbles we're talking about. There is a stumble in the mid-2000 range (usually 2300 to 2500). Then there is the VarioCam Plus transition stumble (usually higher around 2800 or 3000). These are different, and some people have one or the other or both. I very much have both.

 

Baiscally what my dealer told me when he replicated the issue on an identical 997 GTS. I'm in the process of conducting some experiments to get more info via OBD2 about throttle pedal position vs. throttle body position at certain speeds (50km/h is the interesting speed, as it's used for European noise emission testing). There must be some connection between my observations of stumbles at 1'200 RPM, 1'800 RPM and the infamous 50km/h full-throttle noise test...

 

It's definitely rpm related and not speed related.
At 75-80mph at around 2500-2600 rpm in 7th gear (on a very slight incline) I can replicate a prolonged and persistent stumble, which manifests itself in the form of a fishing line tug sensation, or a very mild but continuous misfire.

Unfortunately there is no way to record the sensation, nor is there any definitive change in engine noise.

It feels like fuel or air starvation.

When accelerating moderately in 2nd gear, it stumbles without fail between 2500-2600rpm.

 

That's what my car did. It started doing it right out of the box, brand new, with no miles. So that kind of kills the carbon/coke idea, at least for that car.

My 996 had a very abrupt variocam changeover around 3000 RPM, and it was an entirely different experience. It was also the primary reason that I sold the 996. I noticed it less in the 997.

 

At some point, someone is going to have to get their cars looked at and diagnosed. Without something further to go on, this thread will get forgotten until refreshed every 6 months. You can only talk about the hypotheticals so much.

After all, things are good. They have ZERO incentive to do anything.

http://www.worldcarfans.com/11512091...irst-time-ever

 

I am curious stealthboy how you broke your car in. Did you follow P recommendations? Also at what point did you first change the engine oil?
Jim

 

I guess the same questions to you Duxsi?
J

 

Is your car manual or PDK? Which model do you have?