I've found that in a tunnel or beside a concrete divider with PSE selected I often get a very distinct audible change in the exhaust note.

 

I have requested this twice and they simply will NOT do it.
Perhaps someone else can try their dealership and report back.

 

Sorry to hear that Dux. If you are ever near the Dallas area go to Plano Porsche and ask for Service adviser Issac Madrid or head tech Cliff Blackshear. Part of the problem there is probably that it changes the service scheduling reminders.

ChuckJ

 

I know this thread has gone on for a while.. and has ranged from guesses about software bugs, cam sensors etc..

Just to add another thought to the thread that I've been wondering about...

The 991 boxer engine utilizes an anti-knock system..to accommodate lower octane fuels down to 91.. (which in some states like calif is as high as you can get usually).

And I believe the way it works is that there is some kind of sound sensors which detect the very onset of detonation or pre-ignition.. and then the ECU responds with changes in fuel metering, ignition timing and cam/valve timing.

I'm not entirely sure how much "learning" goes on with this, but I suspect that once the engine starts to detect, it modifies it's timing and probably retains for a while before "testing" the fuel by going back to the original settings.

What I wonder is if there is a failure of one of the multitude sensors involved what would happen? Especially with the "learning" that goes on.. With learning, reloading software or resetting things might fix for a short term..

Just a thought which quite possibly has no bearing in reality. But I suppose if I were experiencing the reported problem (I'm not) it would be interesting to see what happened if you changed the fuel grade to something higher (i.e. find some racing fuel) or even lower to see if it aggravated the problem.

Again, just a thought since no one seems to be getting much traction with Porsche on the issue.

 

Interesting thoughts. I have switched fuel brands (Sunoco, Shell, Exxon) with the same behavior but I have always used 93 octane. Perhaps it would be worth trying 91 or a racing fuel like you suggested. Hey, at least it would give us some more data points or things to rule out.

 

Your thoughts are all valid especially given the fact that a few drivers have claimed that adding Techron (from Chevron) apparently resolved their stumbling issue.

I need to look into this as there are two schools of thought when it comes to additives / cleaners.

 

Given that it only occurs from a dead stop as opposed to rolling start makes the fuel issue seem unlikely.

 

I don't know where you get that. Mine does not just happen from a dead stop.

 

Hunh? Why would that be?

While I'm not sure everyone who is reporting indicates it occurs from a dead stop.. if it did occur only from a dead stop would tend to indicate a higher engine loading condition (given you are moving from a zero inertia state.. .i.e. a stop)..

The thing is with engines.. especially when electronically controlled there are almost an infinite number of combinations of things that interact at any given moment.. In an older non electronically controlled engine of yesteryear.. it was ignition/spark timing advance based on engine loading which was typically indicated by a drop in engine vacuum.. And the mechanical carb could open up secondaries to allow more fuel.

But now you have precise amounts of fuel being injected based on a computer that is monitoring a multitude of things (base atmospheric pressure, engine loading, RPM, engine mode, anti-knock.., engine operating temperature, ambient temperature etc)..

When these factors interact the combinations of things that can happen is very complex.. and when something fails in the system that is used to control it... the failure operation could be very complex and counter intuitive. Which is why you hopefully have monitoring of the sensors themselves to see that they are individually operating properly.

Other things I'd wonder.. are people that are having the problems seeing it at higher altitudes (Colorado?) or just at near sea level environments. Anyone who flies small piston engine airplane will attest to how differently a NA engine performs even over 2 or 3,000 foot altitude change and a little temperature change.

 

This is a perfect description of one manifestation of the issue (assuming you're familiar with fishing).

Yesterday I had the car on a long incline, and while holding a constant speed of ~ 60 mph the rpm's were right around 2500. I had a small fish tugging on the line for several seconds. Definitely not a normal engine response, especially from Porsche.

This made me think; assuming this is a VarioCam related issue, (again, only an assumption), how does the system decide which profile to be in when it's in this transition zone for an extended period of time? Or maybe that's the problem - it can't make up it's mind.

 

My experience is that under hard acceleration from a standstill, at around 2300 rpm the engine hesitates for a moment before continuing on. The acceleration is not smooth. It is as if the fuel is cut off for a brief moment.

 

Intuitively it does seem some kind of failure or misoperation of VarioCam could be to blame..

But I don't believe VarioCam is a directly a profile or mode driven thing.. It is intended to be a continuously variable kind of electro/mechanical thing..

The changes in valve timing and amount of valve lift are altered in real time based on current operating conditions.. and it works in conjunction with ignition timing, and fuel delivery... etc..

That said, when you change "modes" i.e normal, sport, sport+ I would suspect that like everything else in the engine (ignition timing, fuel..) the variocam behavior is altered a bit different.

But yeah, something wrong with it could cause havoc.. If I recall one of the position sensors was responsible for the problems a while back where the engine would just die or couldn't be restarted when warm. Haven't heard much about that in the last few months.. But who knows.. maybe related to that.

 

Which makes me wonder if the crank position or cam position sensor problems are in any way related to this issue. Hard to say, as the Variocam system has a myriad of inputs. In my first several months of ownership, I had three occasions where the car would not start when warm.

What I really wish for is for Porsche to put some of their best engineers to work and get to the bottom of this. It may seem like a trivial problem which affects only a small percentage of owners, but it definitely needs to be resolved.

 

@Grunty - it does NOT only happen from a dead stop.
It makes no difference if it's a rolling start or from a dead stop.
Due to some recent developments I am now convinced more than ever that this issue is S/W related.

 

Well if it affects even a small % of owners it is a real problem.. The problem with a lot of these things is just getting them reported properly back to the mother ship.

What you get are a bunch of "technicians" who aren't really taught traditional troubleshooting and just plug in the diag port and look for faults. . If it doesn't throw a fault then as far as they are concerned it isn't really a problem.. And if it isn't a problem then it doesn't get reported.

So when you get some weird behavior happening it never gets reported to the people who could solve the problem. And things like this are more of a nuance than something that shuts the engine down.. which of course gets more attention because the victims scream bloody murder and lemon law when that isn't resolved..

Thus why it's good to get attention drawn through other channels like this when possible.