I think first we need to figure out what the hell the problem is. As it stands now, Porsche just thinks it's "normal". Until we can point to the exact part that when replaced solves the problem, I don't think there's much to be done.

I'm continuing my investigation. Next step is doing an intake valve cleaning to see if the DFI carbon buildup is affecting airflow. Once that is ruled out (I'm sure that's not going to solve it), it'll be on to the throttle body for diagnostics. So at least I have my short term plan prepared.

don't know if this was ever discussed, but wonder if a cobb tune would help at all

Being a distinct feeling / sensation which DOES NOT affect the safety of the car (but is certainly "annoying"), I suspect that not too many lawyers would touch "annoying" with a barge pole

Also kinda tough when the official position has been "operating as intended" and "driving as designed". This pathetic corporate stance dates back to when the problem first appeared in late 997's.

I realize this is an old thread... but I developed this problem on my 2014 C4S MT and have been trying to trouble shoot it for a while. Tonight I think I may have figured it out... (at least after my first test drive).

I have a iCarsoft Porii code scanner ... and I reset the ETCS (Electronic Throttle Control System?) After one drive it appears to be resolved. We will see if the ECU does some new learning and it comes back... if so... I think I'll just keep resetting this.

Hope this helps someone.

thanks

@NavyDoc333 Being a Doc you know all about the placebo effect, so we patiently await for your trusted feedback regarding the long-term effects of the reset..

I tried it a few years ago. It didn't help. Others have reported the same results.

Has anyone who has the stumble replaced the HPFP?

Seems like folks have tried everything but that?

As far as I know, no one here (or on the other board) has reported replacing the fuel pump.

2009 C2S 191K miles

Yes, a new HPFP did nothing. Yes, I changed all coil packs, did nothing. I changed out those two (one each side) oil solenoids that are in the valve lift system, and did nothing. The pump failed at around 180K miles naturally as did the coil packs around 170K miles. The oil solenoids changeout was just an experiment. This stumble is a complete mystery.

Now after all these miles, I can say the effect is less pronounced and of course, I have no idea why. Also, I disconnected the electrical connection to the air valve (vacuum solenoid) on the right side of the air box and this help make the stumble less... or at least appear less. On S models, we have an airbox that has a little air flap that opens and closes on a sealed chamber in the air box that is there for only aesthetic purposes. It change the sound of air intake. This valve opens at the same moment as the stumble and for whatever reason, makes the stumble less apparent.

I strongly suspect the stumble is due to some poor ECU programming relating to the valve lift transition. In addition to the valve lift, the computer is making a few other things happen in the engine and one of them is the air flap. Maybe the flap causes some air flow flutter but I don't really know. Maybe it is psychological in that the engine sound is more smoother. I dunno. The flap makes the engine sound smoother at lower RPM (low valve lift) and more aggressive after valve lift. So disconnecting it made engine sound a tiny bit more aggressive at low RPM... not a bad thing. Disconnecting the electrical connector to the vacuum solenoid valve will not throw a dash light but will throw a code in the engine management system and you will see the code with a Durametric tool. It has no adverse affect on engine performance at all, it is purely an aesthetic feature. How do I know it is there for aesthetics? I have Porsche tech documentation that specifically denotes it as aesthetic and a non-performance feature.... so don't fret and go ahead and give it a try, just disconnect the electrical connector... the code goes away when you reconnect it. This is a no cost effort. I then covered up the plug and socket with electrical tape to keep the dirt out.

The tech seeing the pressure drop is probably... total speculation... a result of the computer cutting fuel a tiny bit during the valve transition to smooth the transition... but of course, the software programming may not be perfectly implemented. That is total speculation on my part, but what is really odd is that many have the same car as me and don't suffer the issue.

Just to summarize, I have had the stumble issue since I acquired the car with 23K miles and now I have 191K miles. It never got worse and actually got a little less noticeable. It is one of the only real negative experiences I have with this engine... if it wasn't for that, this engine is fabulous and Porsche appears to have really improved it in many ways from the previous M96/M97 version.

Peace
Bruce in Philly (now Atlanta)

I connected a PIWIS 3 and logged/graphed high pressure fuel pump value and rpm together.
I do see the little blips that correlates to RPM drop. But then again, is the RPM drop (because of something else) the cause of the anomalies in the HPFP values....since it's driven by the engine?

The confusing part about all this: if this is a DME programming issue, is this affecting all cars and some people just don't notice it or aren't as keen in observing it?

I have made logging my new hobby....so here's a graph with the lumpy acceleration.
Air flow is lumpy and so is the throttle valve angle. what gives?


https://cimg6.ibsrv.net/gimg/www.6sp...75a5b5c9a7.jpg

2009 C2S 191K miles

First, I am clueless with these types of diagnosis. But... of course... that can't stop me! Regarding air flow.... I have suspected the air flap, not the throttle body, may cause a flutter of air. So.... run and experiment and disconnect the electrical line to that vacuum solenoid valve and re-run the test. I don't know what model you have and if your model has this flap so... that is up to you to discover.

Regarding the throttle body.... I did replace my throttle body (TB) due to chasing an unsteady idle. The unsteady idle can be attributed to a few things that I won't go into here but when the TB is the cause is either it is gummed up with oil or it is wearing out. Remember, I have a very high mileage car so the TB has lived a long time. To clean it, just remove the hose to it, spray the crap out of it with MAF cleaner while pushing open the butterfly, and run a rag all around in there getting the edges of the butterfly. After doing this did not help my rough idle, I noticed the butterfly was hitting the bottom throat of the TB and I could catch this with my finger nail... the pivots(?) were wearing out. So, a quick, easy replacement (also do the gasket) fixed the idle issues. However it did nothing for the cutout issue.

What else is going on in the engine as you accelerate through the suckout zone? Many.... The ECU may be tweaking the ignition timing... the operation of the injectors... and who knows what else. I know in marine applications of my dad's old boat with an outdrive, the computer would kill ignition for just a moment to allow the engine to stop pressure on the transmission so the thing would come out of gear into neutral. I know this has nothing to do with our cars, but, it is an example of how engineers use tricks to accomplish other, seemingly unrelated objectives.

Peace
Bruce in Philly (now Atlanta)

lol, same. I am just curious. maybe it's normal and it is what it is. cars that have it are still living their best lives many miles later with no codes/faults or engine issues.

I will chase down this air flap and see if I can measure it also.

@Bruce in Philly interesting to note that there's been a bunch of COV related issues....and the flap you mentioned is controlled by it?

https://plenums.blogspot.com/2016/10...-overview.html
https://cimg7.ibsrv.net/gimg/www.6sp...b9cb7b90f7.jpg

The flap in my 997.2 is controlled by a change over valve... I think that is what is described as the sound symposer valve in that diagram. That is what I disconnected. This valve type is used all over the car as you can see... all the same part number on my car. When they fail, they will throw a code but only if it an electrical failure... not sure if it will throw a code if the mechanical part fails. Getting back to the stumble, I doubt one of these valves is behind the problem The variable timing, ignition, and valve lift are controlled by other systems. The valve lift is controlled by a solenoid (change over valve) that governs oil flow. On my car (only on S models), there is another butterfly valve in the air plenum post throttle body that modifies air flow prior to going into the cylinder valves... I suspect it is denoted as that tuning flap valve. I suspected that maybe this is sticking... but it is a royal pain to get to as it located in the front of the engine... but this is just an issue of my imagination with no real evidence.

If you are chasing the stumble, I doubt it is the result of the vacuum system... but that is just my opinion.

Peace
Bruce in Philly (now Atlanta)