Did you ask the Porsche mechanic about the connector swap that Rob reported?

 

I did. They did not investigate. Furthermore, I'm finding that they pretty much didn't do anything. I asked for results from the intake smoke test and compression test that I was told over the phone they performed, but it's been radio silence. Also, I just noticed they drove my car a TOTAL of 0.9 miles the entire service (that includes driving up and down from the service bay). I don't see how you can not do several road tests to understand a hesitation and stumble issue that occurs while driving.

I think they kept my car in their garage while waiting on a part I needed (engine lid strut) and then just gave me the same old "it's normal" answer.



I am beginning a communication with the service manager now to get to the bottom of this.

 

[QUOTE=stealthboy;4635587]I have Cobb Accessport and have many sets of data monitoring fuel trim. The data I viewed is limited to S/T as all my L/T values are 0 (not sure why). The data that you captured does not mean much as the F/T varies by RPM and gear selected. If you could capture your data at 2400, 2500, 2600, 2700 rpm, in second gear and third gear, then you would have a solid comparison to a non stumbling car. I would be happy to share some of my data sets if you think it might help, then PM me with your email address and I will forward a couple of sets.

 

Replacing the O2 sensors has resolved the problem (at least for now) for a couple of us. Details on the RL site.

 

Indeed. Following with much enthusiasm. I will be replacing mine in two weeks.

 

Just out of curiosity, are the O2 sensors throwing any fault codes?

 

No. In the stumble situation there are no codes thrown at all. That is why Porsche refuses to look into it.

 

I am done .... for now (!) See my vid below.

Solution to my hesitation: I have now permanently disconnected the air intake resonator flap by simply disconnecting the electrical line to the resonator flap's changover valve (connected to the right side of the airbox).

This has not completely removed the hesitation effect however it removed the worst part, a hard drop out (like momentary fuel starvation). If it was a "10" before, it is now a "2" or "3" now.

This is something you can do, it takes about 15 mnts, does not affect engine power (per Porsche), and can be quickly undone. Go ahead and experiment.... try it.

My theory: the resonator flap in in the air intake is causing a momentary burst of air turbulence causing the drop out. The flap is there, per Porsche documentation, for sound aesthetics only. Screw that.

More theory (fantasy?): There are at least two things going on <3K RPM and maybe three:
1 - This resonance flap
2 - Variocam changeover (said to be enacted by computer given various inputs)
3 - Resonance Tuning Flap in the air plenum after the throttle body.

The working theory out on the 'net, posted on the PCA website Q/A, and stated by my local Porsche dealer tech, is that a few things are happening at the same time given certain situations.... it sure appears this way. I have no insight about the performance of the Resonance Tuning Flap (3 above) so I have no idea if it is in play here. (It will cost me almost $300 in parts to swap out the tube and flap, the changeover valve, and the vacuum lever so I said forget it).

End result: The hard hesitation is gone. But the engine is still not completely smooth. I believe what is left is the variocam transition occurring... but now this transition appears to happen at variable RPM .... as it should given variable inputs. I would not call what I feel now as a drop out but there definitely is a very slight feeling, and change in engine sound when this occurs. Given what I was experiencing, I can live with this.

Placebo effect: I think the engine sounds smoother through the whole range.... I am not sure if this Resonator Flap opens and closes at other times, but frankly I think the engine sounds a bit better. Placebo effect? Maybe.

Power effect: None. I can't feel any difference. This is noted in the Porsche literature that this Resonator Flap is for aesthetics only.

In my vid, I show the flap in operation. I wanted to see if it was working properly. Of course I have no idea how it should work, but I did not experience any sticking... it was smooth. The valve is open at zero electric current (0 vacuum) and therefore open when disconnected. Open is a larger chamber and therefore a lower note to the sound. BTW, it takes exactly 100 mm/Hg of vacuum to hold it completely closed. Increasing vacuum pulls it more closed. I could not measure the vacuum in the line produced by the system either pre or post changeover valve.

This system is just an active lengthening and shortening of a resonator chamber like a flute. The incoming air literally blows across a hole and the valve is just like the valves of a wind instrument making the chamber longer or shorter... but only a two-note system (a two-note Helmholtz Resonator).

Talk among yourselves.

Peace
Bruce in Philly

 

Bruce, thank you for your time, effort, thought and for sharing this with the community. I very well may give this a shot!

 

Interesting. Is this also present on 991's?

 

Yes. This is on the 991s. It doesn’t seem to be affecting the 991.2s though.

 

Anyone know specifically where this changeover valve connection would be on a 991? I'll definitely try this.

 

It is attached to the farthest most point (on the right hand side) of your airbox. No.1 on the diagram.
The only way I could get to it was to remove the spoiler assembly (without taking off the entire bumper etc..).
I tried this earlier today and unfortunately it made no difference.
No errors messages on the dash either.
I also tested my vacuum lines and there was no leak detected.

 

I should clarify that mine is a 991S with a factory sports exhaust.

 

Between this and all the traffic on rennlist how has there not been a lawsuit to force Porsche to do a recall...I mean there has to be at least one damn lawyer driving a 991 right ?