Mufus

it wont make any difference. After replacing a bunch of parts and blaming different components.. i have reached the conclusion that, the source of the stumble is the variocams. There is nothing else to replace, nothing else that could cause it.

My 991.1 Turbo S started to make some chain noises at start up. not always. random. and no difference if the engine is hot or cold. So, this winter i will drop the engine, and replace all that means vario cams, sprockets, chains, tensioners, etc.

So, until then, leave the cars as they are. i reached the conclusion, that this is something particular the 991 has. Live with it, and enjoy it.

I will report back after i change all above mentioned. if the stumble is still there.. well.. like i said.. that's the 991 way.

Ceepe

Ironically I did not notice this in my .1 3.8, but I do notice a bit of a stumble with the .2 3.0 engine at around 2900 RPM. Big hiccup every time it pulls over that range when the engine is cold.
An issue across engine platforms?

koala

Thank you, I appreciate your feedback. I will leave it alone! Especially after watching that video - a ton of work for sure. If I were upgrading my plenum like the guy in the video, then I would probably replace the AOS just because I'm in there.

I personally find I can avoid the stumble (mostly) by driving the car a bit harder when it's cold. I know that's not ideal, either, but if I use more throttle, the issue really isn't as noticeable. I normally like to keep the cruising rev's at 2,000 to 2,500 rpm before the oil is at operating temp, but I'm tempted to just keep them closer to 3,000 rpm. I doubt it would have a massive long-term impact on engine life.

koala

I wonder if you're feeling turbo lag? I have a similar symptom in my Golf R, but it's definitely turbo lag and not the same feel as the stumble in the naturally aspirated 991.1 engines.

worf928

Did you ever pull your fuel injectors and have them cleaned? (I suggested this in this thread last year.)

Have you borescoped the intake ports (i.e. looked for evidence of DFI carbon buildup?)

Ceepe

100% not lag. It does it with little load and zero boost. I had a mark7.5 R so I'm familiar with that throttle delay/lag.

DaveGee

Exact opposite experience for me.

DaveGee

koala

Fair enough! Was the only thought I had on this as I don't think a stumble on the 3.0 turbo has been mentioned before.

Class5Kayaker

The video is a great resource. Others have asked about engine removal. While you don't have to remove the engine to get to the AOS, you do need to lower it a few inches to remove the plenum and access the AOS. The video skips over the AOS replacement and mentions a different video for the actual AOS replacement which is here:

Mufus

no, i did not. will do it in the winter, along with the timing chains. bad parts of this, is that if it disapears, we wont know for sure the cause of it.

Class5Kayaker

Looks like Rennlist member @rnl had his variocam solenoids replaced and it solved his stumble issue. Not sure if his stumble is what we've been experiencing, but it's an easy enough DIY (if you can replace plugs/coils you can replace these two solenoids) and only $270 in parts.

I've been suspecting throttle body/sensor or variocam solenoids are the issue, but leaning towards throttle body based on how the car is acting (stuttering like it's a fuel/air issue) and that the car behaves perfectly fine for about 50-100 miles when you reset the throttle sensor (key in "accessories on" for 60 seconds, then car off, then start car). Also @Dewinator reported success with a throttle body replacement. I know a few have reported that doesn't work, but reading through all the posts all I could definitely see on the "doesn't work" was they replaced the sensor, not the necessarily the entire throttle body and sensor (you can order them separately or already integrated/connected together). I was about ready to pull the trigger on a new throttle body (with throttle body position sensor/actuator included) since it's only a $170 part if you buy the OE Bosch part (0280750473) versus the exact same part with a Porsche logo stamped on it (99760511601 or 99760511600) for $386 (on sale through Sunset). Porsche/Bosch updated the part for a reason, and maybe this was it. I figured if that wasn't the issue I'd only be out $170.

Now I'm on the fence on which way to proceed. I'm not usually the type to just throw parts at a problem, but based on this thread it would take some pretty serious equipment to properly diagnose/troubleshoot during test drives and the dealers couldn't care less. I think I'm going to do the throttle body first, and actually film the old one at different rpms to see if I can see a fluttering of the valve at the different RPMs we seem to experience the stumble (Dewinator reported his mechanic could see the valve not holding a steady position). If that doesn't solve the problem then I'll try changing out the variocam solenoids.

koala

I like your line of reasoning for sure. I wonder if resetting the throttle sensor does actually temporarily help. The last 3-4 days that I've driven my car, it has not exhibited any stumble issues at all. Has driven really well, in fact. I didn't put 2 + 2 together until your description of the process for resetting the throttle sensor. The other day, I had put my key in the ignition and switched it all the way "on" while I went to unplug my battery tender and get the car ready to drive. Then, I realized my garbage bins needed to come in off the street, so I switched the key off to grab the bins.

Could be a total coincidence, but I've been trying to figure out why it has felt like a brand new car these past few drives. I don't think it's a placebo effect as I had no idea you could reset the throttle sensor with that process.

If all of that makes sense, then the throttle body does seem like a better explanation. $170 for the Bosch unit might be worth gambling on. I'd have loved to have changed it last winter when I did my serpentine belt!

Class5Kayaker

Resetting the sensor definitely helps. Every time mine starts getting really bad, that's what I do and it totally fixes the problem.......temporarily.

koala

Well, that is at least encouraging. My morning process of firing up the car almost lends it's self to that reset procedure anyway. My Android Auto integration is buggy and requires some hoops to jump through to get it working 100% so I might just reset the throttle on every drive lol.

duxsi

I can confirm that the complete replacement of the throttle body assembly did not resolve the stumble (in the case I was working on).