Sajan, is your car PDK or 7MT? The AMCO campaign is only for PDK?

I would not be surprised if the improvement for the stumble had been merged altogether into the AMCO campaign (kill 2 birds with one stone...)

 

PDK. I had it done in June and it made no zero diff to stumble.
I know the AMC0 was applied because my PIWIS shows DME last reprogrammed in June and my Sport Plus button now behaves differently.

One thing that made a difference was driving in a different terrain/elevation and weather. In the mountains (10,000ft)......car drove without stumble.
Back at sea level in humid conditions.....back to stumble.

 

So no sport plus, no amc0 recall?
my c2 991 (base) has no sport Chrono or any option at all lol

 

Well. I ordered a oil cap because I needed to drill a hole in the current one to measure the crankcase pressure...to test out the AOS.

I discovered my current oil cap is missing an o-ring! I could hear a little leak near the cap and when I held the cap tight, it stopped.
With the new oil cap (no sound/leaks), the car seemed to idle a little better when warm. Will report back in a few days after further observation.

Now here's the main point: I got a reading of -12inH2O.
According to LN engineering, the correct reading is

• MA1 Nominal crankcase vacuum is -15.0” (-14.0” to -16.0”) of H2O (not HG).

https://lnengineering.com/products/aos.html?limit=all


COMMON CAUSES FOR LOW ENGINE VACUUM READING ON PORSCHE M9X AND LATER WATERCOOLED PORSCHE ENGINES
1. Missing engine oil cap and or oil cap O-ring.
2. Cracked or broken spark plug tubes, if applicable.
3. Disconnected or broken dipstick tube, if applicable.
4. Make sure the oil dipstick is fully seated, if applicable.
5. Anything that can leak air can leak oil and vice versa.
6. Torn AOS boot, broken or cracked oil filler tube, Dipstick and/or crankcase crossover accordion pipe.

COMMON CAUSES FOR HIGH ENGINE VACUUM READING ON PORSCHE M9X AND LATER WATERCOOLED PORSCHE ENGINES
The only thing that can cause an excessively high manometer vacuum reading is a faulty AOS and or sludge.

 

No such thing as a small vacuum leak! Any effect on drivability?

 

The cap being tight brought up the pressure from -10.75 to -12...still off from the 15 value..from my understanding if the number was much higher like -20 or something, it means the AOS is bad but a lower value indicates a possible leak.
The car drives a tad bit better. Slightly more responsive? The stumble is there but with less intensity.

I think to rule out vacuum leak, I will need to do a smoke test. Something I haven't done before. To be fair I haven't done any of this before haha..

 

Well no vacuum leak. Pumped smoke in via oil cap/fill tube....not a single sign of smoke. took off the vent line right after the throttle body and I saw smoke coming out....so entire system seems to be tight.

I returned the variocam solenoids. Going to skip that and the TB. Done replacing parts for now. the list is growing long without much change. hey at least the gas cap leak was resolved! that was a random find.

We might be asking the crazy patient to diagnose themselves...i.e. the DME itself is the root cause (hardware or software) and it thinks it's fine.

 

Any further updates..it's been awhile..?

Mine is still behaving. Thank the Lord!

 

I was one of those that messing with the stability controls seemed to help. Note the past tense in that sentence. The car is put away for the Winter now, so I'm hoping posts a solution before Spring.

 

Interesting point of reference is that after taking my 981 GT4 to a PCA track day and running it hard, I have not had any stumbling issues in the few hundred street miles I have put on it since.

 

meant to say oil cap..
but speaking of gas cap, i am going to replace that too.

so the car is driving A LOT better. the low end torque is there (and it pulls!) and the stumbles are less prominent or non-existent. I will occasionally feel it, ever so lightly but not enough for me to care.
I would even say if I didn't spend months chasing this down and had just gotten this car, I wouldn't even think there's anything "wrong".

So maybe the low crankcase pressure was the issue? My theory is that these are transition points in the variocam tech and less than ideal readings/sensor data/pressure values make them more obvious. after all, look at precise nature of these engines..
the crankcase pressure is affected by higher altitudes which probably explains why the stumble was gone when I was in the mountains..

 

Sajan,

When checking vacuum, did you swap the oring from the old cap to the cap you drilled for the test? And, does the test fitting seal perfectly well? Guess I'm asking if you think holding down the cap sufficient to get best reading given you are only off a couple inches of water column?

 

good question.

I drilled the old cap that didn't have the o-ring. it read -10.74.
when i closed it tight with my hands, it jumped up to -11.81.
so I swapped the o-ring from the new cap to the drilled old one and got the same reading around -12 mmH20....which is technically not a "bad" reading. I think ideal is -15.0" (-14.0" to -16.0").
also you have to take into account the tester itself might have a margin of error.

 

@Sajan So changing your oil cap / o-ring reduced your stumble?

 

I definitely feel a huge improvement. It was a bit frustrating to drive under 3K. Car just felt sluggish and "slowly" climbed up ...stumbling along the way.
Now it just goes. And if I feel a tiny little pop or stumble, it's so instantaneous I don't think much of it. You would have to go out of your way feeling for it, which I have to untrain myself to do after trying to diagnose it for months.

My buddy drove it yesterday and he said car feels better. He said the power is there quicker. He didn't feel any stumbles or hear any pops.