What is the current state of the 'engine stumble' issue?
07-10-2023 | 11:21 AM
#451
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Joined: Sep 2016
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From: Raleigh, NC
Replaced ignition coils and variocam solenoids Saturday, put 30 miles on the car yesterday. My impression is that the parts made no difference. After ~100 miles now on the 93 Stage 1 tune, it feels smoother and stronger (especially in Sport/Sport Plus) but still a bit like it's pulling timing between 2-4k RPMs, noticeably improved over before the tune though. I think I'm fine with it.
07-10-2023 | 04:26 PM
#452
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From: Texas
Not sure if this is the same thing being discussed. Last Saturday I was driving home and as I went through about 2500 RPM, the car stumbled and got rough. I immediately got a red warning light saying, "Reduced Engine Performance - Possible to drive on."
I nursed it home about 4 miles and any time I pushed through about 2500 RPM it got rough so I upshifted to keep the RPMs low and the car moving. Parked it at home and had AAA tow it to the dealer. I e-mailed my SA to let him know it was coming on Monday. Monday morning he e-mailed right back and said it sounds like something they have seen before - a problem with the fuel pump.
After they received the vehicle he called to let me know the seals on the high pressure fuel pump were defective and they CPO replaced it. I've put about four hours and 250 miles on the vehicle and the stumble is gone and it drives like it always has, smooth and direct performance through all RPMs in all gears.
Car is a 2014 911/50 (power kit standard) with 44K miles. I run premium gas and am at sea level. Couple of pictures showing the warning lights I got as well as the fuel dripping near the left rear wheel.
I nursed it home about 4 miles and any time I pushed through about 2500 RPM it got rough so I upshifted to keep the RPMs low and the car moving. Parked it at home and had AAA tow it to the dealer. I e-mailed my SA to let him know it was coming on Monday. Monday morning he e-mailed right back and said it sounds like something they have seen before - a problem with the fuel pump.
After they received the vehicle he called to let me know the seals on the high pressure fuel pump were defective and they CPO replaced it. I've put about four hours and 250 miles on the vehicle and the stumble is gone and it drives like it always has, smooth and direct performance through all RPMs in all gears.
Car is a 2014 911/50 (power kit standard) with 44K miles. I run premium gas and am at sea level. Couple of pictures showing the warning lights I got as well as the fuel dripping near the left rear wheel.
The PIWIS needs to be connected to the car's DME and taken for an extensive test drive with the technician in the car until the problem manifests. Throughout the drive cycle live data monitors need to be observed. The scan tool is capable of live recording. After the symptom occurred you stop the recording and rewind the data to study it from the moment the fault has occurred.
My total guess at this point is that I would pay very close attention to fuel pressure at the moment the fault is occurring. I would look at the in tank fuel pump pressure and at the high pressure fuel pump. I believe the PIWIS is capable of displaying those pressures separately. If not,a mechanical fuel pressure gauge needs to be installed,but I doubt that's necessary on a 2013-14 model.
I would also take a very close look at the upstream oxygen sensors on both banks. In closed loop during part throttle they should go rich observing the extra fuel. If they go lean,it might indicate there's a fuel pressure problem,but for a brief moment and not enough to set a check engine light.
One thing to note : if the condition is occurring briefly and can't be associated with a malfunctioning part,there's a possibility it can be corrected through a software update,by changing parameters in the fuel map.
My total guess at this point is that I would pay very close attention to fuel pressure at the moment the fault is occurring. I would look at the in tank fuel pump pressure and at the high pressure fuel pump. I believe the PIWIS is capable of displaying those pressures separately. If not,a mechanical fuel pressure gauge needs to be installed,but I doubt that's necessary on a 2013-14 model.
I would also take a very close look at the upstream oxygen sensors on both banks. In closed loop during part throttle they should go rich observing the extra fuel. If they go lean,it might indicate there's a fuel pressure problem,but for a brief moment and not enough to set a check engine light.
One thing to note : if the condition is occurring briefly and can't be associated with a malfunctioning part,there's a possibility it can be corrected through a software update,by changing parameters in the fuel map.
Having read a number of threads of fuel smells in the cabin and at least one person's car going to limp mode due to pump seal failure, I wonder if the fuel pump isn't maintaining suitable pressure throughout all load conditions. Maybe just drooping a bit when demand is suddenly increased, not enough to code just a temporary plateau. I don't believe it is a software issue due to folks having software updates or aftermarket coding and the problem remaining, I don't think its the PDK as it seems to occur in both MT and PDK. Its an interesting issue....
There seems to be some correlation to the anomalies on the fuel pressure and the dips in RPM.
I've also noticed that for me to replicate the issue, i need to coast at a steady RPM for a little bit, then demand power. Is this some hint to pressure needing to build up? hmm
here's a 0-50mph run.
Last edited by Sajan; 07-10-2023 at 05:52 PM.
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squidge (07-12-2023)
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Sajan (07-10-2023)
07-12-2023 | 06:43 PM
#455
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Joined: Sep 2016
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From: Raleigh, NC
I appreciate the effort you're putting into logging this. Looks like you're finding some good clues. Are you thinking about trying a fuel pump? Looks like around $1350 from Suncoast. I have a Fidelity waranty which would cover it on my car, so I might also try to do some logging and see if I could pinpoint some similar dips before looping in a mechanic.
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Sajan (07-12-2023)
07-12-2023 | 07:50 PM
#456
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Joined: Feb 2018
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From: Texas
I appreciate the effort you're putting into logging this. Looks like you're finding some good clues. Are you thinking about trying a fuel pump? Looks like around $1350 from Suncoast. I have a Fidelity waranty which would cover it on my car, so I might also try to do some logging and see if I could pinpoint some similar dips before looping in a mechanic.
Since I can only do 8 values at a time, I have to be picky. So far I've done some combination of RPM, Speed, Pedal value, throttle angle, air flow, fuel trims, fuel high pressure..
I did see the lumpy RPM and what else correlates so far are the throttle angle, air flow and some bumps on the HPFP.
I don't know if this is all normal or what's causing what to behave this way..
I am open for ideas on other things to log.
for all we know this is just how the variocam system works and we are chasing our tails. I am not going to replace the pump just yet because I need to see if I can get it covered via the fidelity warranty also. I am not going to spend 2K in parts/labor on a hunch.
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07-21-2023 | 05:57 AM
#458
Burning Brakes
Joined: Dec 2018
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Not sure if it is possible but it would be interesting to plot the ignition timing along with throttle angle position, airflow and fuel high pressure.
Your gas pedal position is your torque request to the engine ECU. The ECU can fulfill this torque request by different combinations of air mass (i.e. throttle angle) and ignition timing for a given fuel high pressure.
At high level, a stumble (i.e. engine speed dip) can be caused by one (or several) of these 3 elements being inconsistent: intake manifold air mass(or air flow or air pressure), ignition timing or fuel mass (fuel pressure and depending on air mass and richness target).
Nice that you could log a lot with a PIWI btw.
Your gas pedal position is your torque request to the engine ECU. The ECU can fulfill this torque request by different combinations of air mass (i.e. throttle angle) and ignition timing for a given fuel high pressure.
At high level, a stumble (i.e. engine speed dip) can be caused by one (or several) of these 3 elements being inconsistent: intake manifold air mass(or air flow or air pressure), ignition timing or fuel mass (fuel pressure and depending on air mass and richness target).
Nice that you could log a lot with a PIWI btw.
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07-21-2023 | 08:54 PM
#460
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From: Texas
Above is a 0-60 ..moderately paced run. speed seems linear but notice the not linear graphs for RPM..
vertical grey line shows the stumble/dip at 2200 or so RPM. see how the rest of the parameters also show something around that...
like the airflow went flat..RPM dipped...ignition angle changed..
vertical grey line shows the stumble/dip at 2200 or so RPM. see how the rest of the parameters also show something around that...
like the airflow went flat..RPM dipped...ignition angle changed..
07-22-2023 | 12:56 PM
#461
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Joined: Nov 2007
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From: Western NY
If I'm reading that RPM scale right, those are incredible drops in RPM, it looks like nearly a thousand RPM. I realize there's a sampling interval involved, so the drop is not truly instantaneous, but still a huge drop. That would be a violent stumble, unlike what I'm feeling. You said a moderate 0-60 run, so are we talking around 10 seconds total on the X axis? It's not clear what the 481.0 ms notation refers to.
07-22-2023 | 01:09 PM
#462
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From: Texas
If I'm reading that RPM scale right, those are incredible drops in RPM, it looks like nearly a thousand RPM. I realize there's a sampling interval involved, so the drop is not truly instantaneous, but still a huge drop. That would be a violent stumble, unlike what I'm feeling. You said a moderate 0-60 run, so are we talking around 10 seconds total on the X axis? It's not clear what the 481.0 ms notation refers to.
The X axis is in ms for some reason ..that's just how PIWIS graphs it. Not indicative of real time. The sample rate is the small steps. Check out the speed one to see how it should look.
It doesn't feel "violent" but it feels like there's a PAUSE and RESUME of engine effort/output. There's def a drop in RPM. You can feel it.
This is less obviously at higher speeds and mild to moderate throttle on the highway.....aka the stumble/stutter.
07-22-2023 | 05:39 PM
#463
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Joined: Jan 2023
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Just want to update my "progress". Since the recent DME / PDK update, and maybe all the other items that I replaced, my "stumble" is pretty much non existent. After the Dealer update, a couple drives were not as smooth as they are now, maybe the DME relearning driver input. There were a handful of times where I "thought" it did it ever so slightly, but honestly didn't know if it was the road or something else. Could this newest revision of the DME / PDK update be the key? At the time of my purchase I had only 1 outstanding recall (Porsche emissions scandal update) 5-6 months later when I had this performed there were 3 recalls, with this update being one of them.
Also, just for reference sake. I live in AZ where it's hot AF now and maybe its running better with warmer (hotter) ambient temperatures than it did when it was cooler? IDK I'll post up if anything changes, but for now very happy it has gone away 99%
07-22-2023 | 05:48 PM
#464
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Joined: Feb 2018
Posts: 737
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From: Texas
Just want to update my "progress". Since the recent DME / PDK update, and maybe all the other items that I replaced, my "stumble" is pretty much non existent. After the Dealer update, a couple drives were not as smooth as they are now, maybe the DME relearning driver input. There were a handful of times where I "thought" it did it ever so slightly, but honestly didn't know if it was the road or something else. Could this newest revision of the DME / PDK update be the key? At the time of my purchase I had only 1 outstanding recall (Porsche emissions scandal update) 5-6 months later when I had this performed there were 3 recalls, with this update being one of them.
Also, just for reference sake. I live in AZ where it's hot AF now and maybe its running better with warmer (hotter) ambient temperatures than it did when it was cooler? IDK I'll post up if anything changes, but for now very happy it has gone away 99%
Also, just for reference sake. I live in AZ where it's hot AF now and maybe its running better with warmer (hotter) ambient temperatures than it did when it was cooler? IDK I'll post up if anything changes, but for now very happy it has gone away 99%
Those 2 and TB are the only things I haven't done.
Other things on mind that could be an issue:
change-over vales for maybe one of the flaps?
HP fuel pump?
Knock sensor?
07-23-2023 | 10:13 AM
#465
Burning Brakes
Joined: Dec 2018
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When the engine dip occurs ( apx 2500 to 1800 rpm with the given sampling), air and fuel paths seem constant but ignition has a big drop from apx 32 to 15 crank angle (given the sampling).
What is the time-step for the X-axis? I would like to understand how much time elapsed between the gear change from 1st to 2nd and the moment the engine dip occurs (where you put your grey cursor).
What is the time-step for the X-axis? I would like to understand how much time elapsed between the gear change from 1st to 2nd and the moment the engine dip occurs (where you put your grey cursor).