Had something strange happen to me last week.

First of all I've been one that as on the fence about the engine stumbling issue that has been discussed in endless threads. I've never had a high level of repeatably of the problem, and I've often questioned if I really had the problem. Usually I'd just notice it randomly when accelerating and I'd kinda go hunh.. that's weird.

Any way I started up the car morning last week and had a yellow ECU fault. First time I've ever had any Engine related fault. Tried a few different things but it didn't go away. Made a mental note to make a dealer appt and when I had a chance get my durametric out and see exactly what fault was occuring. The indication was that you could "drive on".

The one thing I did remember previous to the fault was that on my way home on the night before I got the ECU failure I had engaged in a little bit of "spirited" driving than normal.

Anyway, last night after having the fault for about a week, I stopped at a gas station which wasn't my normal station (where I fill up about 95% of the time). I was pretty much on fumes when I filled up.

This morning when I started up.. No fault.

All of this leads me to believe that the ECU fault was induced by the fuel? Maybe I got a tank of fuel that was really lower than 91 octane (highest available in CA). I'm well aware there is an anti-noc capability of the NA porsche engines to cope with less than the recommended 93 octane. But maybe the ECU fault was telling me I was running something lower like 89 or 87..

It could be that I was imagining but while I had the fault the car didn't quite seem to be itself. (Although I purposely wasn't pushing it very hard).

One of my theories about the "stumbling" issue has been that it was somehow related to fuel octane and the anti-noc handling. And this incident tends to confirm in my mind that there is at least some link...

The other thing is, it would also tend to indicate that my local Shell station (either purposefully or accidentally) filled their premium with substandard fuel. Something I'm not at all happy about. Looks like I will be avoiding that station in the future.

 

I have tried many different brands of gasoline and my stumbling did not change at all. I have always used 93 octane as recommended in the user manual.

 

I'm coming to the conclusion however that the stumbling issue itself as reported is probably multiple problems that manifest themselves similarly. In your case it of course isn't

I also think that those of us that are forced to use 91 octane (which is all you can get in many areas) may be experiencing something slightly different, although it may be similar.

 

Ever considered that it might be a fuel pump problem? A description of a pump that is failing or having issues (but not dead) is that under driving conditions stumbling or hesitation may occur, which appears to be engine issue but actually might be starvation conditions appearing and disappearing very rapidly. Hard to imagine a modern 991 engine having this issue tho, but parts have been known to fail, right?

Why am I bringing this up? I am by no means an expert on fuel delivery systems, especially in the 991 platform, so maybe I am completely ignorant. But I was just working on something recently related to 928's (fuel pump replacement!) and this sounds kinda familiar. Big difference in that case was that the condition was more severe and quickly lead to car not running at all-- because the pump actually failed.

Oh, and something I learned just recently is that when PAG was manufacturing 928s, those cars that were being shipped to hot climates were actually configured with TWO fuel pumps -- one internal to the tank, and one external pump! This was to prevent starvation / stumbling conditions in extreme heat regions, like deserts, and well, maybe even Southern CA! The internal pump was acting as a pre-pump to maintain sufficient fuel pressure at the external pump.

Again, maybe nothing whatsoever related to your issue, but you got me thinking!

Anyways -- just another data point to consider.

 

Certainly anything is a possibility.

The one of the few things about the stumbling problem that seems almost universal is that it occurs during light/moderate acceleration conditions. In the past with traditional fuel pumps it seems that often that kind of problem would occur in more of a WOT condition. But in this case given the electronic control nature of modern engines and ECU's.. I guess anything is possible.

For the particular occurrence I just experienced with the ECU fault I'm fairly convinced it was induced by bad/low octane fuel since it was pretty much a cause/effect that I can isolate to a single tank of fuel at a specific station. But as far as the stumble issue, being tied to fuel that would be more of a theory than anything.

I'm considering doing an experiment where the next time I get down to almost empty, I'll fill up with maybe just a gallon or so of 89 octane and just take a short test drive around the neighborhood to just see if I can reproduce..

 

You are probably correct, @scatkins!

Like I said, I am no fuel system expert! However, I will point out that in the case I referenced (with the 928) the stumbling began at slightest throttle input, not just in a WOT condition. This was being caused by an internal pump failure that caused issues with the external pump....

Anyways, happy hunting!

 

I know that 997.2's also had a stumbling issue- and there were some defective fuel pumps found. In the case of the 991 it's either the cams or the fuel pump. Not sure what else it could be.

 

Had a ton of issues with rattles on my car but never anything engine related. I only ever use shell nitro ( in the UK) which is 99 ron and car runs great. Only once did i use regular unleaded and it definitely felt a much more sluggish / less responsive.