Dewinator

Sounds somewhat similar to what I had. My throttle valve went bad and wouldn't hold steady, it would kinda waffle back and forth. Whenever you floor it, being 90 degrees to the airflow, any slight change would be irrelevant, but at low throttle settings wiggling a couple degrees off can mean a big difference in airflow.

Needsdecaf

Did you have any other symptoms? I will occasionally have a rough idle at cold start.

CSK 911 C4S

Mine is intermittent and it's annoying when it does occur.

I just drop a few PDK gears to get me out of the 2-3K RPM zone and drive on.

Dewinator

I wouldn't describe it as rough, just a little bit uneven as the car tried to hunt for the rpms it wanted.

stealthboy

Yes, this is accurate. I experience these two instances as well. Once you notice it, you can't ignore it.

vwduud

What brand of fuel do you purchase most often?

I noticed this same stumble almost immediately after picking up my '16 Targa GTS new. Was subtle, but exactly what you are describing.

Having come from the hotrod VW scene with dual Weber carbs, I have been used to some sort of hesitation, carrying boxes of carb jets with me in the car. If the weather changed enough, you'd find me on the side of the road changing jets. Thankful for modern day fuel injection.

Back to the 991 hesitation: brief research told me to use Shell's V-Power Nitro+. Supposedly has additives to help with wear, lower internal friction, etc., blah, blah, blah.

So, that's what I used, all the while feeling a slight stumble as described in this thread.

Now, go easy on me here fellas. As a matter of fact, I can't even remember why I tried this. Probably just a low fuel gauge and proximity of the fuel supplier.

So, where did I fuel up at? Walmart. Used their premium fuel of course, telling myself that I'd get to a Shell for the next fill-up.

It wasn't long into that tank that I wasn't feeling the stumble anymore. Thinking that it was a fluke, I filled up again with Walmart gas the following tank. Still no stumble. Then, I decided to go back to the shell gas. And... the stumble returned.

Now, I use Walmart gas more regularly than not. Feeling guilty about the possibility of "lower tier" gas, I throw in a tank of Shell or some other name brand gas. Seems that I did some research back then on "who" supplies Walmart with their gas, its quality, etc.. I don't recall the findings. Seemed it had mixed results. Nothing conclusive.

I drive spirited on mostly 2 lane 55mph highways to and from work, everyday. Gradually rolling through 1st and 2nd where the stumble was most obvious. I haven't even thought of this stumble until seeing this thread. Didn't even realize that it was a possible epidemic.

pfan

Glad this solution worked for you.

However, I would expect bad fuel to exhibit stumbling over a wide range of rpm and throttle settings. The fact that the stumble occurs at a very precise rpm indicates some other culprit.

fast1

Top Tier gasoline requires that the cleaning additives be added uniformly to all octane grades. Since Porsche owners would use the highest rated octane, it could be that the additives in Walmart's high octane are the same as for Top Tier gas. In my case I always make certain that I use Top Tier gas for my Lawn Mower, Snow Blower, etc were I'll be using a lower rated octane fuel.
Hopefully Walmart's high octane gas has all the required cleaning detergents and in the right amounts. I recently read a report from AAA that non-Top Tier gasoline caused 19 times more engine deposits than Top Tier brands after 4,000 miles of simulated driving. So bad gas can cause engine problems and in a relatively short time frame.

vwduud

I am not saying that the fuel was "bad". Just different.

It could be argued that running the Shell fuel is "better" if it does, in fact, have superior cleaning agents in the fuel. Take apart the engine at 100k and compare it to one that doesn't run this fuel. If the intake system is noticeably cleaner than one than didn't run this fuel, then it is "better".

If I knew, for a fact, that the Shell fuel was "better" for engine longevity, and that was of most importance to me, I'd run the Shell fuel exclusively. Maybe there are some independent tests that prove (or disprove) this.

While engine longevity is important to me, enjoying the car "now" is as, if not more, important to me. Why buy this car to be disappointed every time I drive to and from work in it?

There's nothing scientific about ANY of my findings. Not that Shell fuel cleans better. Not that Walmart gas will make the car drive without a stumble. At this point, it just "seat of the pants" findings. And that was all happenstance as well.

What little I know about this car and the fuels available is fine by me. I've had my share of years working on cars and tuning for performance. With both my 911 and my super charged jet skis, I am "told" to run high octane fuel for the best performance. Fortunately, I am also "told" that the engine management system is smart enough to recognize if I fail to do what I am told. Whether because high octane is not available when I need to fuel up or because I have begun pinching pennies. I may not get the highest performance out of the vehicle, but hopefully I won't melt a piston at WOT while having fun, because the engine management system is smarter than I am and won't let that happen.

Is this stumble a result of the engine management system reacting to the "different" blend of gas? I suppose that this is certainly possible. And considering both THIS thread and apparent "other" threads on this topic, no one really knows what the solution is. Not every 991 owner experiences this stumble either.

stealthboy

I have tried all major gasoline providers in my area. Same behavior on each. When I was first encountering the stumble, it really felt just like bad gas, so my first experiment was to try from every station I could reasonably visit in my area. I always use 93 octane.

vwduud

So, by "major", does that mean that you did NOT try Walmart gas?

As mentioned, I did not try Walmart gas "looking" for a solution to the stumble. Based on what I read or hear, I too would consider NOT using Walmart gas for a car like this. My brain cringed when I even pulled in to pump it.

Macster

Coming in kind of late to this but my info is the techs get "a lot" of drivability complaints similar to yours. In almost every case the cure is the car needs to be driven more. Not more of the same type of driving it has been getting but the car taken out and while I hate to use the phrase "Italian tuneup" that is pretty much what is recommended.

Over the years I have found that a highway drive of 40+ miles -- just a steady drive at highway/freeway speed nothing wild and crazy that would get you featured on Cops -- really has the engine running noticably better afterwards. This in spite of the fact both of my cars see regular highway/freeway usage which is part of my 30 mile work commute.

Some things to try: First do an E-Gas calibration. The instructions for this should be in the owners manual. For my older cars: 2002 Boxster, 2003 996 Turbo; with the key off and your foot off the gas pedal throughout this procedure turn the key to the on position. Leave on for at least 60 seconds. Then turn the key to the off position for at least 10 seconds. The next time the engine is started the calibration is complete.

Next consider running Chevron Supreme with Techron. Because a local/handy Shell dealer raised its fuel prices over and above of nearby stations -- why I have no idea -- I switched from Shell to Chevron. After just a tank or two I noticed the Boxster engine running better especially at low end. I talked to the Porsche techs about this and they all said the same thing. Chevron Supreme is the *best* gasoline to run as it really cleans/removes engine deposits. At least one tech said he runs Shell most of the time because he believes it provides a bit more power but even so he runs a tank of Chevron gasoline every once in a while.

You can speed the process up by getting a bottle of Techron -- get the bottle that treats 20 gallons -- and at the gas station then shake the bottle and dump the contents in the gas tank then fill up the gas tank with the proper octane gasoline.

Drive the car normally (though an extended drive on the highway/freeway would help). The general rule is if you notice any improvement from the engine to then when it is time to fill up the gas tank again to dump in a second bottle of Techron, fill the tank and drive this Techron treated fuel out of the car. After the 2nd tank is gone it is recommended the engine oil/filter be changed.

Last but not least be sure the engine air filter is good. My Boxster was running poorly and generating a CEL with a P1128 error code. I talked to the tech and we went over some things. Among them was the engine air filter.

I couldn't remember the last time I had changed or had changed the engine air filter so I decided to change this.

Glad I did. The old filter was filthy. The top of the filter element -- sponge material -- was almost completely covered with large pieces of trash, leaves and such. After I got the filter out I cut away the sponge layer on top of the paper element and smacked the filter top side down on the sidewalks by the car. The sidewalk was black with the stuff from the filter.

After the air filter change the engine ran noticably better. Much like it had when I switched from Shell to Chevron gasoline.

When I used to drive big miles every year I changed the air filter every year. As my annual mileage has shrunk (I put just 10K miles on the Boxster from April last year to April this year) I have stopped changing the filter every year. But from now on I'm changing the air filter every year.

MJBird993

My 991.1 had a bit of a stumble that went away after a few thousand miles. I sold it for other reasons. My 981 S I bought used, long distance, and it had a stumble. I took it in, they said "they all do that" and I sold it a few months later.

Life is too short.

Churchill

Just to respond to some earlier posts (and please pardon me for getting a little didactic) -- it is definitely not the clutch. Diagnosing a worn clutch is easy. The flywheel is a metal dinner plate that's bolted to the engine's crankshaft. It spins with the crank. The clutch disc is made of a wear material, sort of like a brake pad. When the clutch pedal is out, the disc is pressed against the flywheel, connecting the engine to the transmission. The crank spins the flywheel, the flywheel spins the clutch disc, the disc spins the transmission input shaft, which turns gears, which turn the axles, which turn the wheels, which puts the smile on your face. When you press the clutch pedal in, you're lifting the disc off the flywheel, disconnecting the engine and transmission. When the disc gets worn, it will slip on the flywheel. Again think worn brake pads that lose friction against the brake rotor. Therefore, the more power you apply to the disc/flywheel interface -- i.e. the higher the RPM and the lower the gear -- the more slippage you get. In other words, you're simply not going to have a situation in which you get slippage at 2.8K RPM's in high gears on the highway but not all over the rev range in lower gears. So it's not your clutch.

Last week I detected, for the first time ever, the stumble on my 991S. Had it for a year and no stumble. But it was definitely the stumble.

Bud Taylor

Thought I had picked a year and model to purchase but this scares the crap out of me. Does anyone know how to turn this into a poll on rennlist so we get some accurate numbers on incidents ?

QUOTE=Churchill;14282569]Just to respond to some earlier posts (and please pardon me for getting a little didactic) -- it is definitely not the clutch. Diagnosing a worn clutch is easy. The flywheel is a metal dinner plate that's bolted to the engine's crankshaft. It spins with the crank. The clutch disc is made of a wear material, sort of like a brake pad. When the clutch pedal is out, the disc is pressed against the flywheel, connecting the engine to the transmission. The crank spins the flywheel, the flywheel spins the clutch disc, the disc spins the transmission input shaft, which turns gears, which turn the axles, which turn the wheels, which puts the smile on your face. When you press the clutch pedal in, you're lifting the disc off the flywheel, disconnecting the engine and transmission. When the disc gets worn, it will slip on the flywheel. Again think worn brake pads that lose friction against the brake rotor. Therefore, the more power you apply to the disc/flywheel interface -- i.e. the higher the RPM and the lower the gear -- the more slippage you get. In other words, you're simply not going to have a situation in which you get slippage at 2.8K RPM's in high gears on the highway but not all over the rev range in lower gears. So it's not your clutch.

Last week I detected, for the first time ever, the stumble on my 991S. Had it for a year and no stumble. But it was definitely the stumble.[/QUOTE]