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slight hesitation on accel at 3k rpm (991 base, MT)

 
Old 07-18-2012, 01:03 AM
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jester
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Default slight hesitation on accel at 3k rpm (991 base, MT)

Hi guys,

I've been enjoying my 991 base 7 speed MT for 2 months now, coming out of a 997.1S cabriolet.

I love everything about it, and have reviewed here previously why. Except I'm now noticing just 1 tiny thing, and it is starting to bother me.

In 1st gear, usually only upon the first 10min after a cold start in the morning, upon about 30% throttle (so not gentle, but not WOT either), right at 2900 to 3200 rpm, I get a slight hesitation. As if the fuel was just ever so slightly tapered back. Just a little tiny bit. But I notice it every morning. And it's always right in that rpm range. Happens both in sport mode on/off and sports exhaust on/off (all 4 combos). Now, if I go WOT, it seems to be less noticeable, but it's still there. And when warm, much less noticeable, but still there or I'm thinking it's still there.

I'm right on the edge of just bringing it in to the dealer but would rather put up with it till my 5k service (at 2k now).

What do you guys think? Mass airflow sensor? O2 sensor? (are they the same?) If so, I'm wondering if its ok to just put up with it or if other folks have noticed in either 991's or 997's that it got worse over time and I should really just bring it in?

Thanks in advance.
Jester
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Old 07-18-2012, 10:02 AM
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FORENN
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not sure -- haven't noticed that in mine at all
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Old 07-18-2012, 10:20 AM
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Any chance you can stick a video camera on the windshield and video the dash / tach / speedo as this happens?

Emissions are a strange thing. I think I encounter a couple of flat spots under different load conditions for different throttle positions (in a PDK 991S) but short of a dyno visit (which I might do, just for shnitz and giggles) it's hard to say.

Others have noticed hesitation from a standing start in PDK 991's, so there might be a common characteristic in there.

By the way, if there was anything happening in the engine in terms of fuel, air, compression, exhaust, it would throw a code and probably a check-engine light sooner or later.

If you can repeat the stumble at the dealership (not necessarily easy if it requires a cold start) a tech could do a ride-a-long with the PIWIS (guided diagnostics computer) to see if they pick up anything (assuming they're even "allowed" to do this kind of thing these days ...)
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Old 07-18-2012, 01:00 PM
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marcb993
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Jester, I think what you might be experiencing is the variocam adjustment to the ignition timing taking place. Back in the day, when I had a 996, I could feel the adjustment taking place, always at around the 3000 rpm mark.

It was very subtle and i could only feel it during partial throttle.. I had never noticed it until a Porsche driving instructor pointed it out to me.

I am guessing that the 991 still uses some form of variocam, altho my apologies if I am mistaken about that.

Regardless, it should not be so noticeable that it interferes with your enjoyment of the car. I'd go back to the dealer and maybe try out a demo car for comparison.
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Old 07-18-2012, 01:14 PM
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Originally Posted by marcb993 View Post
Jester, I think what you might be experiencing is the variocam adjustment to the ignition timing taking place. Back in the day, when I had a 996, I could feel the adjustment taking place, always at around the 3000 rpm mark.

It was very subtle and i could only feel it during partial throttle.. I had never noticed it until a Porsche driving instructor pointed it out to me.

I am guessing that the 991 still uses some form of variocam, altho my apologies if I am mistaken about that.

Regardless, it should not be so noticeable that it interferes with your enjoyment of the car. I'd go back to the dealer and maybe try out a demo car for comparison.
That's interesting. I never noticed it on the 996 GT3, but that might be different again. Also, I think the 997 went to variable intake and exhaust in the 9A1 and in the 991, I imagine it's now continuously variable.

I'd suspect it's just injection. Since the DFI engines came along, the cold map includes double and triple sequential injection modes with changes at 2500 and 3200 (roughly?) to try to get the cylinder temps up (running lean) to light off the cats. Combined with the array of lift/timing changes on the intake side, I wouldn't be surprised to find some glitches. This is something BMW tinkered with -- making the intake valves become the throttle. I still think that's a cool idea and going to electronic/hydraulic valves seems like the next step -- less parts, more precise control etc.
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Old 07-18-2012, 01:34 PM
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Originally Posted by jester View Post
Hi guys,

What do you guys think? Mass airflow sensor? O2 sensor? (are they the same?) If so, I'm wondering if its ok to just put up with it or if other folks have noticed in either 991's or 997's that it got worse over time and I should really just bring it in?

Thanks in advance.
Jester
only way to say for sure what it is would be to put car on a dyno and connect it to piwis or any other telemetry testing system to see what happens.

probably an independent pro/tuning porsche shop would be better for such research than a dealer, but it depends on the level of free will from a dealer to investigate timing issues.
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Old 07-18-2012, 02:07 PM
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Could it be the rpm blip as the lobes of the cams are shifting for increased valve opening? Its very noticeable on my 997.2, and I think the design of the 991 engine is the same for the Vanos. I don't notice it during heavier acceleration, but it is noticeable under light acceleration at around 2900-3200 like u describe
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Old 07-19-2012, 01:32 AM
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Could it be the rpm blip as the lobes of the cams are shifting for increased valve opening? Its very noticeable on my 997.2, and I think the design of the 991 engine is the same for the Vanos. I don't notice it during heavier acceleration, but it is noticeable under light acceleration at around 2900-3200 like u describe
Chago996 articulated it much better than me, but that is exactly what I was attempting to refer to.

I could sense it on my 996, and possibly on my the 997.1 C2, I don't recall any more.

It never made its presence felt on the 997.1 or .2 GT3s, but they had a different base engine from another branch of the evolutionary tree.
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Old 07-19-2012, 01:55 AM
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I have a 991 7spd cab and I have definitely felt the exact same thing. The car has about 1700kms right now. Occasional it feels like a dead spot while under average acceleration. You almost feel like the car has cut out for just a very brief moment. If you anyone gets confirmation of the cause pls post!
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Old 07-19-2012, 02:46 PM
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Guys,
Thanks very much for all the thoughts and feedback. Seems like a few of you in 997.2's and 991's are seeing this as well, and that it likely is by "design" of the variable valve timing and likely not a faulty MAF or O2 sensor (because it would throw a fault code).
So my plan now is to not rush to take it in but complain about it during my 5k visit... see if there is a software update for the vanos system. Will let everyone know how it goes after that.
Cheers!
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Old 07-29-2012, 09:51 PM
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Any news on this? I recently got a 7 speed manual base 991 coupe and am experiencing the exact same issue. I am going to contact the service manager at my dealer tomorrow.
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Old 07-30-2012, 02:32 AM
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Its hard to say if the Porsche software updates will fix it. An aftermarket ECU tune should do the trick, but right now there is limited feedback on the limited number of available tuning options.
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Old 07-30-2012, 06:13 PM
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Originally Posted by caveatesq1 View Post
Any news on this? I recently got a 7 speed manual base 991 coupe and am experiencing the exact same issue. I am going to contact the service manager at my dealer tomorrow.
It seems u and op are experiencing this with base mt cars. I have not noticed any hesitation in my pdk S 991. Getting it checked out is the best bet.
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Old 08-01-2012, 12:07 AM
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My 997.2 (27K miles) has the same cam lift issue from new. Porsche rep drove the car and told me it is OK however it is not as noticeable on all cars. He said not worry about the engine so I let it go.

Maybe be it has no -ve impact on the engine, but I say it is not normal. Because every now and then it disappears during a drive cycle. Could be a sensor issue, but there many of them in these cars. I am trying to live with it.
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Old 08-01-2012, 12:53 AM
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Originally Posted by dodo1porsche View Post
My 997.2 (27K miles) has the same cam lift issue from new. Porsche rep drove the car and told me it is OK however it is not as noticeable on all cars. He said not worry about the engine so I let it go.

Maybe be it has no -ve impact on the engine, but I say it is not normal. Because every now and then it disappears during a drive cycle. Could be a sensor issue, but there many of them in these cars. I am trying to live with it.
A dismissive response is not surprising. Pitifully weak business, but there it is. I suggest you get it documented in the service record of the vehicle and printed out in your hands, then filed away with every scrap of paper on the car "just in case" ... all you have to do is prove the problem existed during the warranty and it was assessed by Porsche, then it's then liability forever.
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