Scary Power Loss While Cornering

Hi All - So I was putting in another heat cycle on my new engine (such tough work) and taking the spirited drive home. As I was cornering at about 40mph around 4000rpm, pushing pretty good, but not anywhere near breaking loose, it seemed like the hand of god reached down and shifted my car for a split second. It was kind of scary. Here I am cornering, pushing things a bit and for a split second the whole car came unsettled. It took me a few seconds to process, but my conclusion was I lost all power somehow for just an instant and under acceleration and cornering it unsettled the car kind of like a sudden lift of the throttle. This was the first time this happened in this situation, BUT.... I have noticed that right around 3K rpm, when slowly moving through the rpms or holding steady, there seems to be a slight hesitation for a split second, just a momentary glitch. I spoke to a mechanic friend who chalked it up to the engine software still "learning". FWIW - the car is a 2002 C4S with rebuilt 3.8L. I now have about 700 miles on the rebuild. Yes, I did a heard reset of the system before reinstalling. No check engine lights. Where would I start my diagnosis of what this could be? BTW - I have an iCarSoft code reader.

Fuel loss sputter or key off electrical lights out? yes pending codes?

Three questions. 1. What tires are you running 2. What was the air temperature 3. Have you done any suspension work to stiffen the suspension. 3. Do you have a limited slip differential.

Added one more. Sorry about typos

Forget my questions. See it's a c4s.

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Hi - if I'm interpretting your questions correctly, it was more of a fuel loss type of sputter (BTW - new fuel pump, lines and sending unit just installed 700 miles ago with new motor). There are no codes. All previous codes were cleared, and there are no new codes.

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It is a C4s, but it's running in 2WD mode. Running Yokohama Advan Neova tires with about 50% tread left Running OHLINS coilovers w/ Tarret sway bars and links (coincidently, I just had the front struts replaced under their warranty recall) It's California - probably 55 degrees out. The whole thing really seemed more like a momentary loss of power than a traction/suspension issue.

Traction control kick in?

It may not be traction related. However, depending on if you have a LSD or not, tires, temperature and how hard you are cornering and therefore loading the outside wheel, will determine whether you are getting any inside wheel spin. Didn't know what was happening when it happened to me on the track in a relatively low speed but tight turn that loaded up the outside wheels nevertheless. Inside wheel was spinning but I didn't know it. At the time, I was getting on the gas and not going any faster, seemed like I was losing power and speed, not to mention balance. Car was getting very squirrelly. Turns out I did not have an Lsd which was causing it. Inside wheel was spinning and no power was being put down on the pavement. Horrible feeling. May not be your issue, but if you are aggressively cornering in 55 degree weather, with worn summer tires, tight suspension, no lsd, it may be something to look at. As a side not. I do not have stability management on my 02. Glad I don't since I don't like nannies. But it made it even worse. I then added a guard lsd, and then holy he'll, it was like a light went off on what I was missing. Night and day. Can now steer the car with the steering wheel or the rear end or both without losing any power. In my opinion, an Lsd is one of the most important upgrades if you don't have one. If you do, then it's certainly something else. Hope that helps.

Do you have stiffer engine mounts? Could it be a knock sensor picking up some NVH, interpreting it as knock, and pulling a ton of timing?

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I don't think so. I wasn't losing traction. Thanks for the extra insight. I really don't think it was losing traction. It felt more like power was taken away for a split second. I will have to check if I have an LSD. I've been so excited just to get the car running, I really didn't dive into any of the extra stuff.

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This is the sort of thing that it feels more like than suspension issues. That being said, I do have the 911 RS mounts - they are just slightly stiffer, and nowhere near as stiff as aftermarket mounts.

That is what traction control feels like Hard cornering with it on is like hitting a rev limiter for a moment depowering until corrected Its angle and speed related and less about spinning tires on the older cars No expert but this is what it sounds like and with no codes.... sounds like you found it

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Haha - well then... sounds like me and traction control are going to have some issues. LOL On a more serious note, I'm still skeptical. I had traction control on my e36M3 and only had it go off once in all my years of spirited driving and it happened in an "oh ****" moment (ironically on this same road!!!). It was me driving at the limit and with a surprise corner I wasn't used to. My driving yesterday was at about 6/10, maybe 7/10ths - nowhere near as aggressive as I would think I need traction control to save my butt. In no way was I near danger or close to losing traction. Thank you for your input.

It is way too sensitive for track but for street and little old ladys (cant you say that?) it comes on early to help out if grandma cooks a corner

When I first got my 4S some idiot put the wrong tires size on the back, likely because they were cheap and he was selling the car. I'd take some sweeping turns at speed, never loosing traction, and the PSM or traction control would basically try and kill me. I'd check and make sure the tire sizes are correct for your 4S.

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Sounds exactly like TC to me. Did your tires hit the painted line at any point? Painted lines on the road are slippery!

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Hey Bill - Nope. If it is TC, I'm relieved in that it isn't an engine/electronics issue. But on the other hand, this is a bit disappointing if it's this sensitive. Thanks

You should be able to turn off the TC?

I've pushed mine several times like this in on ramp turns. Never bumped into this problem. I guess I need to try harder! Best,

How much of the problem is a byproduct of going from 4 to 2 wheel drive?

Except for the going around the turns part that sounds very close to our failing crank position sensor. Engine would briefly cut out and it would feel like a shock to the drive line, then eventually this progressed to the engine would stall including in the middle of the freeway or a surface street in heavy traffic dangerous. Don't remember the code but it did have one stored, and the Indy did this repair.

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Yes - you can. That's kind of the thing... I wasn't really pushing it that hard. I push it harder on on-ramps all the time. That's why this doesn't make sense. I really don't think it's related. And again, I wasn't pushing the car hard enough to bring out different characteristics of the different drivetrains.

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This is interesting. And seems more in line with what I experienced. Did you get any sort of CEL when this first started happening? I have none!.

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When it was intermittently cutting out no CEL, when it got worse and engine would die we did get a CEL. We had just purchased the vehicle so didn't yet have DURAMETRIC to see if there was a code before the CEL came on.

It happened AGAIN!! I was taking my car out to warm it up for some tests and the scary power loss happened again. It was about 4 minutes into my drive. I was getting up to about 60 mph, just under 3K rpm, and about halfway into a 35 mph left hand turn (which I could easily do at 70mph) under light throttle and smooth steering input, the car had that loss of control feeling again. This is especially odd because the weekend before I had taken it out on a high speed 20 mile rip where I did induce and feel the �nanny� limiter once. That was a controlled reduction of power, but the thing that happened today felt like a second of zero power and a shift of the car. It was scary and unnerving. Any more thoughts?

I read your comments a few times. Does the tach actually drop a little when you experience this momentary loss of power? I saw you said it felt almost like a fuel loss sputter, but your fuel system is new with the rebuild. I have seen and heard mechanics warn of brand new OEM parts failing out of the box... Could it be worth getting a scope on your injectors and watching for cutoff? Not sure if icarsoft has the ability to monitor the injectors. I believe Durametric does. It would be a bummer to drive around with a laptop in your passenger seat until you can recreate the issue... but I wouldn't rule out new parts just because they're new. This happens only when you're turning, correct? It could also be a loose wire or sensor that g-force is "wiggling" out of position. Any sensor connected to PSM will throw ABS/PSM lights... ​​​​​

Look for any loose connections, including battery cables, ground straps, make sure the battery is solidly mounted in the frunk.

Sounds like a PSM issue to me. You have to remember that PSM is not just "traction control"... In other words, it won't necessarily wait for the wheels to spin before it does it's thing. It's reading the speeds of all four wheels in addition to measuring steering angle, yaw, acceleration/deceleration etc. and using this data to "look ahead' for possible danger - ie: "if the driver decides to accelerate more at this point the car could/will lose traction - reduce power"... It's possible that one of your wheel sensors is having intermittent problems (though that should throw a code) of one of the other sensors (yaw or accelerometer) is having same (these should throw codes as well)... As has been mentioned above, PSM also interacts with the ABS system, so it's possible that something with the ABS is causing PSM to react. Have you gotten this "power loss" with the PSM turned off?

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Thanks for the reply and thoughts. I did't notice if the tach dropped as I was concentrating on the corner and lookig ahead. If the rpms did drop, it was just for half a second. I'll have to look innto if the iCarsoft can do that. And yes, this oly happens on a corner, both times on sweeping left. I'm ot so sure about the wigglinng concept, though as I've taken the car through lots of twisties and didn't get this. Your thoughts on a loose ABS sensor has me woderinng, though.

Do you have a lot of keys on your keychain?

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Will do. I actually had this happen once in my Infiniti. All the mechanics thought it was an issue with the ignition. Sure enough, loose positive battery cable.

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Hi dporto - thanks for the reply and the explanation. I have not yet drive the car with PSM turned off as this is my first 996 and I'm getting to know the car. Additionally, since the car is far from stock (2WD C4S, Ohlins, Rentech F&R swaybars, 380+ hp) I'm taking my time gettingn to know it's limits. I'll look into the ABS sensors and anything else I can finnd that sends a signal. Twho things that occured to me is 1.) this disn't start happening until after I had the front shocks replaced (uder Ohlinns warranty), and 2.) my tires (Yokohama Neova Advan 008R) are from Oct. '15 making them 5.5 years old. Is it possible that tires this old are actually harder than a normal street tire now and I'm messetially runing on round "rocks"?

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No, just one Porsche emblem thingy. Pretty minimal and lighhtweight. I think I kow where you are going with the question and I don't think that's it.

Go back to same curve/corner, is the problem repeatable? If so, do that section several times. Then go back with PSM OFF and see if the problem happens then.

Not sure about the cutting out issue, but the slight hesitation when slowly accelerating at 3K RPM is the baffle in the intake opening

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Hi Tom - I did this right after it happened. Tried two more times ad it was not repeatable. I wish it were. Then I'd be less concerned....

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Hey Looneybin - THANK YOU for explaiig this. It's been drivig me nuts not knowing what this was. Can you explain with a bit more detail as to what's causing this?

At 3000 rpm or so, the variocam kicks in. Very slight millisecond pause then kick forward with power. Others are more technically proficient than me to technically explain, but if you are paying attention, you can't miss is. It's not a loss of power. Just a transition in feel.

The vario cam starts to “vario” at 3500 rpm and you usually won’t feel that as it’s gradual the intake flap opens at 3000, it's for the variable length intake runners (like a ram air system) and you will feel that slight hesitation during the intake transition, then you get a kick of additional power

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Hopefully you can get a repeatable issue somewhere, sometime. Until then it is going to be difficult to find. In my earlier years, I used to track (DE Instructor) a 1986 Z51 Corvette. On track, I had a momentary power loss in the esses at Road Atlanta. Then it happened again in a different turn. Turned out to be a loose power connection to the in-tank fuel pump. When the fuel level was at a certain level, with the appropriate G force in the appropriate direction, the connection would separate (swing away), losing fuel pressure and then would swing back and connect. Then would run fine until the scenario repeated with all the right forces. I am thankful there never was a spark... That was a fairly easy fix once I found it. -Tom

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Thank you. This helps.

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Ironically, I had to replace fuel pump ad sendig unit recently. I'll re-examine my installation. Thanks

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Tire sizes are stock: F: 225/40/18 R: 295/30/18

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More like a quick key off pwer loss. No codes thrown.

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Definitely revisit the fuel pump. I know the c4, c4s and turbo share the same designed fuel pump with the split Venturi tubes. I know you mentioned you replaced the sending unit, a lot of guys with the older fuel pumps have seen a split epdm rubber 180 degree hose, that was present in the original pump. The updated version eliminated that hose and went to a corrugated plastic line (like the rest of the pump lines) Easy quick visual inspection on that. Here’s a link that also shares some photos and items to inspect including the Venturi tubes as well. https://www.6speedonline.com/forums/...-pictures.html