Silver Kugel

New owner and new to forum so forgive me if this has been answered/discussed before -- though I have done numerous searches and have not found a symptom match that leaves me with a warm fuzzy...

Symptom: Straightline acceleration (pedal mashed) from light after reaching about 35 mph, and particularly when initiating a highway pass acceleration is noticeably jerky -- almost like tire slip where the tires are locking up and then releasing 3 or 4 times a second.

At first I thought it was tires or the PSM, however turning off the PSM did not seem to make a difference. I recently put on new snow tires and continue to have the issue, so mismatch in tire diameter is ruled out.

I am wondering if it is a cardan bearing starting to go and binding when some driveshaft harmonic is reached.

I have also had occasional error codes thrown indicating cylinder misfires, but the error codes do not occur in conjunction with this behavior -- engine light only goes on when aggressively downshifting or losing traction in snow causing high RPM condition.

I have an appointment at the shop on Monday to look at coils/plugs and the service advisor (who has test drove the car and verified the slip/jerk behavior) says that it might be the cardan going and to baby it over the weekend.

Any other ideas?

Thanks,

Dave

Dennis C

When my cardan shaft went out, it sounded like somebody was under the car banging the body of the car with a hammer. It was a loud, thumping noise.

What you're describing sounds more like a misfire issue (although I would expect to see a CEL with a bad coil pack) or perhaps a transmission issue to me. Or could it be the traction control system taking over to mitigate wheel spin? I don't know if deactivating PSM would impact that or not...

Silver Kugel

I guess I was hoping that there was some benign indication of cardan failure before the midget started hammering on the driveline tunnel. The alternatives appear to be a time consuming trial and error replacement - unless it is some sort of 'normal' behavior related to the PSM and acceleration rate of the Turbo S.

The behavior is correlated to mid to high rpm and boost combined with acceleration. No rattles at idle and no noticeable vibration at any speed when not under acceleration.

I thought I was eliminating wheel speed mismatch by disabling PSM and putting on new tires, but it may be like my BMW that 'off' is really German for 'trust us, you do not really want to try to control this car without our engineering expertise'...

The power fluctuation is at a low enough frequency and high enough amplitude that is just doesn't seem like misfires - assuming two cylinders misfiring at 4000 rpm with fluctuations at 4 times a second would mean 250 misfires followed by 250 good fires. But maybe I am overthinking it.

The other option would be a boost problem. The gauge does not flutter when this happens, but this might be a function of the boost sampling time or gauge hysteresis.

Dave

mcbit

Definitely sounds like an engine related problem to me, spark or fuel.

Silver Kugel

Fuel didn't cross my mind but you bring a good point - could be something as simple as a constricted fuel filter...

Dave

PTEC

If you have misfire codes I can almost guarantee that's your problem. Service interval is 40K on the plugs, how many miles do you have on the beast and have they ever been changed? The coils are a known issue as well.

Silver Kugel

She has 28k on her right now...

endless_corners

The secondary fuel pump kicks in at higher revs... Maybe it's starting to fail.

Dennis C

Good call on the fuel pump(s). When mine failed, my symptoms were similar. That might be it.

TomF

Hey Dennis,

Not to hijack the thread, but I just noticed that you are in Evergreen. What a wonderful place! I have been spending a lot time there as the former CEO of our company also lives there. I got caught in a herd of elk the last time I was there in Nov... right down in town by that great soup place.

Best,
TomF

Dennis C

Hi Tom. We love Evergreen! The soup place is Da Kind. I ate lunch there yesterday!

Shoot me a message next time you're in town and I'll buy you a beer!

JohnnyBahamas

Follow the misfire codes.

Big HINT ...take it.

Silver Kugel

Thanks for the advice. Here is an update...

Prior to bringing the pig in for service, I headed up to the mountains. Acceleration was again jerky and at one time the jerkiness was correlated with flashing check engine light, though the light reset on its own. On the way back down the mountains, the jerkiness set the engine light on, so errors were active when I brought it in to the dealer.

The service shop road tested the vehicle with an analyzer attached and confirmed that the jerkiness was due to 3 cylinders misfiring or not firing. They told me that the coils were at fault, that the installed coil packs were rev2 units, and that current replacement units were rev 30 or something. They also said that Porsche recommended that they be replaced in a set.

They replaced the coils and all of the plugs. Problem solved. Strong and smooth acceleration.

Zoom zoom...

TomF

Sounds great Dennis!

We are actually talking about opening a new satellite office in Denver, so I may be nearby a lot. We are a virtual company, but Denver is convenient to nearly all our staff, so we meet in the middle.

I'll drop you a line the next time I am in town.

Cheers,
TomF

INURGRL951

As i started reading i was thinking coils but i see you got there at the end!