Stumble/hesitation

[the head]

Trying to track down an issue I'm having on my 2013 TT.  It has a stumble when accelerating in the 1800-2500 rpm range in basically any gear.  If I punch it, it will sometimes act like a misfire.

I've changed plugs and coils and looked for vacuum leaks in the front and rear of the engine but didn't find anything.

I hooked up an Edge CTS to it and watched various parameters over the past few days but don't see anything obvious.  Spark timing looks good and consistent, fuel pressure looks OK, Long term and short term fuel trims look OK (see below), and O2 sensors don't appear to be malfunctioning.  Boost may be an issue as the CTS doesn't always read equal to the dash gauge (might be lag in time it samples the data).  I checked hoses and don't see anything obvious and made sure clamps were tight.

I haven't checked under the intake manifold for anything yet.

 

So for data I never see the short or long terms get above +/-10%.  Short terms go up when accelerating but then come back down to close to zero at cruising speeds or driving at a steady pace in the neighborhood.  I've also sat at idle and revved to about 2500rpm with no load to watch them and once I hit a steady state they are almost always at or near 0%.

 

So back to the boost readings, I will see about 0.1-0.3 on the dash gauge and watching the CTS readout it will only go to 3 or 4 psi.  Sometimes when showing 0.1 or 0.2 psi on the dash, it shows 0 or 1psi on the CTS.  So just to be sure I think I'll check the wastegate diaphragms for leaks and then the diverter valves next.  On full throttle I see about 8 or 9 psi, peak at 10 once or twice so I'm not sure how much it's supposed to be making.  But I can't keep it punched for long due to where I live.

 

Any suggestions?

[the head]

Am I imagining things or are there no mass air sensors like there are in the 955s?

[the head]

Read on another forum that a panamera turbo was having similar issues.  The guy changed the diverter valves and fixed the issue so I ordered a set - I'd be replacing those at some point anyway.  Did get the MAP sensor on the CTS to display and it appears to be reading normally - no jumping and is tracking the boost pressure exactly.

[the head]

Replaced the diverter valves - stock ones seemed fine, and as a result no change.  The Durametric doesn't live data the 958 so i'm stuck with a 1-sample per second gauge and it isn't fast enough to catch anything. What is the stock boost level for this engine?

Thinking I'll have to take it to the dealer soon rather than just throw parts at it.  No codes as of yet.

Edited February 6, 2017 by the head

[deilenberger]

On ‎1‎/‎4‎/‎2017 at 6:16 PM, the head said:

Am I imagining things or are there no mass air sensors like there are in the 955s?

 

From the AllData website (info there is from the service manual):

Quote

Like on the Panamera engines, the new pressure sensor on the intake manifold replaces the previous hot-film mass air flow (MAF) sensor. In order to ensure that the correct mixture composition is always present in the combustion chambers, the actual intake air mass is continuously measured using a pressure sensor for measuring the mass air flow. As a result, the correct mixture is always available in the combustion chambers, and any changes in the atmospheric pressure (due to changes in altitude) and outside temperature are compensated. This new pressure sensor replaces the previously installed hot-film mass air flow measurement system and also contributes to increased performance by dethrottling the intake section.

 

NOTE: The load signal of the pressure sensor and the intake air temperature can be found under the DME actual values.

The pressure sensor detects the intake manifold pressure (at the rear of the intake manifold). On the basis of this information, the DME control unit determines the respective air mass and intake temperature from different maps.

 

Edited February 10, 2017 by deilenberger

[deilenberger]

Are you certain the engine is missing? Or could it be a brief driveline slip? The transfer cases on the 958 are known to be a trouble spot, and are appearing on more and more forum posts (on other forums - this one is almost dead on 958 topics.)  The failure exhibits itself as roughness felt when accelerating, and sometimes more noticeable when turning. The failure appears to be in the clutch assembly. Dealers are getting ~$4k to fix this - new transfer case (not hard to put in actually, but expensive to buy.)

[the head]

I'm not 100% sure as I've read that as well. I pulled the fuses to the PTV to keep it from engaging the clutches. Others did this and the low speed grab on sharp turns went away. Mostly these people then had a software update per a TSB.  In my case removing the fuses didn't help, so it seems to not be the issue looking at it that way.  That said, I do have the grab at low speeds but I don't find it that irritating.  I can feel the miss at higher rpm but it isn't as prevalent as lower rpm and light throttle input. Getting close to just taking it to the dealer rather than throwing parts at it

[deilenberger]

One small comment on the transfer case. In another forum in a longish thread on 958 transfer case (AKA-TC) failures - someone posted a receipt from a dealer who tested the TC by unplugging the controller motor (I imagine that does trigger a driveline failure message of some sort.) When unplugged the TC didn't exhibit the symptoms. The dealer seemed to conclude that was a conclusive test - and replaced the TC, to the customer's satisfaction.  A new TC was bought for ~$2,500, labor/parts to install it was around $500. List price on the TC was much higher, but looking at some on-line parts sources, $2,500 is not an unknown price from a Porsche dealer. Strangely - people are paying more than that for used ones on eBay. Big "duh" to them on that..

[the head]

Took it to an independent shop and they diagnosed it as the transfer case.  Nothing wrong with the engine at all, so I feel a little less dumb.  Here in Houston there are 3 Porsche dealers and one quoted $5500, another $4300, and the third I'll never do business with again for service (Momentum).  So looks like $4300 it is for the turnkey expense of getting the transfer case replaced.  Gotta pay taxes first and then get that done.  I'm hoping they let me keep the old one so I can take it apart and see if the clutches are just burned up in it or something simple like that.  

[deilenberger]

On another forum - there are some rather long discussions on the XFer case failures. Some people have experienced multiple failures, most people not at all. Supposedly (wasn't me so I can't swear to it) Porsche changed suppliers (sounds unlikely to me.. but that's the claim their service managers were making) - and one for sure change they've made is the location of the end of the XFer case vent line. Apparently the line used to end low, almost below the bottom of  the case. They've modified the line so it now ends up high above the case. The supposed reason was to lessen moisture being drawn into the case.  If the clutch plates - which apparently are metal to metal - started corroding, they would possibly tend to bind and then release instead of smoothly slipping. It sounds as if this was Porsches diagnosis of the problem (and fix.)

 

I think that also puts some urgency into more frequent changes of  the fluid in the xfer case - to remove any contamination/moisture. It may be why certain vehicles suffer multiple failures and others none at all - environmental conditions combined with use patterns may mean some owners/vehicles will experience more rapid contamination of the clutch fluid, leading to more rapid deterioration of the clutch.

 

I know there was a recall on early 958 vehicles for defective clutch assemblies in the xfer case. The claim was made in the recall that the defect was a machining error on the clutch case. The symptoms were identical to what people are now experiencing with a wide range of production date Cayennes (the newest I've heard of is a 2014 - so far..)

 

Anyway - the PN for the new vent tube is 958-301-047-10, around $29 from Sunset (cheapest I found it on line - I'm giving my local friendly dealer's parts department a shot at it - if it's close enough in price - they get the business.)

Edited March 5, 2017 by deilenberger

[the head]

About a week ago, I changed the oil in the transfer case.  I went with Motul DCTF as it is a replacement oil for the Shell equivalent that Porsche recommends.  BTW, that doesn't mean it's "Porsche approved" but it does cross over from a parts perspective.  Anyway, all of the symptoms have gone away.  It now shifts and drives smoothly.  I'll change it again soon just to ensure that the old oil has been removed as it had a very burnt smell to it - very similar to when an automatic transmission has burned up clutches.  So this might be a fix if caught quickly before the clutches take a lot of damage (and there's nothing saying they aren't in my case).  There was also no sign of water in the oil.