Do your RPM's ever spike on the odd start up? The Cure is here!
 

Hi fellow 997tt Members.

I started a thread on this a while back now asking you guys for advice about this very problem so now that i'm aware there is actually a fix, and this is a genuine problem and, "not normal", I wish to advise all those 997 owners out there that may be curious whats causing this rev up in their cars....and how to fix it.

There is a reason behind everything;).

The exact cause of this problem is an incorrect gap between the the crank sensor and the passing marker....as it rotates past. The sensor is not set correctly within the specified tolerance. Evidently an inherent assembly issue.

As you may know the ecu must read and compute all sensors the moment the ignition key is turned and the starter motor begins to rotate the crankshaft. Voltage levels won't reach system requirements until the engine actually "fires up" so when the voltage is slightly down, as it often is after a period of being stationary, the sensor struggles to register the marker due to the incorrect gapping. (Like a spark plug which is incorrectly gapped too wide, more current is required to bridge the gap).

These occasions of "spiking" and, the frequency of these "rev up" spikes are often determined by how far away the marker is from the sensor after the engine is switched off. A weaker battery can also add to this problem.

For eg. If the marker has rotated just passed the sensor after the engine has stopped rotating, then upon restart, the starter motor must rotate the crank further for the marker to come back around past the sensor again to be registered. So, if the sensor is unable to read the marker due to incorrect gap (combined with the normal slightly lower voltage) the engine may sometimes continue to rotate without starting until the ECU is able to determine a start up ignition environment. The weaker the power the more rotations are likely to occur but once the ECU allows ignition the engine will fire up, the revs will momentarily spike, voltage is then restored to correct levels and then the ECU re registers the correct position of the crank sensor returning the rpm to correct idle speed.....all happening within a split second.

Now to the fix. And, some may not like it:(. Gearbox must come off to access the sensor.....adjust sensor so it is gapped within the specified tolerance....gearbox back on again etc. IMO its probably best to do this when doing your clutch;). You (may) be able to get P to fix it if it is happening ALL the time but that may very well depend on how far out of warranty you are, and the relationship you have with your dealer:). Some cars can be worse than others and this is usually determined by just how far out of tolerance the sensor was installed to begin with....along with other factors such as the battery strength and condition. Its just a guess, but i think P figures that unless the problem is going to cause engine failure within warranty then they have little motivation to go to the trouble and expense of fixing it. The cure isn't that old so most dealers may not be aware of it.....although i am privy to the TSB number.

Hope this helps to settle this concern once and for all:)

Speed21

That best explains the cause of high rpm on startup.
Mine does it occasionally.
+1 Rep to you

I don't own a 997tt yet (still planning), but this info will go into my "one more thing to remember" file. Great information - thanks for sharing it with the members! Definitely worth a rep point. ;)

Great ifo, taking mine in this week, can you fwd me the tsb#?

Great info. thanks for posting. Damn, just had a new clutch done a few weeks ago :mad:

This only happens occasionally and it revs to about 3,500 rpm. What is the danger, and is anyone reving even higher? Are there cars that are doing this on every start up?

Wondering if it is worth the expense if it only happens occasionally

Thanks for the explanation. Mine does this randomly and I've been trying to keep track of any variables that might be causing it. I was even more perplexed since my 2007 TT never did this but my 2008 TT does. Scared the daylights out of me the first time and with an EP2 exhaust my neighbors weren't too happy either. I thought it had to do with outside temp or how hot the engine was when shut down, battery (mine is new) etc. and I just recently concluded that if the start sequence takes an extra cycle (not sure how to define that) to kick over I get the high revs...now your explanation tells me I at least was on the right track. I wish there were an easier fix. Hope someone posts the TSB. Thanks again!!

I would only approach it if doing a clutch. 1/10 times is about typical for me. The only danger I see is the potential lack of oil pressure for that split second during the sudden 3500rpm surge at startup, can't be good.

no, but it sounds good though :D seriously, i'm probably less than 1 out of 10 starts, maybe 1 out of 20.

It has happened to me a couple times but only when the engine was warm. I go somewhere, park, come back an hour later and it would rev up to about 3500. I thought it was the tune... interesting... does it happen on stock cars? I think everyone who commented runs some kind of software on their cars. Tks, Alex

Stock tune - did it before and after install of EP2 exhaust system. Most often occurs after car has been sitting a couple of hours or first start in the morning.

Paul, great post.

Will PM the number. I'm not sure if its a TSB as such, but a Technical report relating to customer complaints along with instructions on how to address the issue. TSB's normally get circulated whereas these may not.

Sorry to hear that.

I believe the further out of spec (wider) the gap is the worse the problem will likely be. But dont forget the condition of the battery may also be a contributing factor as well as it certainly wont help. The sensor must be able to pick up the electrical pulse so the weaker the voltage at start up the worse it will likely be:).
I do know however that mine has never done it since having the sensor's position corrected (over 2.5 months and countless starts) so it definitely fixes the problem....no doubt about that. The distance or gap between the sensor is very very little so even the slightest variation from spec evidently has an effect.
I think its excellent work that P has finally arrested the problem and no doubt they went to all lengths to ensure the problem was clearly identified and not perpetuated into the latest cars.

In so far as the dangers? Well...hmmm. I guess there are a number of oil pumps throughout the engine and, the oil supply is line fed directly from an oil reservoir instead of a sump and pick up pipe so perhaps there isnt a diabolical environment for damage to easily occur, but still, i know what you mean. It certainly didnt make me feel great when the engine instantly spiked to 3500 rpms dead cold or after a week or two away.

Yeh well in a perfect situation it wouldnt do that and agree it cant be exactly great. But again....i guess P feels comfortable enough with the situation instead of recalling all vehicles manufactured within the affected dates/years to R and R boxes. Im sure if the situation was really frequent they would have more of an obligation to resolve the issue in a persons car. I certainly let them know i wasnt happy with mine at all hence them doing the research in their technical files....plus i do enjoy a very good relationship with my dealer especially from a technical position. Anyways.....

No its not software related Alex. The software is always dependent upon the various sensors to function correctly regardless of who's software it may be.:) Im sure if P knew the problem was going to result in blown motors everywhere they would issue a recall notice....if thats any consellation:).

Thanks Bob:). Its good to resolve an issue and this one has been out there unanswered for some time. Its a bit like the smoke on odd start up problem. Anyway I hope i havent set off a mass panic to the dealers as P may hunt me down and kill me for this. JK:)....i hope.

Good info Paul. Thanks for posting. Mine hasn't experienced this but I've probably just jinxed myself. :)

going to guess the sensor is magnetic and would require plastic feeler gauge.?

+1 for someone who's experienced this.

Very random... seems to do it 3-4 starts in a row and then disappears for a few months.

Now... to wait for the clutch to go, or...

edit: I wonder if my battery going south a few days ago has any correlation. Interesting...