996 GT3 stumble or hesitation around 4000rpm

[Qualified Pole]

Hi all,

I finally fulfilled a dream and just picked up a new-to-me 2004 GT3 today (Carrara White, 17k miles) and am absolutely amazed at how different (even more fun) it is than my two previous 996s. Now, after a long time of lurking, I have a question that (I hope) warrants the brains and experience amassed on this forum.

One thing noticed from the passenger seat when friend of mine gave it a quick spin, other than the darty front end that pulls left under braking (due to a poor alignment I gather from the many posts here--which I will have the selling dealer check since apparently that tick mark on the CPO form was not attended to), is this:

Under spirited acceleration, the motor seems to hesitate or stumble around 4000 rpm. Having recently had a C4S and an e46 M3 with all of the electronic nannies, I would liken this hesitation to stability control (in one of those cars) kicking in and lightening the throttle input. It is mild, but noticeable and repeatable.

Has anyone else had this issue, and if so, could you point me in the direction of a post or troubleshooting method?

Thank you very much!

[clord]

Hi SilverC4S....(gotta update that handle now?) Welcome and congrats on your new-to-you GT3. As you've seen and commented on, the GT3 is very sensitive to alignment....and also to tire condition. For instance, if the front tires are worn more on the inside shoulders, the front end dartiness will increase, especially under braking. Measure the tread depth side to side and across the front tread to evaluate. Get a good alignment and keep the before and after alignment specs. Owners have also experienced the alignment changing with, say, pothole hits. For instance, the rear eccentrics that set camber/toe can slip.

As for the hesitation around 4k, I have not experienced that in any GT3. There is something wrong there. It's still under warranty, so have the dealer read any codes and get it corrected. It could be anything from a "vacuum leak" in the intake system to some failure in the variable cam timing adjusters. In any case, it's not normal. Please let us know what you find out.

[Qualified Pole]

Hi SilverC4S....(gotta update that handle now?) Welcome and congrats on your new-to-you GT3. As you've seen and commented on, the GT3 is very sensitive to alignment....and also to tire condition. For instance, if the front tires are worn more on the inside shoulders, the front end dartiness will increase, especially under braking. Measure the tread depth side to side and across the front tread to evaluate. Get a good alignment and keep the before and after alignment specs. Owners have also experienced the alignment changing with, say, pothole hits. For instance, the rear eccentrics that set camber/toe can slip.

As for the hesitation around 4k, I have not experienced that in any GT3. There is something wrong there. It's still under warranty, so have the dealer read any codes and get it corrected. It could be anything from a "vacuum leak" in the intake system to some failure in the variable cam timing adjusters. In any case, it's not normal. Please let us know what you find out.

Hi Craig,

Yes, I guess a new handle is in order. Thanks for the tire point, although the tread depth appears almost full (they have been replaced) I'll double check those as well when it goes up on a lift this afternoon. A follow-up question on the alignment: is least aggressive (least neg. camber) within factory specs the way to go for street driving on the snowless days between now and my next track event in the spring, or is there a consensus on different specs?

It's under initial warranty, and then CPO beyond that, so I will have them do that. It was my understanding that a DME dump and fix was part of the CPO process, I wonder what is going on. Thanks for the info and advice. I'll post the results.

Michael

[clord]

SilverC4S...... Ah, yes, the old "what alignment?" question. You will find many opinions on GT3 alignment in these forums and others. I'll share my own opinion with you. There is a good alignment for the street and there is a good alignment for the track. They are different and each has problems in the other environment. Bottom line for me is, I have figured out what I like for each, and re-align the car for each use. Track use on the street alignment is not the best for track handling and bad for tire wear. Street use on the track alignment causes more twitchy handling and is bad for tire wear. I decided long ago that I could get three 4-wheel alignments for the price of one rear tire worn out prematurely due to track alignment on the street, or vice versa. To your question on street alignment.... Yes, for my purposes, for a street alignment, (and for decent tire wear) I spec it toward the conservative end of the factory recommended adjustment tolerance. That is, minimum negative camber within the spec and maximum toe in (for stability) within the spec. I found that rear tires last up to twice as long that way than with a more agressive alignment spec.

[Qualified Pole]

Craig, thanks for the info. I would also prefer multiple alignments to new tires. I am awaiting a service appointment shortly and will have them set up the car to the least negative/max toe in for the non-snowy days I go for rides before spring. From Loren's 1/24/05 post, it looks like that will be:

F Toe (unpressed) +6'

Camber -55'

R Toe +11'

Camber -1*45'

leaving front caster at the 8*

Does that sound right?

Thank you again,

Michael

[clord]

Hi Michael..... Personally, I convert all the alignment numbers to decimal degrees. That just makes it easier for me and the alignment tech to see where the settings are since we tend to think in decimal numbers easier than degrees, minutes, and seconds. Any good alignment machine can be set to read/print out in decimal degrees as well. Here's my most recent conservative street alignment specs:

Front Camber: -1.0 deg

Front Caster: Can't be adjusted but mine come in between +7.7 deg and +8.4 deg.

Front toe: +0.06 deg to +0.08 deg per side for a total front toe of about +0.14 deg.

Rear Camber: -1.7 deg

Rear Toe: +0.23 deg each for a total rear toe of about +0.46 deg.

Try to minimize the thrust angle by getting each rear toe very close to the same, side to side.

Have the tech really tighten down the rear adjustment eccentrics....they are know to slip if not tight. If you've got the time, get to know the tech and sit in the driver's seat while he dials it in. Depending on what machine he's using, you may have to remove the front brake cooling ducts from the lower trailing arms so the front wheel alignment heads can see each other under the front end. I've gotten pretty good at removing and replacing the ducts....mine are the cup car ducts that are a little larger too.

Remember, this is a conservative street-only alignment that maximises stable handling and decent tire wear. You will get other advice for best agressive handling....but, the darn thing is already is one of the best handling cars on the road!! Let us know how it turns out.

[Qualified Pole]

Craig, thanks a lot for the info. Sorry, I was being lazy and did not grab the calculator to convert from mins. I'm going to try your setup.

Michael

:notworthy:

[Qualified Pole]

The "stumble" disappeared. A couple of days before taking the GT3 in for service at the selling dealer, the stumble just wasn't there. I took it in anyway since they realigned it and they checked for faults but no codes were stored. I know I was not dreaming, but the only thing I can guess may be the problem is fuel. I'll post if it happens again in the future.

Craig, the dealership realigned the car and what a difference! Unfortunately, I had to drop the car off and run so my carefully written list requesting items to take care of (including the alignment specs you gave me) were ignored. The end result was that they brought the care to within minimally acceptable factory specs (making as few adjustments as necessary). So, although the rear toe was the problem and the twitchiness has been significantly reduced with the changes, there is some room for improvement. I'm going to take the car to a decent shop to get it set right after the new year.

Thanks again,

Michael

[clord]

Hi Michael.... I'm glad to hear that your stumble has gone away...but, a little uneasy that we don't know the cause. I hope that, if it was serious, that there would have been fault codes logged. Also glad that your alignment job fixed some of the dartiness. Yes, these cars are very sensitive to alignment. It's a good learning experience to know the before and after alignment specs and to associate those with your experience of the car's handling. That way you you get to know what YOU want in an alignment.