Johnb55

I've put 35,000 very enjoyable miles on my '07 997 GT2. Made a few minor upgrades along the way. EVOMS tune, which immediately made the clutch slip. Then GT2RS single mass clutch and lightweight flywheel(cured the slip). Guard diff, GT2RS Intercoolers, That's about it. I drive it pretty hard occasionally and it had been running flawlessly for years. Just recently I started feeling a brief hesitation for about a half second after going to full throttle / full boost in say 3rd or 4th gear. The car would just slightly hesitate then get right back to business. At first I thought nothing of it, maybe bad gas or something, but it continues to happen after a couple fresh tanks from different sources. Last weekend I set the dash display on digital boost to have a look at what was going on (which can be tricky taking your eyes off the road at full boost in 4th). I saw 20psi initially, then the stumble hit and boost went to 19, then right back to 20 and business as usual. Has anyone here experienced this before and found the problem? It could be a VTG problem, waste gate, boost leak, even coils?

nwGTS

Have you checked codes?
Not a Porsche, but my last turbo engined car had a stumble on full boost just like you describe. Turned out to be bad bearings on the turbo impeller so the turbo needed a rebuild.

redmonkey928

Was going to say turbo bearings as well just from initial impressions.

OP, nice description! Can very much visualize the issue with your description.

Tom@TPC Racing

Spark plugs & ignition coils is good place to start. Pretty common thing with tune; more power = higher cylinder pressure = drawing more current from spark plugs & ignition coils.

MC 968CS

Had a similar problem in my 996 GT2 and then more recently my 997 GT3, was coils starting to go as Tom suggested. They are getting older.

Johnb55

Thanks, you guys are the best.
I just scheduled a 10,000 mile service and will have the plugs replaced and coils checked/replaced. Then I'll try again.

TB993tt

Could you report back what if any codes you have ?

My first thought was spark plugs also, they tend to wear out quite quickly on stage 1 (style) tuned 7GT2s, lots of timing and quite lean produces the extra hp IIRC.

Igooz

OP,
Out of curiosity, with that kind of tune what kind of peak boost do you see on the digital read out?

(I think that my stock 7GT2 I've seen 21psi or maybe 20 psi? It has been a while. I think that i get the same on my GT2RS, I was told due to large IC. And on my 997TT it was 17 psi on over-boost, this car is now sold).

TB993tt

The stock GT2 should max out at 1,4 bar (21PSI) and that is as high as the dash will read, it doesn't have a 1.5 bar reading unless you get it modified (I understand it can be modified to read higher)

The RS runs at 1.6 bar max (23/24PSI) from factory so presumably its dash readout can read higher although I have no idea if it does ? the increase to 1.6 bar has nothing to do with the fact the intercooler may be physically larger but that larger intercooler (because of its efficiency) does allow it to operate at higher boost pressure and produce power (I think you probably know/meant that )

Igooz

^Thanks. More boost and more power. But I've never seen 24psi on the digital dash read out. I've seen 17 and 21 on the other cars. Hmmm...

Snowboarder54

Boost leak👍

Johnb55

I have the basic EVOMS EVT 600 tune, software only. I think it changes the timing and fuel maps, adds a couple hundred rpm to the rev limit and increases boost a little. They claim +62 hp which I took with a grain of salt. They told me I would need to upgrade the clutch as the added power would make it slip. I replied, "yeah, we'll see..." Well it did immediately start slipping under full throttle in 4th and 5th so I upgraded to a single mass clutch and lightweight flywheel. I now take their claims a little more seriously. They have other tunes paired with headers, high flow cats, intercoolers, and even different turbos that go up to 754 claimed hp.

997 GT2's have two different boost readouts to select from on the dash. A bar graph that goes from 0 to 20 psi, and a digital readout that just displays the number which is probably not limited. The highest I've seen on the digital display is 21 psi. I don't recall what it was stock, maybe 19?

Igooz

^Thank you.
I was curious and so I went out and tested for myself. Stock 7GT2, 6,000' and 65F and it showed 21psi.

TB993tt

It is the torque at lower revs which slips your clutch the standard GT2 clutch is almost at the limit and the same clutch in the GT2RS will slip (with its ~700NM) if the temp gets sub 10C, happened to a couple of pals with RSs.
Tuning the stock GT2 is about temperature control with those pesky VTGs creating lots (of heat) A GT2 will run the 620PS of the GT2RS with just the addition of RS/997.2 intercoolers and appropriate mapping but even the RS will lose its 620PS rating if used aggressively in higher ambient temps, the ECU is highly efficient at stealing your hp oce the IATs head over 40C.
The dash numbered boost gauge is also limited to 1.4bar, won't read any higher.

Johnb55

Update:
I picked my GT2 up from service today. New plugs and coils, plus oil change and cabin filters.
The tech said most of the coils were cracked and there was evidence of some arcing. The car has just under 36,000 miles and the coils had never been replaced previously.

I gave it the old boost test and it ran stumble free showing 21 psi of boost the whole way. Here's what I saw in the mirror

Good call on coils and plugs you guys, I hope this thread is helpful to someone in the future.