My GR stumble: What I've learned what my car likes.

[STIGR-Wagon-Dad]

I have a 2008. It's your typical Stage 2+ELH and I'm setting it up slowly for a bigger turbo swap. My stumble was registering on my boost gauge. The parts I've added are installed to support my future turbo are:
1. DW65C-The car ran rich and I had my tuner clamp down on the duty cycle for now. What I did notice was a slight hesitation part throttle in 1st gear.

2. IAP fpr stumble fix-The hesitation went away and the car runs more smooth. It got more smoother after zeroing out the load comps.

3. 1150cc injectors- I installed these and got it tuned just to run for now. After a few days of driving, the stumbles came in hard at part throttle 4th gear and up. My tuner tuned it out but it eventually came back a few days later. He did what he could on a limited time.

4. Cobb fpr reference relocation kit- This tapped into the bypass valve hose to get an even pulse from all 4 cylinders. The stumbles got worse to a point of getting a #3 cylinder misfire code.

The outcome:
I drive to work this morning and ran in all gears in the stumble area. When I did the Cobb reference relocation, I had the stumble much worse than the stock location vacuum source. So bad that I had a CEL for #3 cylinder misfire. I noticed strong fluctuations in my boost gauge in the stumble area. I went with a long 12" vacuum hose and kept the vacuum source at the bypass valve. Stumbles are very faint, but much more smoother than the short 2-4" hose (experimented from 2" to longer) and tolerable. I then added the Ixiz Concept ixizMADblk and keeping the fpr vacuum hose 12". I drove in the stumble area again and my boost gauge once again registers smooth build in boost and no fluctuations! Stumbles gone!

 

[Grinder34]

It's been a while since i was on the ixiz website, i'll have to check out the MADblk.

Glad you got your stumble figured out, I hope its fixed for good. IIRC there was something about 08+ (maybe all EJ257s?) and fuel hose lengths causing standing waves or something along those lines? Has that come up in your research? Could that also be a/the problem? I've been too busy lately and have forgotten so much, so I'm hoping to (re)learn something!

 

[STIGR-Wagon-Dad]

It's pretty much the 08+ STI have reverted back to the Legacy 2.5l turbo fuel rail system. The 08+ STI are based on the Legacy chassis. There are reports that the Legacy folks are experiencing this stumble phenomenon also. There are some folks going Stage 2 and experience the stumble. The only time I had the stumble on a stage 2 tune was on OTS 3.33 and 3.34. I went back to 1.02. The 3' hose does work from what I've read but it came back once they went with bigger injectors, especially with 1000cc. I would have done it but it looks terrible. I did IAP for kit and it worked well with the stock injectors and pump. Based on my researching the forums, I find that it may well be both the Legacy fuel rail system and the intake manifold resonance that is causing the problem. But I'm curious to know if just installing a longer vacuum hose might do with the stock fpr/damper. Vacuum lines are easier to hide and cheaper.

 

[HolyCrapItsFast]

I think the Stumble is due to a combination of issues, those being Fuel System, Induction/Vacuum and Tuning. If you don't address all three you will almost always have a little bit of stumble. It looks like you hit all of the marks during your trial.

So far my formula for squashing the stumble is to A: revert the fuel system to the 04 design and use a good after market FPR or replace the feed lines altogether with a series feed system. B: Ensure there are no leaks in the intake system by performing a boost leak test and zip tie all your lines. low and mid load vacuum is usually where the stumble occurs and even the smallest vacuum leak can have profound effects. C: Meticulous tuning, especially the load comp tables. I have noticed that the OEM is doing a fairly good job of tuning the 2015's where load comps are concerned. Their tables are very close to mine and need only minor adjustments. So it seems they are following the tuning communities concepts to some degree.