"STI Stumble"

I think I've seen it barely mentioned once or twice on here, but are our cars affected at all by the STI Stumble? Seems like the fix is an FPR like the Cobb unit?

Not sure about the answer to your question, but the tuner had to do several tweaks to get fuel to not to be 'flat'. It could be true, but now I seem to be good with no fuel pump regulator upgrade.

a quick search and I found this that explains what I thing you mean. http://www.iaperformance.com/product_info.php?products_id=3155

Yep, that's it. Cobb has one as well.

 

https://www.cobbtuning.com/products/fuel-system/subaru-fuel-pressure-regulator-kit-sti-2008-2016

Something else you could try first, which seems to be a good fix on a 4th gen and has more or less cured the stumble I had (although it's most noticeable on cold start, so the real test will come this fall), is moving the FPR vacuum reference off of the cylinder 4 runner on the intake and teeing into the bypass valve signal line. I did it by running a little 3" stub of line out of the BPV port on the back of the intake manifold, and sending one branch of the tee in each direction. Not quite as pretty as on a 4th gen, where the BPV and FPR are neighbors, but it seems to work.

 

Edit: check out this thread- couple of pics of the install on a 4th gen in post 223, and I think there's a better description of what was done within one of the surrounding posts.

I have not heard much about the stumble in the 5th gen. I know I have never experienced it on my car. From stock to stage 3 I have never had a stumble.

I rarely notice it, but shifting into 2nd at about 20 mph is usually the best way to make it happen. I'll occasionally get something in the 2100-2200 RPM range if I give it more than grandma throttle input, but that's much less frequent.

So then it is possible for it to happen. So maybe investing in one of these for next set of mods would be a good idea to possibly avoid an issues?

Moving the vacuum reference is easy and cheap enough that it'd be worth doing that first to see if that cures the issue (assuming you notice it). If you're still having issues after that, then maybe look into a new FPR.

I have always had the stumble pretty bad on my McDowell S1 tune. The Cobb OTS S1 tune there was no stumble at all, same with the stock tune.

 

It seems it rears its ugly head when certain fueling parameters to a more aggessive state. I keep meaning to try the reference line mod first, but another work around is to coil 3' of fuel hose between the FPR feed and hard fuel lines by the left strut tower.

Funny you mentioned that because on my Mcdowell S2 tune, I also had what felt like a stumble as well.

Yup, about 2400 to 3200 rpms when giving it light throttle, 2nd gear its most noticeable. If I get my foot in it a bit in those rpms, it goes away and screams like a bat out of hell. Reminds me of a carbeurator when the idle jet and main jet dont seamlessly overlap a little slice of rpms. I kept sending him logs and he never saw anything so I gave up.

I haven't noticed any stumble with my stg1 COBB dynotune. I was thinking about an e-tune if I went to stage 2 purely out of convenience, but I might have to bite the bullet and make the 8ish hr roundtrip to get another dynotune

 

 

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I recommend a dyno tune, lol.