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Hey I have a 2013 WRX with 41,XXX miles and I am getting some weird hesitation/stumble between 3500-4000 rpms on quarter to half throttle.
I am tuned on *93 pump gas Shell or BP only) by Graham Gaylord who is the owner of Boosted Performance Tuning who is a very reputable tuner. He told me it is most likely a mechanical issue which is why I am now here to ask for help!
Oh also no check engine lights or codes.
Mod list:
Cobb AP V3
Perrin top mount
Invidia catted downpipe
Perrin 1.75 uppipe (NON Catted) (NO EWG)
Grimmspeed 3 port ebcs
KS Tech 73 MM Big MAF CAI (With Heatshield) (Ceramic Coated)
Tial QR Recirculating BOV
ID1k injectors
DW65C fuel pump
Perrin Turbo Inlet
TGV deletes
Air Pump Delete
NGK 1 Step Colder Plugs

So far I have replaced my MAF Sensor, Front O2 Sensor, I have done a Smoke Test (no leaks), I am hitting target boost, my vacuum shows -22 at idle, I replaced my plugs with 1 step colder, my battery voltage is between 13.2-14.4 (optima yellow).

This only happens on partial throttle between 3500-4000 rpms and I don't feel any hesitation or stumble on WOT
I don't know what to do anymore... Please help!
 

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It may be something silly - AC/defrost kicking in? Does it do it every time?

A log might help a fair bit.
 

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Not sure bud, sounds like the stumble I get in my car which is supposed to be (which I haven't tried yet so cant confirm myself) able to be eliminated by an IAP or COBB (or homemade) Fuel Pressure Regulator and a tune. Not sure if the WRX (Non STi) has the same issue. Maybe someone can chime in to confirm if the 13 non STi's can suffer this same bug.
 

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I don't see anything that looks like an issue, or really any indication of where the stumble happened - it all looks smooth.
 

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OK so I had an issue like this 3 times. Each for different reasons. To start log looks fine. Would be better to upload one at wot. Also incluse stuff like avcs. Ocvs. Injector duty. And some.other important ones. First off tune. I had this issue before but mine would always be around 4500. And started happening after I got my tune. When I got a retune it went away. Next is that grid peed ebc. I got this issue again cuz one of the 3 lines to the ebc got bent. I didn't even notice it at first looked fine. Wasn't bent or crimped at all. But I redid all lines for the ebc and gave them more slack. That time it pulled but I got a hesitation again and a misfire. Changed coilpacks and was all good.

Also another thing I hear is a bad engine ground can cause that. Dirty sensors like maf or o2. Knock can pull timing and do that. Check knock sum in all 4 cyls after a 20 min drive. Cyl 4 should have the most. Also try a full reset and relearn disconnect battery. For a true reset for a 10 min.. on and oil change would be good if you're close to one. Maybe a filter or banjo got a little clogged up with old oil.
 

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I'm wondering if you resolved the issue with your vehicle, I just test drove a 2013 WRX that seems to have the same issue as your vehicle ..... hesitation when engine hits 3500 - 4000 rpm, heard turbo kicking in about that time as well.

Hope you were able to resolve this
 

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Other than the current issue you're battling, what's your opinion of the vehicle, the one i test drove seemed to lack the power i expected out of the WRX, it felt more like the off the line acceleration of a toyota corolla.
 
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