EJ257 Stumble 2015 STI

Hi folks,

I have a 2015 Subaru STI. I recently had my short block rebuilt with forged internals. During the installation, I upgraded the injectors to ID1050x. I am currently tuning with DimeMod. However, between 2500-3000 rpm, I am experiencing a stumble. This was not an issue when I had the stock injectors. Currently, my tuner is trying to figure out the issue, and adding timing seems to help. I am getting less stumbles after adding a bit more timing. This happens only when I am on light throttle, less than 25%. DAM never dropped, nor did the feedback knock. The car does have a stock FPR. I know Cobb offers some stumble fixes. Any experience with this issue or suggestions would be much appreciated. I have attached some cruising data logs as well.

These are the current mods:

COBB SF intake

Grimmspeed catted downpipe

Invidia q300 catback exhaust

NGK 1-step colder spark plugs

Cylinder 4 cooling mod

IAG AOS

Grimmspeed 3-port

Turbo smart external wastegate 40 mm (Comp-Gate40)

ID1050X injectors

AEM340 fuel pump

This is a well documented issue present on all 08-21 STI models.

While those learning to tune may not be aware, if you're paying someone to tune your car, they should absolutely be aware of this.

The issue cannot be resolved by tuning alone since it's a mechanical issue.

I worked with COBB's hard parts team to create the COBB fuel system which resolves the fuel pressure oscillation that causes the stumbling.

It must be used in its entirety to work properly. The FPR kit by itself is not a complete solution.

The complete setup is their lines, rails, FPR, and it must be installed as described in their instructions, including the correct FPR vacuum source relocation, which stock parts are removed, etc.

With that done though, and proper tuning which someone familiar with this issue will know how to do, the stumble will be gone.

The parts I'm referring to are all linked in this package:

https://www.cobbtuning.com/products/subaru-nexgen-stage-2-power-package-sti-2015-2018

You don't need the intake etc. to solve this, it's just the easiest way to link you to the fuel parts.

Thanks Mike, I was able to resolve the issue. I ended up with buying Radium adjustable FPR regulator kit and after installing it tuner made some adjustments and it is fixed now. Thanks for your help.

I'm glad all is well. You're welcome!