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Old 06-25-2019, 11:57 AM   #1
Scooby Newbie
Member#: 348886
Join Date: Mar 2013
Chapter/Region: MWSOC
Location: Louisville, KY
2004 Subaru FXT

Question Replacing ECU ID in 04 FXT and running as 04 STI with AP

Hello all.

Currently working on a 2004 USDM FXT build. The specs are not particularly important to this discussion, but the rough cut is an HTA 86, PPG trans, and a closed deck stroker.

I am excited to try out the flex fuel tuning features and really wanted to utilize custom sensor logging via Accessport.

I previously tuned the 04 FXT ECU open source, and modified the electronic ECU ID by flashing a 2004 STI ROM to the 2004 FXT ECU to get easier access to previously identified definitions for some tricky tables and logging parameters.

When switching to accessport, i realized that the current 2019 revision of accesstuner does not enable these features on the 2004 FXT platform (CCF, including custom sensor logging, flex fuel, and others). I was disappointed, as the reason i switched to COBB was for the simplicity of not having to reinvent the wheel or install odd workarounds in OS every time i hit an undefined area or wanted greater ease of functionality. COBB is just easier, in so many ways.

I contacted COBB about my scenario, and Bill informed me (thanks Bill!) that it is unlikely that the 04 FXT will receive these features at any point (though the 05 FXT and 04 STI have them enabled)- and of course mentioned the path that some have taken- swapping the 04 STI ECU into the 04 FXT (as they are plug and play), then treating the car as an STI from the AP perspective and moving forward. To note, this is an unsupported path and was not suggested by COBB, only noted that this is what some have done, and i was warned sincerely to be careful regardless.

I purchased a used 04 STI ECU to set aside in case i completely ruin this FXT one, but decided to move forward by keeping my re-imaged FXT ECU and installing the 04 STI accessport software into my electronically swapped environment to see what happens. If it works without a hitch all the way to tuning out my HTA86 in SD, then this could potentially open a door for other 04 FXT owners to access otherwise unavailable COBB features.

The 04 FXT ECU, which thinks it is an 04 STI ECU, now has the 04 STI accessport installed with COBB custom features activated. There were no errors during install, and so far it appears to be reacting appropriately to my inputs in Accesstuner.

I am in no way suggesting that anyone else do this! I have an extra ECU handy and am performing this testing on an engine that i plan to throw in the trash in short order in favor of my new built swap, so all potentially affected parts will be cycled out anyway, or can be.

My question is, has anyone else tried this method instead of a full ECU swap? If so, did any quirks or problems come up throughout the process that i should watch out for? Does anyone have any notes to mention regarding actual physical or memory differences between the two ECU's that might potentially leave me stranded when trying to enable certain features or actions?

Best case, id like to test and figure out the quirks while my engine is a throwaway, then transition to the new engine with confidence that my ECU is reacting appropriately everywhere that matters.

In open source, the FXT ECU has no problems running STI roms, as the 32-bit base is the same, definitions and addresses are just different. Memory size between FXT and STI roms typically reads within bytes of one another, and all pinouts match physically on the ECU. Just trying to get ahead of anything that might come up, unless i'm the lone wolf trying this out...

thoughts anyone?


Last edited by Louisvillewagonmafia; 06-25-2019 at 12:23 PM. Reason: corrected title
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