Flex Fuel Set Up

Question on Flex Fuel;

Can it be run through ROMraider/Carberry for my USDM 04 STi by repurposing the TGV solenoid control units?

Or would I need a seperate stand alone ECU to run flex?

*ECU Flash, ROMraider, Carberry, Flexfuel sensor connected through repurposed TGV solenoid wires?

The flex sensor is just a way to understand the fuel composition, it can't actually make the car flex fuel. The tune needs to be modified on the fly to allow this.

The factory ECU especially of that era will not really care about fuel type as its assumed so it usually isn't changeable in a factory ECU of that generation.

So if you want to run the car permanently on say E85, then you could retune the ECU to deal with that, but that isn't flex. Its just retune to allow the car to use e85 100% of the time, but it then wouldn't be able to run normal petrol.

Flex requires something to alter the amount of fuel being delivered to deal with the lower energy density of e85 relative to normal premium petrol, and it needs to do this dynamically as if you had 10 litres of normal petrol, then top up the tank with e85. Its not running e85, it might be e70, and thats assuming the e85 is actually 85% ethanol which it often isn't' from the pump, it can be as low as e60 or 65 i beleive and this changes during the year to help with cold staring in winter. e85 is just the max it can be, no more than 85%, but can be less.

I'm not aware, nor have i heard of anybody being able to do this to a factory ECU without some other aftermarket gizmo like COBB to do it

Things like COBB access port can do this and a few others as they can make corrections to the tune based on the flex fuel sensor feedback on the percentage of ethanol in the fuel. These can integrate in with the factory ECU to achieve this, its not perfect but does work.

Problem is, COBB last year discontinued these flex kits in the US

Aftermarket ECU's like Link, Haltech etc do this far more easily out of the box and can offer far more adjustment for everything from boost, timing, and many other things on how the car runs all on the fly.

I believe Ecutek offer flex fuel via an input on their flashes, but not 100% sure how it works. Might be CAN based. Worth looking into though

COBB reintroduced flex fuel for 08-21 STI so far with plans to extend coverage to additional Subaru models as time allows. They had to redo the whole system to make it compliant by getting ethanol content and fuel pressure data to the ECU without replacing TGV position data. This was a large technical challenge that takes time.

Ecutek offers flex fuel via CAN input if you have a CAN based flex system you can integrate with it.

Many standalones offer flex fuel as well.

There may be an open source option for your vehicle, but that's not something I am familiar with.