Well guys after weeks of combing through wiring diagrams and creating endless spreadsheets, I believe I have found a way to convert your 99 EJ25 harness to accept a EJ222, or EJ222 top end. Instead of pulling the whole harness out from behind the dash and replacing it with the EJ222 harness, I am working on converting the harness with very minimal changes to the EJ25 harness. The harness, for the most part, is 75% the same. There are 19 small changes that need to be made to make it work. Best part about it is, I am working on a plug-in conversion kit so you can go back and forth between a EJ25 and EJ222 ECU and electronics with no further modification.
There are a couple of issues that I am trying to work out on the OBD connector (Every Subaru I've seen has a pin 10 that is linked directly to the ECU, except the EJ222, which has pin 8 and 10 and I'm trying to find documentation on what pin 8 does).
Also, I am trying to source a EJ25D ECU Header for nearly plug-n-play setup of the system. So far, there are only 4 wires that need to be run from the EJ22 ECU to the engine bay.
More details to follow!
Disclaimer:
The reason I am doing this is because I got a mid-90's EJ25 bottom end for free and have a perfectly good EJ222. I am combining the EJ25 bottom end and EJ222 top end for a higher compression engine that is ECU friendly. The downside is that you must use the EJ222 ECU and engine harness. Those of you that know about the 99 know it is a rare breed in terms of electronics.
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