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I'd like to swap the old ej18 with a clean ej25 sohc I'm confident it will bolt right in I even have a new kevlar clutch for it but I'd like to know about the wiring harness will the ej18 harness work with the ej25 motor and ECU

(1993 impreza base model front wheel drive 5 speed)
 

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Biggest thing you will need to make sure of is the crank and cam trigger wheels are the same as the 1.8L. The early 2.5 SOHC should have the 6/7 setup. You can verify by checking how many teeth are on the crank timing cog. If it has 6 it will work but if it has way more then you need to swap. That is straight forward and is the same. The cam cog mounts different than the 1.8 so you will need to obtain the correct one. I dont know the correct PN off the top of my head.

I would use the 1.8 ECU and run the 2.5 as it seems to work without much issues. If need be you can run any kind of piggy back on the 1.8 ECU because it doesnt care about feed back like the new ECUs. I personally think the older ECUs drive ALOT better if only for that reason alone.

You can put the 1.8 harness on the 2.5 and make everything work. The idle air controller might be different. Some got 3 wire and others got 4 or 6 wire. The coil is different on the 2.5 as it has the ignitors integrated where the 1.8 has them seperate and mounted on the fire wall. Use the 1.8 coil and make need spark wires if needed.

I know there are several other people that have done this but that is all I can think of right now.
 
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