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Is the coil internal to the distributor? You should be able to check the resistance of the terminals of the ignition coil to see whether it’s out of spec.

 

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The fuel filter might be clogged too.

 

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The injectors may not be matched to the ECU. What ECU are you using and is it tuned?

 

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I’m pretty sure a stock P28 isn’t a good fit for a H23a unless it’s tuned.

No idea how the car ran well before with the injector box, but if it really did, it might be a good idea to reinstall it.

 

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You’re right about the main relay’s failure mode.

If you want to test the integrity of the ground, you could temporarily make an additional ground with a jumper cable. Connect one end to the battery and the other end onto the engine, then see if things improve.

 

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You’re methodical about this. I know you’ll be able to sort this out properly.

 

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Way to go man. I knew you could figure it out. Thanks for sharing your resolution!