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Can someone provide guidance to me about the engine swap from a D15Z4 to a D16Y4 in my 1997 Honda Civic?

Can I use the old ECU with new engine?

Do I have to buy a new ECU with the new engine?

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Assuming that the ECUs are identical in pinouts and wiring you still have the issue of a 1.5L vs a 1.6L engine so the ECU programming would need addressed to allow for the size change to insure engine run at best performance.
Normally when I do an engine swap I buy the ECU from the donor vehicle along with the engine. I will also try to get the wire harness if needed for any changes to the location or extra hook ups.
If the ECU and harness is not available then you need to compare ECUs base numbers and if it is considered a suitable match have it reprogramed.

ECU ID

One other consideration is the transmission being same or different (manual or automatic)

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