I have a 96 civic with a 95 motor I swapped in.. Everything was running perfectly fine. Started OK and drove good.. but when I went home from work to start it up . It just kept cranking but won't turn over.. I checked the spark it was fine. I can hear I fuel pump turn on.. so don't think it's the relay. BrAND NEW BATTERy and used a different ECM.. don't know what's wrong all of the sudden. I have strong spark when I checked it. HELP PLEASE all my other friends just tells me buy a new bottom block because the piston, which I doubt.. it should still just start right? HELP ME

Check the error codes first. Also check the main relay.

Perform tests, in Sections A and B, below:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563.html

Nope I here fuel pumping.. so it can't be the main relay... and I have a lot of spark on every spark plug wire so I don't think it has to do anything the ignition... any thing else

. Check each cylinder's compression.
. Reset ECM.
. Clean main ECM ground on thermostat housing.
. Clean MAP Sensor connector of any corrosion.
. Distributor may be bad--you can have spark, but at the wrong time, if CKP, CYL, or TDC sensors are damaged.

Remember, 90%+ of all "crank but won't start situations" are electrical-related, not fuel.

All my sensors are still good.. I think I have no compression.. think my bottom block is gone you think so? That's what everyone is saying to me