Hey just put a CA18DET in my '93 240sx Vert. It was running fine then after sitting (just overnight) it wouldn't start. Turns out it was flooding. Pulled the fuel pump fuse and started up. If you put the fuse in after it starts and runs it stays running fine. But once you stop the car it floods again and you can't start it. A couple of times it completely lost fuel pressure while running.

Have you checked for codes? It kind of sound like maybe a bad FPR. Can you check fuel pressures? What size fuel pump is in it?

Is that the engine from the s12? FPR takes vacuum, maybe you have a leak from swapping? Did you ground the engine block? I'm just wondering if that would affect the injector pulses. Oh, and fuel return line and evap lines would need to be in correct order and could have been jumbled during a swap...