Having issues with 1998, 454l all sudden initial start issue, turn ignition and no 5sec fuel pump, pump is good, new relays, jumped pump and no start. Put new ic module, pump started as should and fired right up, sat for a bit came back and turned ignition , no pump? The harnes is no longer available. Just sent out the mifi 2 to get checked out. Question is, want to drop a 454 mag in, can get the engine with harness. Would it be a plug n play with the harness plug? Or going carb, efi is great when working, carb never left me at the dock.

should be plug and play.

Since it works and then it doesn't its an electrical problem. Not out of the realm of possibilities, but don't think its the ECM, I would check at the connections from the starter post to the key and back to the motor.

Starter post
50 amp breaker
10 pin connector
key switch
kill switch
10 pin connector
Coil, fuse main power relay
Fuse, fuel pump relay

And don't forget the pins on the ECM

Assuming when this happens and pump does not turn ON, the motor will still crank?

Yes motor cranks, new ignition switch, checked all pins, j1 n j2 ECM. Getting pwr, to j1 on the 3 pins manual says. When hook up computer at obd no communication thru ECM. Un plug ecm n plug back some times get communication, that's why leaning to ecm? Not getting 5amps from ecm to obd. Previous owner said was a fresh water boat, beg to differ after digging into wiring.

My info list the MEFI 2 (16210729) was used on the 350 mag and 7.4 L29
Being on a 502 someone has changed things and wonder if it was retuned?

It's in a 454 l29 1998., Should have backed away from beginning, bought from owner that had it for a week, said beam was to narrow for what he wanted, n said engine only had 20 hrs, previous owner told him marina didn't winterize correct, n put a new long block in it. Looked good till got into bilge, freaking hack job!!!!

Dang my bad, don't know where I got the 502 thing