1996 5.7 that has been put in a hot rod truck by another individual with the original wiring harness cut and spliced to fit into the truck. The truck will crank and crank until the battery goes dead an wont ever fire and run until you give it a breath of ether the it starts and runs great. Shut the truck off and restart it no problem with no aid. Wait a couple hours and it wont start again unless you give it aid. I have gone thru everything I know and tested fuel pressure, spark, air. Its has a new opti spark, plug, wires and like I said it starts with aid and then will restart everything you shut it off unless you let it sit for an extended period of time. I have verified power to the ecm of constant power and key power and have my 5V ref. but Modis wont communicate with just the key on. It will communicate while the truck is running though. Its like it loses its memory until you force it to start and then it comes back to life. I am pulling my hair out has anyone ever run into a similar situation or have any advice? PLEASE HELP ME!!!

Thanks Terry

 

Did this '96 ECM come out of a F-body or Corvette. If it was a Corvette the stock calibration will be looking for a CCM handshake(which only the Corvette uses) in order to fire up.
Might be something to consider

 

Thanks for the reply,
using a snap on MODIS, ECM is showing the serial number of a 1996 Pontiac Firebird, best we can tell someone has disabled the security in it because like we were saying it starts and runs great after you start it the first time with a breath of ether, then it will start everytime after that till you let it sit over night then will not start.

 

perhaps this site may give you some help
https://lt1swap.com/all_lt1.htm