L76 starting problem , cranks but won't start
 

This issue is with a L76/6L80E swapped into a 69 Camaro . Drivetrain has been up an running for a year or so with no issues with not starting . Car has been sitting while finishing up on other components , battery has gone down a couple of times and had to be charged. A couple of weeks ago attempted to start it , engine turned over fine , noticed voltage was down a little , but no slow cranking or starter drag , engine just turned over and over without starting . This continued through the day with no change , I suspected a fuel pump issue but was by my self and frustrated so I left it alone for the day . Showed up the next day with help and determined that the fuel pump relay and fuel pump were both functioning although I did not check fuel pressure , pump was cycling on for 1 second or so . I continued to try and start with the same results , put battery charger on the battery for a while but still no start , just crank over and over . Finally tried a jump box , same deal , no start . Next tried a big charger with a 40 amp start feature , instant start and ran perfectly ! Let it run for a few moments , killed it , cranked it back up , no issues . ?????? Fast forward 2 weeks , car had been sitting and owner tried to crank it for a friend to hear .... no start . He called me , told me issue , I told him to put the big charger on 40 amp start setting on it , he argued that it was turning over fine so it must be a fuel issue . Owner ended up just leaving it alone and I went to the car , tried to crank it , no go , so put the 40 amp start on it and it cranked instantly .....again , and continued to start afterwards .

Alright guys , WTH is going on ? Does the ECM need to see a certain minimum voltage before it will fire ? It's been a learning curve but this one has me kinda stumped .

It means the battery is not holding enough charge to properly operate the computer, even though it turns the engine over. Looks like the battery is getting weak.

Is there a certain voltage that it needs to see before it fires ? That's seems to me to be the best reason for it to act like it is . The battery is 3 years old or so and has spent the lion's share of it's life sitting and slowly discharging . Think that the best move at this point would be to replace the battery , and I really don't trust it at this point . Thanks !

At least 12 Volts