Starting off, sorry for my ignorance, I've tried combing through some forum posts but haven't found an answer to my specific issue...

I have a 64 nova with an ls1, standalone harness by speartech, stock 2004 ecu dyno tuned, driven 900 miles no problems. The other day the battery died after dropping my kid off at school. Had battery tender on overnight and full charge when i left the house. Drove maybe 15-20 minutes no issues and when i tried to restart, wouldn't even turn one lug. Jump started it after about 7 minutes of charging from other car, ran fine until i got about another 7 minutes down the road and it started popping and back firing and then just power cut off. towed it home, after checking both alternator and battery at local parts store the battery was toast (2 month old battery). Got the new battery and charged it up fully 13ish volts. Ignition on it only dropped to 12.98 volts, but when I hit start (push button) it fires up and dies all in one cycle. Maybe a half second and cuts off. I checked all fuses, I checked all connections from battery (in trunk) to alternator. OBD shows no DTC's on powertrain.

I'm guessing...there's a short to ECU or ECU is somehow blown from battery dying?? I'm not an electrical guru but have built this car from empty shell to here. So I'm about to rip out all the wiring and try and find some sort of ground issue.. Pulling my hair out since it ran just fine up to this point. Even cleaned the map sensor thinking it got oily from the engine cutting off and decel since I was on the freeway. Once again I'm at the point of conspiracy theory trying to sort it all out. It feels like an ecu issue with ignition but that's a stab in the dark. Any help would be appreciated.

 

Can you keep it running by giving it some throttle? One of my cars does this whenever the battery is disconnected for a long time, but if I keep it running, eventually it learns where the idle should be. Has done it for years, but once it's run for a bit it goes back to normal so I never bothered finding out why it does it.

 

I tried giving it a little throttle right at as I pushed start but it didn't do anything. I can try again when I get home from work. I have a remote battery cut off switch for anti theft since I don't have a keyed ignition. So my battery stays cutoff whenever I'm not in the vehicle.

 

That might be at least part of your problem. With the battery disconnected, the ECM forgets any self-learning.
I know when I've disconnected the battery for a while, it takes a couple of starts to figure out how to run.

 

It's just weird cuz I only drive it on the weekends so it stays in that shutoff state during the week. Problem started when it just died on the freeway. I'll play around with a few more attempts to start tonight.

 

Yep..My 99 build would not hold idle after disconnecting the battery. Best to keep it connected and on a battery tender.

 

Maybe I will rewire the cutoff switch to my ignition and have my computer hard wired to have constant power? Wouldn't that draw from the battery though all the time?

 

Check the pinout from the stock setup, but there should have been at least one direct connected (fused of course...) B+ wire to the computer as well as the ignition switched wires. The GM ECUs typically have multiple power wires for different purposes.

Based on your stated 2004 ECU on an LS1, I'm guessing a GTO sourced engine? Just used that as a starting point for a harness lookup on LT1swap: https://www.lt1swap.com/2004gto_pcm_pinouts.htm and the power schematic: https://www.lt1swap.com/pictures/Mod...&%20Ground.gif

Looking at that pinout, it would look like you'd want constant power to the blue connector pins 20 and 57. You could switch power on pin 19 (as that is where the stock ignition switch interrupted power.)

You need to verify this matches your harness and ECU though, as this is my guesses based on the info in your post.

 

ECM has a battery of its own to retain its memory, for the learning part of its job. Most of what it’s programmed to do is burned into a hard drive, but obviously is tunable. The small battery goes bad after a number of years, and the learning portion of the ECM has to re-learn how you drive. This Is very common with older ECM’ s. Usually takes 15-20 years for this battery to fail. Happened to me once with an older work van we had. As suggested, you may have to feather the gas pedal as soon as it fires, to get the ECM tables to figure out where it is. After like a short 3 minute drive, it will be back to normal. Just be ready to “catch” the vehicle with the gas pedal at the first stop sign or two. That’s usually when the vehicle will stumble the most.
For what it’s worth, as a disclaimer here…I’ve never opened up an ECM and put my eyes on this battery, but had a GM power train engineer tell me this info.

 

It actually turned out to be a pretty simple fix. Started with my intake to check components before getting under the dash and it ended up being a faulty MAF. Wasn't registering at all. Problem solved. Thanks for the help!