ECU issue

So I have a 02 2wd manual with the 2.4. I hit a deer a few months ago and pulled the motor and transmission to fix the body damage. After I got the motor back in the truck starts and runs but has intermittent idle issues where it'll lose all spark and my dash will shut off and the coolant gauge will peg high and the truck dies. I've checked all fuses and grounds and it all checks out clean and have a brand new battery. This problem doesn't occur on initial start but until after about 5-10 secondsof idle. I do have a p0171 lean code which the truck had since I bought itand hasn't really caused issues before. I'm at a loss on where I should even start.

 

I know u said P0171 on b4 deer and engine removal
But a good list of things to start with

 

I encountered this ‘starts NP and then dies’
It was a timing issue
(I put my distributor in 1 tooth off after changing D. O-ring)

 

Definitely sounds electrical. Bad ground(s) would be my first guess.

Do you have a multimeter? What is the voltage at the battery while this is occurring?

 

This truck is coil on plug

 

It's staying around 14v while it's running and drops to about 12.8v after it dies. No hard spikes or drops in voltage whatsoever.

 

The o2 sensor is the only thing it could be at this point I've changed or tested everything else and have been chasing that code for 2 years.

 

Can anyone tell me where all the grounds are in the engine bay I'm thinking I might have lost one in the collision. I have the one on the back of the head, the one on the inner intake manifold, the one on the block by the starter, and the one to the fender next to the battery I can't remember if there are any others.