Info: 99 XJ had a 4.0 originally, now has rebuilt 4.0 out of a 04 Grand Cherokee, 99 wiring still. Ran good for a few months. started crank no start sometimes, but will start other times like there was no problems at all. Generally will start after sitting for a long time (several hours) but if i go to the store and come back out it wont start. (issue somewhat resolved, unknown cause of resolution) now is occasionally running really rough at start, until given throttle then clears up for about 10 minutes. Afterward is super hesitant and doesn't wanna do anything, running rough.
Fuel and oil pressure is good, spark is blue, good compression.
Brand new: distributor cap and rotor, TPS sensor, crank sensor, fuel pump, injectors, fuel rail, MAP sensor, O2 sensor, Idle control valve, water pump, all new gaskets for intake and exhaust, oil pressure sensor, coil, spark plugs and wires.

Really confused as to what it could be, I've tried pretty much everything i can think of. Looking through here a lot of people are saying TPS and CPS sensors, fuel pump as well but changing those had no effect.

 

trouble codes?

 

Check engine light was saying o2 sensor was bad even though it was brand new, did some looking. apparently they like to fry O2 sensors sometimes. seems to be running alright right now, after putting a new OEM. Will update if anything further happens.

Update 12/28/23: ran really good for about a day then started acting up again. still better than it was but not much better than the main post. went over the renix tips and nothing really changed. I talked to an old mopar only garage and they said they havent seen anything like it before so im really stumped.

 

Renix ? Check Cruiser54's tips.

The engine harness multi pin plug through the firewall bit me a couple times. Cleaning it once a year isn't a bad idea.