**Just check the cap and rotor next time 1st thing. Save yourself some trouble.
*Actually upon further experimenting, it's doing it when cold in open loop too

Been awhile since I'v posted here. Was looking through the other thread on this too. The car was running fine with no problems and suddenly as I stomped on it getting on the freeway it started stumbling and backfired. I limped to the next exit and checked things in a parking lot. Found nothing unhooked or loose. Started down the street and suddenly it ran fine again for a few seconds, then got bad again. Ran almost normal doing 50 all the way home as long as I didn't accelerate to fast from the occasional stop.

Runs fine when cold, but once warmed up, it starts running rough.
No codes.
Fuel pressure good.
MAF and O2 I'm pretty certain would throw a code.
I don't remember if a temp sensor for the ECM will throw a code or not when those go bad.
I'm going to ohm out the injectors here in a bit (should be 12 iirc), but what else am I forgetting that can cause issues in closed loop mode and not throw a code?

Do you have the stock injectors? When you check the resistance make sure you check them when they are hot and cold. They're prone to coil failure especially when they are hot

They're Accel injectors I got when did the engine swap. I didn't test them yet. Need to come up with a way to access them, or I'll just have to take the upper plenum off really quick. Worked on the other two cars to get them drivable. I did test how it acted when it went into closed loop mode, and saw no emmediate change. It has to get warmer yet before it starts acting up.

Putting in a new temp sender today.

Well, it's doing now when cold too before going into closed loop. I'll have to modify my list of possible causes.