A little back ground on the set up. Swapped a stock 5.3 into my fairmont with a used swap specialties harness, and a 0411 tuned by a guy from a forum I'm on. Had flashing check engine light while cruising. Codes was p0300. Replaced plug wires and o2s with oem units. Turned out to be a bad trans causing a bad vibration. Took car apart and sold the trans.

Fast forward a few months ago and purchased a rebuilt trans with a stock converter. Took the car out last Thursday and the car ran amazing. Tons of power, no vibration and no CEL. Got gas that night and drove home. Two days later the car started again with the flashing check engine and p0300 code. Would also randomly throw B1S1 and B2S1 low voltage codes and even a p0507 for high rpm IAC. Used brake clean to check for vacuum leaks and didn't find any. Checked my coil grounds and thy're tight and clean. Pulled intake and cleaned gaskets tightened intake in the correct pattern. Now i'm getting p0304 cylinder 4 mis and a p0354 cylinder 4 coil code and the car runs terrible. Ive tried unplugging the coil and injector and get a noticeable miss. Im at a loss how the car ran amazing one day and now it runs like garbage.

 

Update.

Changed the plugs back to the original oe plugs, fixed a pre o2 exhaust leak, and got some fresh gas. Car is close to running like it did the first day out. Took it for a twenty/thirty minute ride last night, and still have a persistent flashing p0300 while cruising at 2000ish rpms. Each time i cleared the code it took longer and longer to reset, but kept coming back. As i pulled into my drive way it also threw the p0151 code. I'm going to build a smoke machine and check intake for leaks.

 

What kind of scanner do you have? HPTuners access?

Can your scanner tell your the current LTFTs for bank 1 and bank 2? and STFTs while car is running?

Is your alternator charging the battery to 13+ volts while engine is running?