I have a 04 monte ls 3400 with a p300 code i cannot solve for the life of me!! I gave it a tune up, new coils, new ignition control module, new egr, new MAP, new 6th cyl injector which cleared the 306 code, & with the new injector the upper intake gasket was replaced..new fuel pressure regulator, i literally replaced the whole ignition system and it didnt fix !! The car doesnt turn over as quick as when i first got it...used to start with a flick of the key, now the other day the car stalled, started after 5 mins... Today it cranked but wouldnt turn...20 mins later it turned over n im still good. The car only misses at IDLE . My battery is fine and starter seems operable & no noticable alternator issues. On the scan tool every system reads OK . I never loose power while driving. What else causes this unsolvable misfire & new stall/turn over issue? Help please !!!

 

Check the plug wires for a white spot, spark is leaking to ground.

 

Have you cleaned the MAF sensor? Dirty throttle bore momentarily hanging up the butterfly at startup?

Edit: Vacuum leak at PCV or lines from throttle body to fpr and purge valve?