This has been an ongoing issue. I have a 2008 Cobalt Sport and am constantly getting misfires. Occasionally, it will trigger the P0300, but most of the times it just constantly misfires @ idle and WOT!
I've replaced all the spark plugs (4 times), all of the coils, the MAF, Ive checked for vaccuum leaks more times than you can count on the intake manifold, the throttle body, the maf, any vaccuum lines; ive replaced the fuel filter, checked the fuel pressure, and have changed the injectors! The engine in it now isnt the original. It got shipped from LKQ as the last engine i had the timing went off and a piston smacked the valves. But according to them they compression test it before it leaves. Im inclined to believe them as they dont want any warranty claims.
So, to say the least it has been straight HELL with this car. But for a while there i was gut feeling it that it was from messed up fuel injectors. Well, before this engine i bought a supercharger kit and was waiting to install it. Well it came with new injectors. So to justify it i thought ill just put the kit in since it has new bigger injectors.
To make a long story short, i got my computer flashed to be able to run the supercharger kit. Ive changed the gaps on the plugs from .042, .040, .035 , .032, and .028 to eliminate the possibility of spark blow out. It misses at Idle ( I have an aeroforce scan gauge and it will be anywhere from 2 to 12 misfires at idle) and at WOT now with the CEL flashing.
I have been on this for months guys, MONTHS! and now my old lady is getting ready to get back to school and my car is still not on the road.