So far, I have replaced the following things after getting two p00300 and a p00400 code. The car was running rough and the engine was surging at an idle. After replacing the Fuel pump it backfired through the upper plenum gasket.
Fuel filter
Fuel pump
Plugs and wires
Coils
MAF
had the catalytic converter checked, very clean.
Fuel injectors
Throttle body and intake gasket(plenum gasket)
After fixing all of this the idle is still a little rough, when it's put into gear, it skips and bogs down. If I leave it at an idle now it eventually dies. At least there is no backfire through the intake now, but I dont want to run it anymore to avoid anymore problems. I know my next step is o2 sensors and possibly a bad fuel pressure sensor. But after each of these problems being fixed, and still getting just the misfire code I am beyond my wits and need some guidance or wisdom. If anyone has had similar issues, or if they can confirm the possibility of a bad battery or any other possible cause it would be greatly appreciated.
Get a new EGR valve.
Thank you for the help, I will keep you posted
I had a "pro" che k it out at the buick dealership. They told me that the fuel injectors were bad. That was the third thing I replaced. Misfire can be a tricky code for any mechanic. So far I've listened to two professionals one being a family member and got the wrong answer twice. Thanks for the advice though, it was nice.