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Got it figured out this weekend. Quick recap of the symptoms...runs rough with an obvious miss at idle, runs okay at mid to full throttle, P0300 Random misfire codes.
It seemed to run okay after a cold start and running in open loop (default sensor values) and the miss only started after warming up and going into closed loop (starts reading O2 values and such). Did another cylinder balance test by disconnecting the coil pack at each cylinder. Disconnecting each of the drivers side made it run worse so they were working, but none of the passenger side bank seemed to make any difference. Watched the O2 sensor on the passenger bank and it wasn't responding the same as the driver's bank. Pulled the passenger side plugs and they were all black and carboned up by running rich.
Basically the passenger side O2 was giving a false lean signal and thus dumping a bunch of fuel which fouled out the plugs at idle, but again no O2 sensor code. Swapped in a new O2 sensor on that side and went ahead and put a new set of plugs in and now runs fine. Plan on swapping out the drivers side O2 also but the parts store only had one in stock.
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