It has a miss at idle and driving.Check engine light on. code po 300 and po 307

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Number 7 cylinder plug was real black and burnt. rest were tan. And the cap and rotor was real corroded. Changed plugs cap and rotor. Cleared computer. Still got same miss and check engine light came back on with same codes. I can't believe the one injector for all 8 cylinders. Would make just number 7 misfire. And the plug wires were new last year. AC delco. Maybe a compression check on that cylinder.