I recently changed my plugs in my 2008 gt 4.6. Broke the plug on the rear drivers side. Got that out and when I started it had a bad idle. Took it to ford and they said it was catalytic converters. Replaced the converters and still misses. Checked fuel pressure it's around 30 to 35 lbs. Checked coil packs with old spark plug and they fire. Used a sthescope on each fuel injector and can hear them click. Can't get a code to read on 3 different plug in computer. Took it back to ford and they said it was mufflers. Took flow masters off to make sure they wasn't any blockage. My new spark plugs have black carbon on them.