Cylinder 4 misfire
Then the coil pack is not the problem...Possible causes; Faulty spark plug 4, Clogged or faulty fuel injector 4, Faulty ignition coil 4, Fuel injector 4 harness is open or shorted, Fuel injector 4 circuit poor electrical connection, Ignition coil 4 harness is open or shorted, Ignition coil 4 circuit poor electrical connection, Insufficient cylinder 4 compression, Incorrect fuel pressure, Intake air leak.
I've has the same problem, two coil packs, one injector, three spark plugs. Ended up being a dirty fuel filter (believe it or not). If you look at the fuel rail on the entire 4.6 motor, the line make a strange little turn to go to #4 Injector. I'm guessing that #4 has the most restricted path for fuel. As soon as I changed the filter the truck ran like a Swiss Watch. (If you look above, incorrect fuel pressure, was the problem, but it just effected #4 cylinder first.
I had the same problem with a 2008 (old heads) and the fix may be free if you have any CRC Contact Cleaner. I took off the coil and sprayed the cavity and the connectors. I think the act of "forced nozzle" spraying over and over, may or may not have been overkill but I did not want to do it twice. Truly amazing that engine runs perfect after that. I came within an inch of buying a fuel filter and that special 'line disconnect" tool. My symptom was: P0304 misfire on increased load where downshift did not occur. For some odd reason it misfires then most often. It makes sense that it is fuel availability related but I think it is a frequent misfire that is more "pronounced" with the increase load, uphill, etc. where the code reaches the threshold to be captured. Also, this is the reason gas mileage was poorer overall regardless of loads. My gas mileage has already gone back up after a few miles. I am still scratching my head.
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