First time here,
Bought a used 2003 F150, 4.6 with 87,000 miles on it, ran great until 92,000 miles. Service light came on, took it my local Auto Zone to read codes, it showed P0303 code, number 3 cyl. misfire. I moved the coil to the number one cyl. to check and see if coil ok, reset light. 130 miles later it came on again, still showing number 3 code. I replaced the plugs all eight, with motocraft Plat's., checked the gap all set at .54 also cleaning all connectors and coils, checking for corosion. After completing replacement of plugs and starting the truck the light did not come on, so great problem solved. The truck still seemed to have lag when I stepped on the gas but other wise seemed ok. 106 miles later the light came on again, re-checked the code's and sure enough the number 3 cyl. code showed up. MPG is at 8.6 but running smooth, every once in a while still has bump when ideling. I doubled check to see if I connected hose and wire's up, all ok. No smoke, or fumes seem to be happeneing.

Thanks for any help,
Bigbluebox3

 

When you swapped the coil, did you also swap the spark plug boot? My mother-in-laws 2002 Crown Vic had a mis-fire that turned out to be a pin hole in the spark plug boot (tiny white spot on the boot's side wall) that allowed the plug to arc to the head in the plug well. Bought a set of plugs and boots from Advance Auto and problem is gone. Check your plug boots for small white spots.
All I can come up with.
Good luck.
Let us know what you find.

 

Swapped boot and all, but heres the strange thing again, I drove the truck and turned it off came back hour later restarted and the light was out. I did nothing to it. Strange, almost makes me think theres a short somewhere.

 

If you swapped the coil and changed the plugs, but are still getting an intermittent mis-fire with the same cylinder, maybe checking the wiring / plug that connects to the coil could be the next step. If the connector is broken, or a wire at the connector is broken, (sometimes making enough of a connection to get the signal through) it would explain what you've experienced. Other than that, I'm at a loss for suggestions. If it weren't electrical, I'd suggest SeaFoam poured into the Brake Booster vacuum line while running to clean the carbon build-up from the backs of the valves for improved flow and sealing of combustion chamber, but the trouble code doesn't suggest that.
Good luck and keep us posted.