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

 

check for a heater hose leak above #3 and 4

 

Bigbluebox3,
Just went through the same issue. Replaced plug wires. Test your old plug wires in dark and spray lite water mist on them. Good luck