jesseelias

i have a 1999 4.2 f150. i recently rebuilt the engine myself. ive only drove it about 2 miles. the check engine light came on and gave me p0171. i drove it again and now the check engine light started flashing. there is a big lack of power mostly in the mid range rpms or when i give it full throttle. once when i started it up it gave one quick backfire but that was the only time it backfired. what do i need to do to take care of this problem (flashing CEL and lack of power)