What will cause #8cylender not to fire at all on a ford f150

If your getting a no spark condition at the sparkplug the I would look at the ignition coil, try swapping the number 8 coil with a different cylinder such as 3. If the no spark moves to cylinder 3 then replace the faulty ignition coil. If cylinder 8 still has no spark the I'd look at fuses, and the wireing between the coil and pcm (power train control module) since the pcm sends the signal to the coil to fire it. Check to see if the sparkplug is fouled or properly gapped. If it's due to no fuel then I'd check the fuel injector, swap it just like the ignition coil test I described above. Check the wireing for the injector using a noid light, if the light pulses while cranking the engine the voltage is getting to the injector. I could also be caused by not enough air wich could be due to a worn camshaft lobe or lobes not allowing the cylinder to draw in enough air to properly mix the air fuel mixture which should be 14.7-1 (14.7 parts air to 1 part fuel). Check to see if you have good compression on that cylinder. If you don't it could be caused by faulty piston compression rings. Hope this helps.