2004 F150 6 cyl - engine misses

A fouled or worn plug (spark gap too big) can do that. Also a bad ignition wire can do that. These are especially suspect if it is a particular cylinder that is missing. You may be able to ID it if you get a check engine light and the code is misfire cyl no. 3 or something like that. If it gets worse in rainy / damp weather, suspect a coil pack, spark plug boot or spark plug wire getting wet. If it is not a specific cylinder it may be a clogged fuel filter causing insufficient fuel pressure during high demand such as acceleration or pulling a hill.

Wet with fuel means no spark. If you suspect the coil,swap it with one from another cyl and see if the miss migrates to that cyl. Possible you have a defective plug too. If you have a "noid" light tester, check if you have injector pulse.

wow no year , impossible to help really , save generics.
what engine, name it, what country, some countries have other engines and other EFI controls.?
is spark good to ALL cylinders ,not just one?
if yes, are spark plugs relics or fresh, and are they dry or soaking wet (tips) with fuel>?
getting fuel means zero if where is not said, spark tips?
it only takes 3 things. to start. most skip compression , id never do.
but yes, a bad engine WILL not start.
150PSI on all cylinder W.O.T , not 80, if
good spark on all cyl. and timed to spec, "timing light time)
last is fuel ,never 1st or second.
last , is sparks are wet its flooding, (lots of tests ask)
last if spark tips are dry , then introduce test fuel
run of 3 second and stall now, (ask for how to preceed.)
all this is called BASICS. we can peel the onion as deep as you want
ask.

is #8 the first or last injector on the fuel rail ? I have heard of fuel rails getting partially plugged or not enough fuel pressure to make it to last cylinder in rail. have you pulled valve cover and watched the rockers , or cam finger followers to see if they move the same as the ones next to them, and also you might still have the miss at higher rpms but the eng balances it out and is not as noticable. check your electrical connections at injector and coil to make sure they are not spread, to where they are only making partial contact this is common problem too. is the spark plug from that cylinder any different color from the rest, example Black sooty, clean, wet, white ,etc; could be lots of things, cam lobe,coolant leak in that cyl.,elect.con.,ecm,bad grd. wire to coil,spread terminal, sounds like you are on the right track,keep plugging along you will find it. good luck

if you've changed the spark plug and no difference, then it's time to swap the coil with another cyl. coil and see if the miss goes to the other cylinder, if it does'nt follow to other cylinder, then I would replace that injector !

Pull the vac hose going to fuel pressure regulator and see if it has fuel in it. If it does replace it. Just fixed one with multiple misfire codes.

You have coil over spark plug ignition and one of your coils is bad.You need to find the bad coil and replace it.

sounds from afar like a problem i had after installing after market spark plug cables on a diff. car. make sure you have factory-quality/ even mits. brand cables. (right parts vs. after mkt.?- no contest.)

Most likely needs new coil pack for cyls 3 & 4.They fire together.

Matter not whether plug fires out of engine. Put it back in and clamp a timing ligh to it. If the light won't fire, you don't have enough spark to fire cylinder, hense miss.