I feel like i've been thru it all but i havent fixed the problem. Its a 5.4 three valve and during hard excel anywhere from idol to sift point the valves start to rattleon #8clyinder and thecomputer shuts down that cylinder. the check engine light does come on but flashes and goes back out.I have changed the plugs,coils,cam phasers,cam phaser sensors,fuel pressure check,oil pressure check,injector swap and induction cleaning and problem continues.during the rattle there is a tramendis lose of power.I've also done compression and leak down tests and results are within range.Any advice?

 

Possibly valve float. Even though the #8 is code fault flagged, check the before/after #8 fire ordered cylinders for a mifire issue or base engine fault.. Replacement plugs should be the latest level released PZT-1F-F4 or equivalentper the TSB06-3-3..