OK I have a mis-fire. #4 CLY. for 2years. change the plug, ok for 200-300 mls. Last time the new plug didnt help, but the new coil did. This fix was also good for an 20mls. per tank increase. But it was short lived 2800mls. #4-#8 mis-fire. fixed #4 with new plug... But #8 no comp. If I put air to the cyl. I hear air at the throttle-body. If I turn the crank to open intake-vavle, the air is stronger at the throttle-body,like it should be, turn the crank to check exhaust,air is strong at the tailpipe like it should be, but still faint at throttle-body.Sonds like intake vale is stuck open, or wedged open, or ???? I dont think that the vavlespring is at falt, 'cause the air I put in the cyl. should have closed it. Anyone ever seen a rocker break and cause this???

 

According to what you have stated you have done with your truck it seems to me that your fuel injectors might be the culprit. If a fuel injector fails to open and spray as it should it will create a cylinder misfire, even with new plug boots,coil, an spark plug. I would suggest swapping the faulty cylinder injector to a different piston that shows no fault to see if the misfire moves to the swapped cylinder.