Have a 2005 f150 5.4 lariat and it started shaking and not going over 20 miles, had to turn it off and on then it worked fine got home then turn it on later that day and it started to shake more then got check engine with code 0308 misfire cylinder 8. I had an extra coil and spark plug change them and nothing change. Any clues what it could be mechanic came by he check coil and injector both good he says it might be its not getting compression on that cylinder. Any ideas would help. Thanks in advance leave south of texas if anyone knows a good mechanic

 

If you know the coil and plug is good then Testing compression on that cylinder would be your next step. My old engine had a misfire on 3 that ended up being a seized cam follower that wiped out the cam lobe.