what makes engine mifire at all speeds on 1996 Ford F-150

engine has a slight miss every 15 seconds at idle. at all driving speeds the engine misfires badly... changed plugs and fuel filter and added lucas injector additive no help .. the plugs were light gray and burning clean .. no check engine lights ... any ideas ?

Asked by for the 1996 Ford F-150
i would continue and replace the cap, rotor and wires. cover all the basics before going somewhere else. if this does not work, get a diag from a shop with a bid.

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Thanks Roy, I will do that .. i have dual gas tanks on truck and a co-worker had the same problem and he told me to check cat converter as switching between tanks can clog converter !! I have never heard of that .. I will change all tune up parts before I get into that..
your welcome
1 more answer
if you have two tanks replace the fuel tank switch on the dash fixed mine 13.99