after 20 to 30 miles my truck picks up a bad miss at lowend and overdrivw on 1999 Ford F-150

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missig bad after 20 miles or so a small miss otherwise only code evap leak but has been there before miss started
(3) Answers
Also you can have a cylinder contribution test performed with a scanner, most likely an ignition coil or coils failing.
From the description it sounds like you have a type-a ignition misfire, the threshold for setting dtcs on older fords is broader so they won't set a code as fast as a newer ford. You'll need to take it to a shop that has a power balance test capability that will show exactly which cylinder is misfiring to get an accurate diag.
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