Q&A

 Master

When you say it "misfires at idle every 15 seconds", do you mean that there's kind of a "catch" in the idle? A misfire is more of a constant rough running, on less than your 6 cylinders.

If there is a catch every 15-30 second, I have seen in the past where the idle is affected by a fault in the PSOM (programmable speedometer odometer module). Seems odd to imagine, but that unit is tied into the engine processor operation. Instrument clusters are obsolete from Ford, so if this ends up being the failure (have the truck tested!!!), you will have to seek out a used unit.