Carbon tracks down the side of the plugs will make them arc to the head . It will look like a balck dust on the plug porcelin .
The race then idle rough sounds like a vaccum leak . Listen for a hiss sound around the throttle body or the intake manifold . There are many things run by vaccum on the rangers as well that could have a leak .You can pin point it by using a can of WD 40 . It will seal the hole for a short time returning it to a regular idle . But be careful as it is flamable don't squirt it onto a hot manifold .
Some can learn by others mistakes , others have to whizz on the electric fence for themslves .