I just hit 90,000 miles on my 99 f150 with the 4.6. I figured that I would go ahead and change the plugs. The ones I pulled out were MC single plat. gapped at .070 insted of .054. I put in Autolite double plat. and gapped to .054. I bought the truck with 60,000 miles so the plugs were changed sometime before 60,000. Any idea why someone would change out the factory plugs so soon and gap them incorrectly? Maybe they just screwed up and didn't check the gap before installation. Or is there a reason for gapping so big? It was running ok before and ok now. Although now I feel something different through the accelerator pedal...not really vibration, but just a grrrrrrr...can't hear it just feel it through the pedal. Any ideas?

 

As plugs are used, the spark slightly wears the electrode and also pushes the metal prong on the plug away from the electrode....with regards to the grrrr, check the plug wires to make sure they are all very tight with the connections, highly recommend using dielectric grease on boots and electrical connection points.