Good, honest answer, thank you. My 2000 Explorer started missing about 4,000 miles ago. Hole number six plug top porcelain cracked top to bottom. Replaced all plugs with Bosch iridium (sp) (trusted pre-set gap). Also replaced plug wires while I was at it. Ran good for a couple thousand miles then started missing again - number four hole. Nearly new Bosch plug was missing 1/2 of the bottom porcelain (around the stinger). Clean vertical split, 1/2 gone. Figured manufacture defect and replaced it. Still missed in number four, all plugs looked good so I replaced the coil pack. Still missing number four. Did a compression check. #4, 60#. Bad. Had the heads (and a few other things) replaced- (both heads were cracked)! Still missing but now hole number one. Pulled the plug and checked it with a magnifying glass. Saw what looked like discoloration but with a dental pick discovered it was a crack in the bottom porcelain. My mechanic (for major things) said he did not put the plugs back in the same holes which explains the miss going from number four to number one. Replaced all 6 plugs with Motorcraft plugs but checked the gap this time. All, including the Bosch were preset at.045. Reset them to.054 and installed the Motorcraft plugs. Has run perfect for one hundred miles so far but hope to pull a trailer three hundred miles in a couple weeks and would like a little more confidence. Either I got two new Bosch plugs that were bad, or having the gap wrong managed to crack the bottom porcelain, or?
And yes, I double checked both that my engine is OHV and that my plugs are the right ones for it, not the longer ones for the SOHC engine.
With it running great right now this obviously is not a critical situation but it is sure got me curious and I do not like unsolved mysteries - especially if there is a possibility there is something I am missing.
Thanks for the feedback.
(I hit "update" because I cannot figure out if this sent or not - sorry if it is a duplicate)
Tuesday, August 28th, 2018 AT 8:30 AM