99 Ranger 4WD 4.0L! Keeps cracking plugs! Help!

Had engine replaced about 10k miles ago. Since then, cylinder 4 spark plug cracked at the insulator and half the insulator was gone upon removal. a few weeks later, cylinder 3 started missing, took the plug out, the insulator was cracked in two places and about to shatter like the first one. what can cause this? they are brand new motorcraft plugs.

Had engine replaced about 10k miles ago. Since then, cylinder 4 spark plug cracked at the insulator and half the insulator was gone upon removal. a few weeks later, cylinder 3 started missing, took the plug out, the insulator was cracked in two places and about to shatter like the first one. what can cause this? they are brand new motorcraft plugs.

I've experienced the same thing. busted centre electrode porcelene. Bosch plug. swaped out to Autolite and ok now. Where the piece went ????? who knows. I thought bad plug but who knows

had same problem with both bosch and autolite platinum plugs, then found out some parts stores only list iridium plugs for 98 rangers changed one plug so far, so far so good, didnt want to change all again if they would`nt make a difference

had same problem with both bosch and autolite platinum plugs, then found out some parts stores only list iridium plugs for 98 rangers changed one plug so far, so far so good, didnt want to change all again if they would`nt make a difference

The iridium part of a spark plug is micrograms on the electode tips. I don't think that is the answer.

Predetonation is the usual cause, a hot spot in the cylinder somewhere causes an early bang on the compression stroke. Still a mystery I think.