Years ago I had a leaky head gasket. It steam cleaned one cylinder, so checking plug was obvious. If only I'd checked the head bolts, I'd have found a loose one but replaced the gasket. Speculated when it was rebuilt a few months before some oil got in the hole. It would torque to the oil but later loosen up. You should have seen the huge white plume when I took off from traffic light, didn't have money then to make pay for a tow.
Beater of the pack has wise advice.
Mark
