I would guess you were loosing compression.
But I would think if the plug was that loose it would sooner or later get blown out of the threads and take them with it.
After you have new plugs installed, how does it run ? If its fine don't worry.
What brand of gas do you use ?
miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. CHECK your oil, these cars use it.
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