2004 Subaru Other

Engine Mechanical problem
2004 Other Subaru Models 4 cyl All Wheel Drive Manual 100000 miles

I have a 2004 Subaru Outback. It blew a spark plug out. Had it towed to a garage. They replaced the plug and checked compression said it was okay. Drove vehicle for 100 miles stopped for coffee when went to start the vehicle it blew the spark plug out again. Vehicle currently in the shop. Any ideas what may be wrong. I'm thinking head casket but hope not.
R Belear
December 21, 2009.

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