What causes my engine oil cap to blow off? My spark 6 blew out 2x's strippedthre

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Ead and melted the boot and the exaust blew out white smoke for like 5 seconds and it was running perfect just not even 24 hours the wheather got cold it snowed and i started it up drove about 7 miles and the engine light start blinking and did a tuneup on it still happen but only to that one