If you didn't get any smoke with the sea foam, id say your engine is pretty clean then. Maybe check that your throttle valve isn't sticking? I've seen them get gummed up on the backside of them on other cars and stick every once in awhile. However, if you've gotten to the point that it backfires, that almost sounds like a timing issue. Cam position sensor maybe? I've also heard of some guys having problems with the timing chains stretching prematurely on these engines.
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