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i have a 04 grand am gt. the other night was on my way home pulled away from a stop went wot and the second it downshifted it started missing. i got it home just barly felt like it was running on 3 cylinders and sounded like it had a upper end tick. pulled off the air filter housing and now i think what i was heiring was backfiring through the intake. did a compression check and everything checked out fine, but has a p300 multiple cylinder misfire. havent pulled valve covers yet......dont know maybe a broken rocker, valvespring, or broken cam maybe.....anyway sorry for the long post thanks in advance jeff