Well that backfire isn't a good sign. You may have to double-checked how you set the marks on the pulleys for the timing belt. If a valve is timed wrong in the combustion cycle, you'll get that very symptom. I know it's a pain to do that, but at least you will have eliminated the obvious.
I cleared the code of 300 and ran the car for about 20 min at around 1500rpms than checked for any codes and got no codes.but when it gets to 2500rpms it still boags out.checked for codes a few times and got nothing.
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