problems... pls help!

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my check engine light came on and it was diagnosed as the mafs code. it's come a couple days after i put in my injen cai. i'm not really sure what to look at. i'm gonna clean my filter tom and see if that helps any, but i'm stumped. here's the problems i've encountered:

1. sometimes i'll engage the clutch and the rpm's will just drop all the way down past idle and stall

2. when engine gets warm and i'm in 3rd, 4th, or 5th gear, the engine will kinda stutter when i'm in the 2.5k to 4k rpm range. like it'll kinda jerk along. this only happens when going at constant speed, not when accelerating.

3. LOTS of black smoke. i guess that means i'm running too rich. i'm thinking maybe my filter's dirty, so not enough air is getting in. that's why i'm cleaning my filter.

so yea... that's what's wrong with my car. anyone have any clue? thanks for any help.