I have recently bought an 04 maxima that had the check engine light on but ran fine. When I cleared the codes, it barely ran, stalled when stopping and now has p0021, p0300, p0158 and p2111 codes. And also the aftermarket exhaust went from sounding powerful to puny. I attempted to clean the throttle body and the plate inside but it was like tar. Now if I try to push the gas pedal down any more than around a half-inch, it seems to flood the engine and try to die. Any thoughts or suggestions to help me fix this?

 

Dealing with your codes would be an excellent place to start. Those are all bad codes.