Ok so here is the situation. My wife's 01 Altima gets about 20-22 MPG in the city which I think is pretty low for a 4 cylinder but I could be wrong. The main problem however is an extremly rough Idle, about 600-700 rpm. Initially i though that it wasn't get spark and misfiring so I replaced the plugs and wires and also the cap and Rotor but it still was rough. I took the car to Autozone to have the codes read and they pulled codes that were about the knock sensor, the mass airflow sensor, a misfire on cylinder 4, and lean exhaust if I remember corrctly. They said that it could be a bad MAF sensor. So i bought some MAF cleaner and cleaned it out but the car still Idles rough. I've tested compression and all four cylinders have about the same compression. I've thought that It could be the fuel injectors so I put some lucas engine injector cleaner in it for a few tanks but that doesn't seem to affect it in anyway. Part of me wants to say that it is something in the fuel system but I'm not sure if its the injectors or the regulator. Is there anyway to tell? or do you guys have any other ideas of what the problem could be

Thanks in advance for all your help
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