I changed six (6) fuel Injectors, Six (6) spar plugs, Two Ignition coils and am using Premium grade gasoline in my Nissan Maxima 1999. My car is still rattling at 40 MPH. When I drive car, the speed reach at 40 mph at 2100 rpm. At this point rpm drop from 2100 rpm to about 1300 rpm(changed gear at this speed), and car start to rattle. when speed reach at 50 MPH, stop rattling. Is any body have this kind of problems on Nissan Maxima. Please provide how to correct this problems. Also the engine light show with code P0300.

 

are you using oem injectors, sparc plugs? NGK sparc plugs is a must, and injectors, you have to use the right ones..im sure other members will tell you.

otherwise it will run like crap and like you said, rattle