e_NiNo

miss fire cyl #4 at 5000 5500 RPM, the cat in the up pipe trouble the dealer found my probleme... 65 PSI in back pressure at the )2 sensor and normaly it's 31 psi MAX ... the up pipe cat are stuffy

I add my car to your list miss fire cyl #4 up to 4500 5000 RPM only, idle RPM cut a litle bit at cold , ok when is hot Have a brand new short block change injector ,spark plug ans coil pack the trouble are alredy there. I clean the MAF,... ok for 15 minutes ... alredy the cyl 4 possible a bad valve ?