ive been getting these codes for about 10 months now and still cant find out whats wrong with my car. p0300 p0301 p0302 p0303 and p0304. i changed the spark plugs and ignition coils, and it was been through 2 mechanics. neither of them found the problem. one of them thought it was the timing and he fixed that but its still misfiring. and sensors are sending positive feed back, and passed compression test and NOTHING! it has a rough idle but i cant actually feel the misfires when im driving, but one of the mechanics said its misfiring at around 1500 RPM. the misfires started happening after i changed my catalytic converter to get rid of p0420 which it did but now i get these misfire codes. one of the mechanics thought that was causing back pressure but the other mechanic said its not (he checked that). I DONT KNOW WHAT ELSE TO DO! i need to pass smog test next month

 

what yr tc and how many miles

 

Its an 05 with 91k miles

 

fuel misfire?

 

Did you replace the exhaust manifold or did a shop? Was anything else worked on at the time? Were any vac hoses removed or knocked loose?

 

A shop did. And the manifold wasnt replaced. Just the catatilyc converter was cut off and a new one was welded in place along with an exhaust with custom piping and a magnaflow muffler

 

Fuel pressure was also checked and it checked out fine