Ive got the po300 misfire code. Ive changed the plugs. coils look brand new. assuming they have been changed by the other owner. also moved them around and unplugged one at a time with no change. Runs rough as soon as you cut it on and smooths out after a few minutes. its at the point where the CEL is NOT on anymore but my code reader still picks up the code. sometimes it doesn't pick up a code at all but comes back before the computer is at readiness. sometimes the code wont clear at all. Not sure what to do anymore. Anyone had this happen to them 08 nitro btw

 

I have an 08 nitro too. Randomly started misfiring a few months after I bought it.. took it to a few different places, they all told me it was something different. One place even wanted to put a whole new engine in my car that costed more than what I bought the car for lol. Anyway, had a guy do something called like a decarbonization or something which is supposed to clean the engine. Ended up helping a little bit. My check engine light never goes away but my car doesn’t misfire quite as much as it used to. I would notice it would happen after I filled with gas and so a mechanic I knew told me it may be an evap solenoid causing the issue. I still haven’t figured it out fully but for now it’s slowed down.