So my 1998 Dodge Neon is still throwing a check engine light for a random misfire (P0300). And I have done many things to attempt to fix this. But alas, after all of my work the code has come back to haunt me again! Here is a list of everything I have done:

  1. Changed spark plugs and wires (properly gapped too)
  2. Changed the coil
  3. Changed the Crank position sensor
  4. Changed the rear O2 sensor (got a separate code, now gone)
  5. Had a shop replace my fuel pump (car wouldnt run, they told me my intake gasket had a leak which was causing my P0300 and P0301 codes)
  6. Changed the intake gaskets (both the upper and lower gaskets) and also cleaned out the intake while I had it off to clean the passageways. Those old gaskets were REALLY bad. P0301 has gone away and has not come back.
  7. Turned the car on and it ran like shit, like a huge vacuum leak. Sprayed brake clean around the intake and injectors. Found that Cyl 1 injector was leaking. After pushing it in a bit more, spraying more brake clean and running it, this problem went away. Then idles smoother and quieter than ever before!

So then I drove home from the hobby shop I do my car work at and it just ran beautifully. Before it ran like dogshit when revving it out and now it happily went out to 6k and actually felt kinda quick! First time I got to enjoyable drive the car and really beat on it a bit. Made it about 25 minutes into my 30 minute drive home and then the dame flashing light came back on for P0300. No noticeable change in performance, just that damn light. Had it blink at me again the next day on the way to work, around the same amount of time into the drive. Now I am stuck scratching my head trying to figure this shit out.

  1. Maybe the problem is when it heats up enough, a small vacuum leak occurs somewhere, causing a random misfire?
  2. Maybe that injector that gave me trouble needs to be replaced? Bad o ring or just faulty in general? Not sure if I need/should do all four then.
  3. Maybe something entirely different is at play here? Messed up exhaust valve springs? Some other crazy problem? A wiring fault? A different sensor?

At this point it runs pretty good so I am at a loss as to why it would be throwing the code. Im gonna try playing with brake clean some more and unplugging cylinders while running to see if any particular one affects the idle. It will require some much finer troubleshooting to figure this crap out. And really the only reason to fix this is for resale value.