(This is a continuation of a previous thread: http://community.cartalk.com/discussion/2294781/trouble-starting-and-rough-idle-on-start-up )
I have a 2005 Nissan Sentra with 75,000 miles on it. Several months ago (see above link) the check engine light came on with a P0304 cylinder 4 misfire code. The only symptom appears to be a rough idle/shaking every 3rd or 4th start for the first 10-15 seconds, but runs smoothly afterwards. My mechanic replaced all my spark plugs (I had new plugs put it at 50,000 miles for the same issue) of which the one from cylinder 4 was worn down much more than the rest. The problem persisted so they replaced my fuel pump and fuel filter and switched the spark coils from cylinder 4 to a different cylinder, but the code still read cylinder 4 so it wasn’t the coil. That was back in October.
I took my car back in last week for the same problem and without starting my car (because the problem is worst on the first start) they pushed my car into the bay and did a smoke test, no leaks in the fuel lines. Then they checked the cylinder and said that there was “excess” fuel in the cylinder and replaced the fuel injector. Then after ~3 days the check engine came back on with the same P0304 misfire code. Previous repairs/diagnostics rule out spark plugs, coils, fuel pressure, and injectors. What else might it be? Are there other symptoms I should pay attention to aside from the trouble starting?
Thanks for any help!