Chevy Sonic P0304 Misfire on 4

[Sonic2711]

2013 Chevy Sonic LT Hatch only 99,000 miles

Car died one day while driving. engine light blinking.

replaced ignition coil and spark plugs. got it running.

dealer replaced fuel pump. but still rough. said would need to investigate misfiring. quoted and suggested oxygen sensor and t-stat.

ran codes myself.....
P304 misfire on 4
P219A air fuel ratio
P598 thermostat

i replaced valve cover and vapor canister purge valve. P219A is gone. engine light does not blink any longer. now just stays on. runs better.

i still get 304 and 598/597.

what could be causing the misfire????

 

[Z71]

Misfire is most commonly caused by a faulty ignition coil. Or, a faulty or cracked spark plug could also be the culprit. You said that you replaced the ignition coil. Did it cure the original misfire? Large ignition coil packs these cars use are very sensitive to impact and rough handling of even a new packaged coil can cause it to crack internally and misfire on one or more cylinders. I picked up a spare coil for my 2013 Sonic from a wrecked Sonic some time ago. When I installed it to check if it was good, it misfired terribly on at least two cylinders. Installing the original coil made the misfire go away instantly.

If this is not convincing enough, I bought a new AC Delco ignition coil from O'Reilly's auto parts two days ago. When I installed it, the engine misfired visibly, and I got engine check light and pulled a misfire code on no 2 cylinder. Reinstalling my old coil instantly got rid of the misfire. This means the new coil, still in a sealed box was most likely dropped from some height by someone and got damaged. So this proves that these coils are damage prone. I ordered a new coil from the local GM dealer and will pick it up tomorrow.
Also make sure that none of the pins on the coil connector itself are bent.

A malfunctioning oxygen sensor will typically only cause a rich or lean running condition, not a misfire. I had this happen in my 2009 Impala LTZ.

 

[ib2loud]

Could also be from a failed valve cover gasket letting oil seep into the spark plug tube.