Q: Car misfires when driving 45+ mph for extended period of time
The car only misfires when driving 45+ mph for an extended period of time. I get error code P0304.
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The code P0304 is a misfire on cylinder number four. The reason it is misfiring at cruising speed is that is when the engine is running its leanest and the coil is finding an easier path to ground than through the spark plug causing the misfire. The most likely cause is that the valve cover is leaking oil into the spark plug holes and causing the coil to short. I recommend you have a mechanic like one from YourMechanic diagnose your misfiring issue. In this situation, the mechanic will have to remove the coils and inspect for oil in the spark plug holes. The valve cover spark plug tube seals on some Nissan engines require a new valve cover to repair.
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