2014 BRZ P0303 Even After Replacing Spark Plugs and Coilpack
 

I have a 2014 BRZ, changed all the spark plugs at 70000km due to occasional rough idle and a P0303 code, cylinder 3 misfire. Swapped coilpack between cylinder 1 and 3 to see if the coilpack was bad.

Car was fine through winter and spring, come summer, 30+ Celsius weather the rough idle came back occasionally when coming to a stop. This would be most apparent after driving on the highway.

Cleaned the throttle body and MAF, and car drove really well for 3 days.

Rough idle returned and was worse than it was ever before. Threw a code for P0303 cylinder 3 misfire again. Checked advanced multiplier and it’s stuck at .26 with the CEL on.

Car has catted UEL headers and OFT tune, any ideas what’s the cause of this issue? Injectors? Direct injectors?

Car is currently in the shop pending a leak down and compression test to check for engine damage.

Misfire worse when warm makes me think of a fuel delivery issue, particularly in the face of supposedly good coils and plugs.

Between OBD2 data and some time with an oscilloscope, hopefully all will be made clear by the mechanics at the shop.

If the miss is truly a single cylinder miss, it will make things much easier to track down (basically eliminate air supply and exhaust issues, anything with MAF, MAP, most timing, upstream fuel delivery (pumps, lines, filter), and most of the electrics).

Low compression in cylinder 3, time for a new engine or engine rebuild.

I have no idea what could have caused this. The car was not abused or driven overly hard.

Try changing all the coils had something similar with a bugey . Code reader kept saying miss on 3 cylinder , I swapped the coils around and the miss did not move which was odd, I did do a compression test that was fine , I changed all the plugs and coils and that fixed the miss on cylinder 3 , perhaps try the cheapest things that are doable before making big financial decision ?


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These coilpacks are known to fail esp when under influence of extra heat from header. Change all as mswbrz has suggested first. Swapping may be misleading given mileage. Good luck.

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