Check engine light flashing, codes P0304 and P0303

Tiny
BCU29291
  • MEMBER
  • 2011 KIA RIO
  • 4 CYL
  • AUTOMATIC
  • 85,000 MILES
Received error code p0304. So I changed ignition coil. Then received pending error code p0303. Changed that ignition coil now running even more sluggish and has flashing check engine light is steady when car is first turned on then starts flashing and not showing any error code.
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Thursday, June 7th, 2018 AT 6:35 PM

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Tiny
STEVE W.
  • EXPERT
Both of those are misfire codes. However, misfires can be caused by many things, not just a bad coil. First I would connect up a scan tool to verify what codes you actually have at this time. If the light is on or flashing there will be codes stored, but some code scanners work well with imports and some do not so it may be you need a different one to read the codes if the one you tried does not see them

I would then pull the coils from the "bad" cylinders and move them to the "good" ones to check if the misfires follow the coils. If not then pull the spark plugs to see if they showed any signs of damage from something like a blown head gasket or lean burning. You could also swap them to test them. Just do not move both the coils and plugs at the same time. Say you move the coils and get no change. Then you move the plugs and the misfires move with them, replace all four plugs.

However, if you moved the coils with no change and the plugs with no change and still have the misfire in three and four I would then do both a compression test and a leak-down test, because at that point you are looking for compression issues or leaking valves or head gasket and trying to eliminate those as mechanical problems.
The test equipment can usually be rented or "borrowed" as a loaner tool from many chain parts stores like AutoZone, O'Reilys and others.

Here are a few items that cover the above testing:
Compression test.
https://www.2carpros.com/articles/how-to-test-engine-compression
https://www.2carpros.com/articles/head-gasket-blown-test

Now suppose you do those and still find nothing. Then we have to look at the fuel system and possible fuel contamination.
However, the above should be done first. As you know how to change the coils and the plugs are just as simple.

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Thursday, June 7th, 2018 AT 10:01 PM

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