If this relates I had a 2009 Hyundai Accent, also with a 1.6L ICE, although not the Miller cycle engine, but probably similar in lots of ways. I bought it used, and noticed the Original owner had brought it in for mis-fire/rough driving at ~11k mi. It had check-eng. light. It had code of random-misfire. They replaced the/a spark plug wire and rebooted. About 10k miles later I had same problems, like a slight ping occasionally. They replaced all spark wires. I would still get it intermittenly, the worst being when it felt like I was driving over mini railroad tracks. Brought it in and they rebooted. After awhile, when it happened I noticed the codes were never the same. Always random-misfire, but would call it cyl.#1, cyl.2&3, 1&4 etc. After warranty I even replaced plugs myself and no change. Rebooting ECM worked for short time, then back to problem and ce light.
I noticed when I was in Hyundai dealer they had their own branded engine additive, but it was Chevron Techron(same shape and cardboard label. I bought the Chevron additive, waited till low fuel, then added. It worked for awhile. Then I added again, but STOPPED using the **** non-Top Tier WaWa gas (Sheets too.) I never got the problem again in 10's of k's of miles. This is the same additive Kia calls for. Even though I never use non Top Tier gas now, I put in a bottle ~every other oil change. I've felt a little ping/hesitation here and there, usually when I'm letting off the pedal a bit. How much of that is elec motor to ICE conversion, or o hybrid related I don't know. I've never had a check engine light and still get 52+ MPG. For what it's worth..
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