Piston Slap: Misfiring on Mazda 3 Skyactiv's Diagnosis?

Brian writes:

Hi Sajeev,

You are my go-to guy for in-depth automotive knowledge. (Wow! Honored. – SM)

My brother has been having a heck of a time with his 2012 Mazda3 Skyactiv. It’s got about 70k miles on it, and in the past 2 years it’s been in and out of the shop. It started with high engine oil consumption (more than a quart per 3k miles, and going up to this day). Then the ignition coils needed to be replaced. Then the MAF and oxygen sensors needed to be replaced. Right after those were fixed, he had to take it back to the shop because there is now a misfire in one of the cylinders.

What’s going on here? My only guesses are worn piston rings causing engine oil to run into the combustion chamber (then down the exhaust and ruining the O2 sensor), or malfunctioning PCV, a broken EGR system, intake valves with a lot of carbon deposits, or simply “bad luck” with this car.

Hope you are able to shed some light on this mystery. Thanks!

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