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!

Sajeev answers:

As we all know, Skyactiv engines improve fuel economy without the usual side effect of a wheezy, wimpy powerband. But you can’t have your cake and eat it too: low tension piston rings ensure this truth. But is this really a problem?

I’ve postulated that oil burning is likely a net positive in modern engines and — unless a Dieselgate-worthy scandal comes upon us — this will continue. So tell your brother to not fear the oil consumption issue, unless the rate doubles in a fraction of the current time/mileage interval.

The other issues are worrisome, but less about the car and more about the diagnoses. What engine codes were generated to merit replacing all those bits? Armchair quarterbacking is a dangerous profession, but it seems like this direct-injected motor got a lot of parts thrown at the problem. Parts that either went bad because the motor wasn’t de-carboned, or they weren’t bad to start.

I’ve been spilling digital ink about de-carbonizing Direct-Injected engines for years ( at least 2012) and that’s likely what your brother’s Mazda needs. Perhaps the traditional chemicals/walnut shell/whatever Mazda recommends as a treatment won’t cut it now, and perhaps the intake/EGR system needs a good physical cleaning on the workbench?

What say you, Best and Brightest?

[Image: Mazda]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.