Since the 4 cyl engine is direct injection, I have heard of potential carbon buildup on the intake valves. I would like to know if this engine has experienced this problem and what might be done to avoid the problem.
I assume this is “Skyactiv-G” engine and YES indeed it is direct injected (DI) one.
Carbon buildup is not really plaguing Mazda DI engines of this series, but they do recommend cleaning up valves every once in a while, something like 50K miles.
To reduce deposits, use only good gas and good oil, that will do 
Cleaning up valves - is that a DIY process?
This product was developed to address this issue.
Tester
I’ve used this Sea Foam Spray on my 2013 Chevrolet Equinox with the 2.4 4 cyl DI engine twice (30k and 60k miles). I’ve had no issues before/after each treatment. To be on the safe side I only used about 1/2 the product for each treatment. I consider it preventative maintenance. It has 79k miles and runs fine.
Ed
I’d be sure to choose stations that offer Top Tier gasoline if I had concerns about potential carbon build-up on valves. Supposedly, additives included with Top Tier gasoline are formulated to help keep combustion chambers cleaner.
CSA
Top Tier gas won’t keep the intake valves clean on a direct injected engine.
That’s because the gasoline is injected directly onto the cylinders on a direct injection engine, instead of the intake runners in non- direct injected engine where the gas comes in contact with the back-sides of the intake valves,
Tester
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To help avoid the issue Tester notes, some manufacturers are using a dual-spray or dual injectors.
In the dual spray, fuel is sprayed at the intake valve while it’s open to clean the carbon, and again while it’s closed for the main combustion. Or the spray is started while the intake is still open so that some of the spray washes the back side of the valve.
For dual injectors, there’s a small port injector whose spray keeps the back side of the injector clean.
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You could install a PVC oil catch can.
What about the EGR gases and leaking valve guide seals?
Tester
At least one study shows catch cans don’t help:
It seems intake valve deposits come from exhaust gas flowing back, not PCV fumes.