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Discussion Starter · #1 · (Edited)
I'm posting my experience on here so that others who may be having the same issue as myself can hopefully learn from my mistakes and experience.
I understand there are other threads touching on this topic. But this is simply my individual experience which I think others can take away some knowledge from.
This is for those who have Mazdas with Skyactiv/direct-injection engines who are not already aware of this potential issue:


(Mods, please don't close this discussion. My hope is that individuals who are experiencing this same issue can post their own experiences here and use this as a discussion board for what they are experiencing themselves.)


If you regularly drive in slow-moving traffic, city/stop & go traffic, or sit with you vehicle idling for even a few hours per month, then you especially need to pay attention to this.
And even if you primarily do highway driving with limited low-rpm driving or idling, you should consider taking these necessary precautions as well.

Mazda's Skyactiv engines are awesome, there is no doubt about that. They are powerful and efficient for being such small units. However, this comes at a price that most of us are unaware of until it rears its ugly head and plagues the vehicle we enjoy driving so much.

I have recently become all too aware of this issue that I previously had no idea even existed.
Around summer of 2022, I began to notice a gradual drop in power and fuel efficiency in my 2016 Mazda 3 2.0L Skyactiv-G. At first I didn't think much of it, but once fall came around, I finally realized that in my five years of owning the car, I had yet to change the spark plugs, and the car was just then rolling over 90,000 miles on the odometer, so it was definitely past due.

When I pulled the old spark plugs out of the engine, I noticed that all four of them had a noticeable amount of fouling on them, but the one that came out of cylinder #4 was especially bad. It had gasoline in its threads and lots of fouling and carbon built up on its end.
I figured that this was because the spark plug probably stopped working properly a few months before, and had left my engine running mostly off of three cylinders. So I replaced all four of the spark plugs with NGK Iridium spark plugs (all gapped and torqued properly) and immediately noticed an increase in power and live-mpg readings. I also began to use Techron and fuel injector cleaner in an attempt to get rid of any carbon build up within the combustion chambers that might have formed from the misfiring caused by the bad spark plug. I though I had solved the problem! But I still wasn't getting the same power and fuel-efficiency that I had gotten for the vast majority of my time driving the car.
So I began digging further into what the issue might be. I bought an infrared thermometer to check the temperature of the cylinder wall on the engine block. The first three cylinders read right around 175 °F, but the fourth cylinder read at around 130 °F, so I knew something was still wrong with cylinder #4. I figured that if it wasn't the spark plug causing the issue, then it was likely to be a bad ignition coil. So I replaced the ignition coil on cylinder #4 and again noticed an improvement in power and mpg! I also remeasured the temperature of each cylinder and found all four of them to be around 175 °F now. This was good news! I thought I had taken care of the whole problem.
But I hadn't.

About a week after replacing the bad coil pack, I noticed that my power and fuel-efficiency was once again struggling. Needless to say, I was starting to get pretty frustrated.
So I pulled all four new spark plugs out of the engine and found all four of them to have a significant amount of fouling on the ends of them. This absolutely astonished me because the plugs had been in use for less than a month at this point. So I brought all four of the brand new, fouled spark plugs into the house and soaked them in a cup of Techron for two days and gently brushed them with a small brass brush to remove all of the fouling from the ends and threads.
At this point, I knew something was wrong with my Mazda's engine, beyond just bad spark plugs and coil packs.

So I scoured the internet, looking for information on why this might be happening.
I read that it could be a number of things: dirty air filter, bad mass airflow sensor, bad fuel injectors (god forbid), bad O2 sensor, or, drumroll please...

...excessive carbon build-up on the intake valves. This can lead to a restriction of airflow into the combustion chambers and cause the engine to run too rich. This would explain why I found gasoline in the threads of cylinder #4's spark plug!
It would also explain why the brand new spark plugs had experienced so much fouling so quickly, as well as the car's lack of power and hurting fuel-efficiency that just kept getting worse and worse.

With Mazdas, excessive intake valve carbon build-up is born from a combination of things:

- Skyactiv engines have the world's highest production-gasoline engine compression ratios in the world. The Skyactiv-G has a compression ratio of 14:1, which is the same as a lot of diesel engines. Not only does the Skyactiv-G engine operate at a higher than normal chamber pressure, but it also operates off of 87 octane (US) gasoline, which is kind of insane. This means that even though the engine's combustion is more efficient than typical gas engines, it still isn't as complete as it would otherwise be with 91 octane (US) gasoline.
But more importantly...

- Skyactiv-G engines are direct injection, which means the engine's intake valves never come into contact with the fuel which is injected into the engine (unlike with port injection). Because the intake valves never come into contact with gasoline, there is no self-cleaning that takes place for the tops of the valves.
Over time, carbon deposits on the tops of the valves compound more and more, until they begin to restrict the airflow into the combustion chambers. When your engine can't get enough air, the air-fuel mixture becomes too rich, throwing off the carefully planned balance of gas and air which makes your engine operate properly.
If this goes on for long enough, it will lead to different problems, such as misfires, fouled spark plugs, and exponential creation of carbon deposits within the engine.

So how do you fix this?

Well, the best answer is to not let this happen in the first place! It is more likely to happen with vehicles which see an excess of low rpm driving and idling, i.e. stop & go traffic/city driving.
But people live where they live. You can't expect them to exclusively drive at highways speeds at 2,500 rpm.

So instead, every 10,000 miles or every oil change, you should seriously consider performing an intake valve cleaner treatment. This is an excellent preventative measure that you can take to make sure the inevitable carbon build-up never gets away from you.
There are a number of GDI intake valve cleaners that work well at removing carbon deposits from the intake valves of a gas engine. They include 'CRC intake valve & turbo cleaner', 'Lucas deep clean GDI', 'Liquimoly valve clean', 'Royal Purple max-blast intake valve cleaner', and a number of other products just like these. All of them seem to work quite well at removing carbon deposits from intake valves. From what I've seen, the Liquimoly Valve Clean seems to do a little bit better than most others, but they all seem to work pretty well.
It's quite easy to use them. Typically, you simply remove the intake box from the throttle body and spray the valve cleaner into the intake throttle while the engine is revving at about 2,000-2,500 rpm.
Each brand's treatment has varying directions for their specific application procedure, but they all seem to include the steps of spraying the cleaner into the throttle body while revving around 2,000 rpm, followed by letting the engine heat soak for an hour or two, followed by driving the vehicle at highway speeds or with a sustained rpm of at least 2,000 rpm for 15 minutes to an hour. DO NOT, I repeat, DO NOT let your vehicle reach above 3,000 rpm while performing the initial highway-speed drive! I learned this the hard way! When you do this, the engine sucks in too much of the valve cleaner all at once, which causes too powerful of a combustion in one or more cylinders. This will cause engine knock and can damage your engine if it happens too many times! Luckily when this happened to me, my Mazda 3 went into limp mode and forced me to pull to the the side of the road (this was on a busy, steep mountain road with an 8% grade - not fun).

The valve cleaning will be best to do as a regular preventative measure, however you can also do it when the carbon build-up has already become an issue. This is primarily what they advertise it for. That's how I am doing it anyway. But from here on out, I will incorporate an intake valve cleaning every 10,000 miles on my Mazda 3 and any other direct injection vehicle I have in the future. I highly recommend you consider doing the same.
I have primarily been using the Lucas Deep Clean GDI because it is the most readily available of the cleaners that I have been able to find. I ordered a box of 9 cans, so I should have more than enough to take care of a majority of the intake valve carbon deposits, as well as some for future preventative treatments.
I have also ordered a new air filter as well as ANOTHER SET of spark plugs (f*** my life). I think that despite cleaning my month-old spark plugs twice, they have already lost their usefulness from the rich fuel-air mixture, misfires, and fouling. So I'll be replacing them with NGK's Ruthenium HX spark plugs, which are supposed to help with a more complete and efficient fuel combustion. I will swap them in once I think I have sufficiently removed enough carbon from the intake valves (probably after two or three more treatments).

Additionally, I would highly recommend using a high-quality fuel additive every 5,000-10,000 miles such as Techron, Sea Foam, STP, Lucas, Gumout, there are tons of great options to choose from for adding to your gas tank.

Our Mazdas are amazing little cars, with better than average performance for a typical passenger vehicle. But that means they need to be taken care of a little more than the average boring Corolla or Civic.

Now you know!
 

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Discussion Starter · #4 ·
Thanks for your post, because of it I bought some engine Octane Booster, Bottled Fuel System Cleaner from Amazon. I have never used it so I hope I bought the right stuff.
Either buying Premium gas or adding octane booster is probably a good idea. I don't personally know if octane booster really works, but if it does, then it's a win in my book! Adding some fuel system cleaner is always a good idea at least once in between every oil change. I usually add some every other time I fill my gas tank.
As preventative maintenance, I will start spraying a can of fuel injector cleaner into the throttle body at least once every 10,000 miles.
Thanks for participating in the discussion!

Cheers
 

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Discussion Starter · #5 ·
Thought this should not be a major issue, as Mazda claims having an Air-Oil separator before the PCV valves. Per this video, at around the 13:50 mark:

I'm not sure if what they're saying is just wishful thinking or if it doesn't apply the the Mazda 3's 2.0L engine (I thought it was exactly the same as the Miata's but I could be wrong) but I can confirm it is indeed an issue. Maybe it is just my own personal circumstances with how I drive the vehicle, but at 90,000 miles on the odometer, the carbon build-up on my engines intake vales is astonishing. I will post some pictures in my next comment...
 

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Discussion Starter · #6 ·
I decided to do things the hard way, but the right way. I got everything I needed to pull my intake manifold off and walnut blast the s**t out of my intake valves.
When I pulled the manifold off, I was absolutely blown away at how much gunk was on the valves. Way more than I thought there would be (see included pictures).
But once I started getting rid of the gooey, oily gunk from the valves, I found out how bad the valves truly were.
On most of the valves, there was a solidified layer of carbon deposit about 2mm–3mm thick. That may not seem like much, but the intake valves are TINY. About the diameter of an American nickel or quarter. Go get a ruler and see how thick 2mm and 3mm are. On such a small surface area, that is quite significant.
There was so much carbon build-up that was basically fused to the valves, I had to walnut blast a valve, then soak it in valve cleaner for a few hours, then blast it again, then soak it again, followed by scraping any remaining deposits off with a plastic pick. It was way more involved than I thought it would be.
It is now abundantly clear to me why I have been getting such abysmal power and fuel economy from the car for the last five or so months, and why my brand new spark plugs became so fouwled so quickly (ruined in about a month).

I'm not sure that this is going to be a potential issue with EVERY Mazda engine. Maybe the 2.5L Skyactiv-G doesn't suffer from the same problem. But I can confirm that it has the potential to be an issue with the Mazda 3's 2.0L Skyactiv-G engine (at least the 2016 model). Especially if you live or commute in the city or regularly drive in stop & go traffic, as I do.

Take a look at some of these pictures I took while restoring my Mazda 3's intake system (there are some side by side comparisons of dirty valves and clean valves):
Hand Hood Fluid Automotive lighting Sunglasses
Tire Wheel Hood Automotive tire Tread

Pollution Gas Electric blue Waste Plastic

Automotive tire Motor vehicle Hood Bumper Automotive exterior

Automotive lighting Hood Automotive exterior Bumper Gas


Fluid Liquid Window Font Electric blue



Automotive tire Motor vehicle Font Bumper Gas

Liquid Window Sky Font Tints and shades

Eye Mouth Jaw Fluid Liquid

Automotive lighting Liquid Sky Tints and shades Font

Liquid Window Sky Font Tints and shades
 
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