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Discussion Starter #1 (Edited)
The intention of this post is to clear the air on direct injection (DI), specifically the issue of carbon build up. I noticed that many people have concerns regarding the long term reliability of these engines, due to the issues plaguing early DI engines, and decided to make this handy reference to explain DI, why these deposits form, and how to prevent them. Hopefully this will clear the air, and if anyone has any helpful information to add, please do so.

What is Direct Injection?

Direct injection is a form of electronic fuel injection (EFI) where the fuel injector sprays fuel directly into the cylinder, as opposed to port fuel injection, where the fuel is injected into the intake tract ahead of the valves and after the throttle body. Direct injection offers many benefits over port fuel injection, including increased power and fuel economy, due to it's ability to contribute to an extremely lean burn. Direct injection engines require a high-pressure fuel pump to create enough pressure to properly operate, and use a common rail fuel system.

Carbon Build Up Problems>

Direct injection is actually a fairly old technology, with the 1955 Mercedes 300SL using direct injection, and diesels using DI for quite awhile. Gasoline DI only became mainstream around around the early 2000's, and many of these early engines had issues with carbon buildups forming on the intake valves; an issue that was rather uncommon for other EFI engines. The reason for this is fairly technical, but I'll shorten it for clarity and provide a link that explains it in more detail. Basically, combustion byproducts get recycled into the intake tract through the PCV valve, and will cling to the intake valves on their way into the combustion chamber. With port fuel injection, fuel is constantly moving over the valves, in effect "washing" the valves and preventing carbon form building up too much. With DI, the absence of this fuel allows the carbon to accumulate, and if left unchecked, can prevent the valves from seating properly, causing a number of issues.

How to Prevent It

Simple: rev the crap out of your engine.

No, seriously. Mazda was one of the first auto manufacturers to put direct injection into mainstream (read:economy) cars back in 2005 with the 2.3L DISI engine. They've been messing with DI for awhile and so they were obviously aware of the issue when designing the Skyactiv-G engines. What they found was that carbon deposits form when the intake valves get below 400*F, so when designing the block and cylinder head, they routed coolant passages away from the intake valves to keep them above that threshold. You can do your part by taking your car out on a nice drive and putting the engine through it's paces, and taking it up to redline a few times.

This article provides some excellent information on the Skyactiv engine, as well as information about valve deposits (thanks, DradernH!):
http://www.motoiq.com/MagazineArticles/ID/2105/PageID/3529/131-Compression-and-40-mpg-on-87-Octane-fuel-Introducing-Mazdas-Skyactiv-Technology.aspx

Oil Catch Cans

An Oil Catch Can is a container put in between the PCV valve and the intake manifold that separates oil and water, preventing it from going back into the intake. For many cars, it can reduce (not eliminate) the potential for valve deposits forming. It is very much a beneficial mod, and I would recommend it to anyone who plans on keeping their car for an extended period of time. However, don't let other people say that you "need a catch can", because, as I discussed above, the issue already has a solution that doesn't cost you $165.

I hope this post helps answer some questions, and if anyone has questions, feel free to ask and I'll answer it or point you in the right direction.

For more information on fuel injection, here's a helpful video:
Another good video explaining catch cans and the PCV system:
 

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At least once a week I take 'Lexie' for a spirited drive and rev the crap out of it.

This is specially important in winter, when the engine takes a lot longer to get up to temp


Sent from my iPhone using Tapatalk
 

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hahaha is it a coincidence that mazda builds engines that are happy to rev to redline? think rotary....
 

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The wife's MZ3 will have 40K on it sometime around February, and at that point we'll take a look at the intake valves to see how they look. We plan to keep the car indefinitely, so if we find that the valves are a mess, we'll clean them, install a catch can, and then make it a point to work the engine from time to time.

At present, the car can't have been revved to redline more than just a few times, and we'll keep driving it that way until the 40K inspection of the valves. That should give us a pretty good indication of what 40K's worth of ordinary grocery-getting does or doesn't do to the valves.

A brief mention is made in this piece about Mazda's effort to keep the carbon buildup down and the intake valves hot (here, above 400° Centigrade):
http://www.motoiq.com/MagazineArticles/ID/2105/PageID/3529/131-Compression-and-40-mpg-on-87-Octane-fuel-Introducing-Mazdas-Skyactiv-Technology.aspx
 

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I plan to scope mine at some point just out of curiosity. When I eventually get around to it, I'll post some pictures. My guess is that I will have very little buildup if any at all. Almost all of my driving is highway at around 120 km/h.

Shooting for highest mileage mz3. Ask me in a few years :p
 

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Discussion Starter #11
JinZ, you should be fine with redlining it whenever, but a good way to take care of the engine would be to race it at an autocross event!

arathol, you're absolutely right, it has been discussed before, but by making a post dedicated to the issue, it'll allow the information to be more accessible. The thread you linked to has a typo in the title....
 
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