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Mazda Skyactiv Engine

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.

I haven't been able to find the link to the article that details Mazda's findings, but I'll update the post when I do.

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.

Stole this from M3R user yobo9193
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