2019+ Carbon Buildup? (merged thread)

Crud build-up on the intake valves is a 'feature' of direct injection engines. It comes from the EGR and/or PCV systems feeding into the intake manifold/pipes. In the old port-injection, throttle-body injection and carburettor systems, the fuel (petrol) itself acted as the solvent to (generally) remove these deposits.

One way fix is to have another injector in the intake manifold which periodically squirts fuel in to clean the inlet valves/ports. This sort of defeats the whole purpose of having direct injection, but anyway, the other way is to have periodic cleans of the inlet manifold using a suitable port/valve cleaner.

CRC produce a cleaner specifically for DI engines. If you (or your dealer) uses Subaru Upper Engine Cleaner (SUEC) this I suspect is likely to do the job for you. It's quite simple, the stuff gets squirted into the inlet manifold whilst the engine is running. A bit of revving, a bit of soaking and perhaps some driving and the job's done.

I asked my local dealer who indicated they didn't think the engine has the extra injector. So you are likely to require a periodic clean of the inlet ports/valves. My local dealer indicated they run SUEC through cars they service as a matter of course. SUEC is quite highly regarded, it seems, and a lot of other people use it on other makes & models too.

If you want a bit of an intro to it all, see the video below which explains it at/near the beginning- just be warned, John has a rather abrasive, if also rather entertaining and informative style.

The truth about oil catch cans: Should you fit one to your car? | Auto Expert John Cadogan

With direct injection, carbon buildup is going to happen. It’s just a question of how soon and how often.

And, to some extent, the rate of carbon buildup is directly related to how long of a period that the engine runs. Engines that are run shorter distances, allowed to rest for an extended length of time (hours .vs. minutes, as an example), then run for another short distance are going to see carbon buildup on the intake valves much faster that the same engine run for an hour at the time, a period of rest, than another long running period.

Why? Oil temperature, for one thing. Hotter oil cooks off moisture. Short “run” periods never get the oil warm enough to cook out the moisture. Higher moisture contented oil residue coagulates on the intake valves, causing oil buildup.

But, that’s not the only reason. Engines that have dual injection systems, or systems that inject the fuel directly into the intake manifold (above the intake valves) gain the benefit of having the fuel spray above the intake valves act as a cleaning solvent, helping dissolve the carbon...ie., preventing carbon buildup. There is no such cleaning on direct injected engines.



There are endless explanations of these conditions, as well as explanations of how different injection systems function all over the internet. Therefore, there’s no need to reinvent the wheel here...

Google “direct injection carbon buildup” and marvel at all the explanations, in written, verbal and video presentations.

tomcoul said:

The main issue for me not buying the Forester Sport yet is the potential for carbon buildup on the new 2.5 DI motor on short trips in a cold climate area. If I decide to buy the car, I will get an engine block heater installed.

In searching this issue with the Crosstrek 2.0 DI motor, it doesn't seem to be an issue (yet?), when did the 2.0 motor get DI? Or is the motor still too new for carbon buildup to happen?

Click to expand...

Although there were some problems with GDI early on, most (not all) manufacturers have learned lessons to prevent catastrophic failure. We have a 2013 KIA Soul Base 1.6 Automatic with GDI. It has 87K miles and is running like new. I've used synthetic oil almost every change since the synthetic emits less "fumes" when heated, therefore reducing buildup in the PCV/Intake tract.

Contrary to what some folks say, running a bottle of Techron or Seafoam thru it is NOT going to clean off the valves. The fuel is injected downstream and never touches the intake valve on the "outside" face where buildup occurs. Many KIA folks run Catch Cans to supplement the PCV, but there are lingering questions about how that affects the warranty. I still think synthetic oil is good preventative maintenance for the carbon buildup issue.

If you think about it, almost all diesel engines the past 30 years have been running on the exact same principle as GDI up to the point of ignition. They are also direct injection, with PCV fumes being sucked into the intake tract over intake valves that never get "washed". There are a lot of diesel trucks out there with 300K-400K-500K miles running strong. So, I think the manufacturers can design around that problem.

Question is: Did Subaru do its homework? Let's hope so. I have a new 2019 Forester base model sitting in the garage with just 800 miles on it. I'm hoping to hit 200K miles someday.

Diesel engines work on almost the same principle as DI engines up to the point of ignition. They also have Direct Injection that does not "wash" the intake valves. They also have PCV fumes being sucked into the intake tract and past the intake valves. They've been doing this for the past 30-40 years. I don't hear any big hoopla about diesels self-destructing (aside from the GMs in the 80s - LOL).

Point is - with proper design, DI engines can last as long as Port Injection engines. I drive a 2013 KIA Soul 1.6 with GDI and it is coming up on 90K miles. Still runs like new and used not a drop of oil in our last 1,700-mile trip. I use synthetic oil because it emits far less vapors when hot, resulting in fewer deposits on the intake valves. DI in and of itself is not a strong reason to blacklist any car. Except maybe a Yugo, if they ever come out with it.

I would think (hope) that Subaru has done its homework and researched the DI engines from other manufacturers over the past several years as it was developing the new engine. I'm sure they don't want to have to pay to replace engines.

Regardless - I bought the 7-Year / 100K-Mile Bumper to Bumper warranty with Zero Deductible, so I don't have to worry about it - Subaru does. LOL


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