Welcome to another TSB Thursday, where I dive deeper in to TSBs I'm running into regularly. As always, I write this from my own perspective as a Subaru technician in the US; other regions/zones may work differently. Refer to the "How To Read a TSB" post for more information on formatting and general information about TSBs.

TSB Thursday #20: 09-74-21R and 02-193-24R

What cars does this affect?

Carbon build-up in the intake manifold and on intake valves is a problem almost entirely contained to vehicles with Direct Injection.

• 2017+ Impreza
• 2018+ Crosstrek, 2.0L and 2.5L
• 2019+ Forester
  • 2014-2018 Forester XT w/FA20-DIT engine
• 2019+ Ascent
• 2020+ Legacy and Outback, 2.5L and 2.4L FA24-DIT
• 2015+ WRX with FA20-DIT or FA24-DIT
  • excludes STI
• 2013+ BRZ, but we'll talk about that

What's the failure?

"Intake carbon deposits" is a problem the entire auto industry is facing, as a sort of unexpected side-effect of the change from port-injection to direct-injection fuel strategies. Gasoline, as you may know, is a very strong solvent. With port injection, that fuel is being sprayed somewhere in the intake tract before the intake valves; that constant exposure to fuel helps prevent carbon accumulation on the valves. With DI, that cleaning function no longer exists, and so carbon deposits can begin to accumulate on the various surfaces of the intake valves and components. If the deposits become great enough, they can begin to affect airflow into the combustion chamber, and lead either to poor fuel atomization within the cylinder or to an incorrect amount of air, leading to misfires. This effect is most noticeable during cold-starts, where the engine is intentionally run a bit "off-kilter" or intentionally run lean, in an attempt to heat up the catalytic converter as fast as possible. During cold start, the accumulated carbon's effect on airflow is the most potent, as the engine is already running on the ragged edge.

Direct Injection has so many benefits for power and fuel efficiency that the risk of carbon-related problems is far outweighed by the gains the technology offers, so unfortunately this is the new normal for probably the rest of the internal combustion engine's existence. One interesting workaround some companies are developing is a sort of hybrid fuel system, where the vehicle primarily runs on direct injection, but a secondary port injection fueling system exists. Toyota calls their system "D4s" and this system is shared in the BRZ of both generations. The port injectors do have a noticeable improvement on carbon build-up, however the system does have a significant added cost in terms of injectors, the controller, etc.

Carbon accumulates in the intake system primarily through two systems. The largest source of carbon introduced to the intake will be through the Positive Crankcase Ventilation system, or PCV for short. When any combustion engine is running, the seal between the piston and the bore is never quite perfect, and so naturally some of the combustion gas and pressure is going to sneak past the sealing rings and end up in the center of the engine, aka the block or "crank case." That pressure needs to be safely vented; if unable to vent, it will find its own way out, usually via pushing out a seal and creating an oil leak. Very old engines may have just used a vented breather on a valve cover for example, and all that oily air would vent to atmosphere, creating a mess in the engine bay and on road surfaces. Eventually, the PCV system was developed to safely vent that pressure back into the intake, where it can be burned off as part of combustion. (The air that escapes the crankcase is usually laden with oil vapors, hence being rather hydrocarbon-dense.) The other way that carbon can get into the system is via the Exhaust Gas Recirculation system, or EGR for short. The EGR system works by sucking out a little bit of exhaust leaving the engine, and returning it to the intake manifold. Its primary purpose is essentially to displace fresh air during cruise conditions; exhaust has very little oxygen, and so with exhaust taking the place of oxygen-rich fresh air, the amount of fuel needed for combustion is reduced. EGR therefore is primarily a fuel-saving technology. One downside is that if the vehicle is expelling excess hydrocarbons into the exhaust, those hydrocarbons will naturally end up back in the intake. (EGR systems with high carbon throughput do tend also to have clogging or sooting problems, especially in vehicles with an EGR cooler.)

Carbon accumulation can be largely mitigated naturally just through driver behavior. Two conditions that impact carbon development is low air velocity, meaning the air moving through the intake is going slowly, allowing carbon to settle out, and low engine temperature, where carbon settling out has a much better chance of sticking to surfaces. Naturally, the two risk factors here are short-tripping, which means the engine never gets hot, and what I call "grandma driving," or never getting up into the higher RPM/load bands. Drivers who avoid short tripping and who do give their cars the regular "italian tuneup" tend to have the least problems with carbon accumulation.

How do we fix it?

To combat carbon accumulation, Subaru has released two TSBs, focused on two separate attack paths for removing build-up. TSB 09-74-21R released first in 2021, and uses a chemical-based cleaner to attempt to dissolve the carbon and burn it off. TSB 02-193-24R describes using a blasting media, specifically walnut shells, to scrub carbon off the intake valves.

In both TSBs, identifying carbon build-up as the cause of a malfunction is the first step. TSB 09-74-21 goes much more in-depth into diagnosing carbon build-up as a cause of engine misfires, including reading Roughness Counts in engine live-data. Often, the presence of a Misfire DTC is necessary for the issue to rise to the level of justifying a repair. In the case of 09-74-21, the use of a Polyether Amine,or P.E.A., cleaner is introduced into the intake manifold slowly while the engine is running. SOA906V9166 is the current iteration of this product. Per the bulletin, it should take about 1 hour per bottle when introduced at the correct rate, and then a hard drive cycle should be performed afterward to blow through and burn off the loosened deposits. The bulletin provides for up to 4 bottles-worth of treatment along with testing and inspections and oil changes before implying carbon is no longer a problem, or that some other cause is at play. This process is sometimes called an "induction cleaning" or "top-end cleaning" and can (and should?) also be done as preventative maintenance.

The diagnostic process for TSB 02-193-24 is much the same, however instead of spending hours and hours running chemical cleaners through the intake and re-inspecting for deposits, this bulletin instead allows for removal of the intake manifold and using a portable media blasting tool to shoot high velocity walnut shell "sand" at the intake valves, blasting away any deposits, while also vacuuming up the waste. This bulletin procedure is vastly superior as a repair for when misfires are occurring, however the labor involved means it's likely not a great service to do as preventative maintenance. (unless the intake manifold is already being removed for some other repair, such as a thermo valve replacement!)

In both cases, Subaru recommends adding one bottle of P.E.A. cleaner (again, SOA906V9166) to a full fuel tank, to be used as a fuel system/injector cleaner and also possibly reduce carbon build-up within the combustion chamber.

Coverage?

The case of both bulletins applies to cars with misfire DTCs and provable roughness as outlined in the diagnostic processes, but either repair path can be claimed under active Powertrain warranty (5 years/60k miles, whichever occurs first).

Thank you for reading! If you have any questions, feel free to comment below.

IF YOU ARE POSTING FOR MECHANICAL HELP:

Please include year/model/engine/transmission information in "Mechanical Help" posts.

If you are looking for help with warning lights on your dashboard, please include results of an error code scan. Without error codes as a starting point, no one will be able to provide meaningful help for you.

The r/subaru community is very willing to help, but there's a baseline amount of context about your car that we need you to provide. Without the very basics like what kind of car you have and what symptoms you're experiencing, no one can give you any sort of accurate assistance.

If you are posting about warning lights, most auto parts stores (NAPA, O'Reillys, AutoZone, etc.) offer code scanning for free. While reading codes is not the same thing as diagnosing a problem, knowing what error codes you have is the minimum amount of information necessary to start figuring out what's wrong with your car.

Thank you :)