With your 2021 direct injected engine, there is NOTHING you can add to the fuel to prevent carbon buildup on the intake valves.

That being said, consider running Top Tier detergent gasoline to keep everything else clean:

TOP TIER™ Gasoline Brands - TOP TIER™

TOP TIER™ Gasoline Brands. All retail locations within the licensed country must meet the high standards of the TOP TIER™ program.

www.toptiergas.com

Keep using 87 octane. 91 is just a waste of money.

 

Continue with plain regular, 87 octane and nothing else. Read your owner's manual or any owner's manual. You won't find any recommendation for using fuel additives at all.

 

 

 

If only "fuel additives" didn't result in 111 hits when using the mysteriously magical search fuction....

Fuel additive | Subaru Crosstrek and XV Forums (subaruxvforum.com)

Techron - How much and how often? | Subaru Crosstrek and XV Forums (subaruxvforum.com)

Potential new idea for oil carbon buidup in the intake tract/valves | Subaru Crosstrek and XV Forums (subaruxvforum.com)

Carbon build up on valves | Subaru Crosstrek and XV Forums (subaruxvforum.com)

 

For GDI engine. fuel additive on gas tank maintain cleanliness of injectors and combustion chamber, but to clean need the cleaner run through air intake. for gas tank additive so far my best bet is redline si-1 or gumout regane high mileage. for cleaner through air intake, i use crc gdi ivd cleaner and penrite catalytic system cleaner. so far this combo never fails to make me smile on the new engine feeling after cleaning.

 

See post #4 by @CRAZYHAWK and post #6 by @Bluefoton.

Also, drive your lease buy-out Crosstrek "like you stole it" for a week. It will help with horking-up those nasty, carbon build-up, loogies.

 

See take my advice, you two things to lower carbon build-up:
1. Fuel additives to clean injectors and pistons from carbon build-up. If you don't use it you will end-up with engine replacement.

2. Add catch can which helps alot to collect carbon before entering combustion room.

If you follow my advice your engine will run smoothly without any issue 👌 😉 💯

Amount of carbon collected after 3K miles only (5K km)

 

2. Add catch can which helps alot to collect carbon before entering combustion room.

Apparently, you don't operate your Crosstrek anywhere that gets anywhere from -15°C to -30°C ambient temperatures for 7 months of the year. Catch can contents, and/or associated plumbing, freezes-off.

 

the oil vapor starts accummulate on the combustion chamber from the pcv line once you rev the engine at high rpm, e.g. past 4k, so engine oil that burns clean also another consideration to reduce the deposit.
someone say low SAPS and high ester oil may fit the requirement best to maintain the cleanliness and fuel economy.

 

There is zero proof that a catch can will help keep intake valves clean

 

if you want to totally screw up your O2 sensors, use fuel additives

 

You want to add an additive? Just pour a can of Coke into your fuel...that aught to clean out whatever you want to clean out?

Other than keeping injectors clean, adding any additives to help with the carbon deposit cleaning is pointless.
Subaru does have an intake cleaning kit that you can use once in a while, but remember the more crap your engine tries to burn, the more your expensive CATs and O2 will wear out faster. So maybe the occasional intake cleaning every 30-60k is fine?? The inevitable walnut blasting or whatever method of valve cleaning WILL happen. No one can say for sure, as we all drive differently in different scenarios.

Personally, I use one bottle of HEAT to dissipate any water/moisture in the fuel lines to avoid my fuel from freezing during the winter months. I usually put in a bottle in late fall, and I'm usually good to go. Otherwise, 87 octane is it!! But you are welcome to put in anything you want into your fuel

 

Hmmm...so some of you are saying products like Techron, Lucas,Seafoam...not only do not help but they can actually cause harm in a Subey??

 

Its a trade off. You are allowing the engine to ignite and off gas various types of chemicals. Those OTHER burnt gases react differently in the CAT and sensors vs. just the fuel it was designed to burn. Doing this too much will cause issues. It's a fine line. And as always...depends on your use, operation and environment of the vehicle being operated.

Some folks may benefit more from the intake cleaning chemicals, while others may not.

As mentioned many times in many forums, modern vehicles operate better with today's oils and fuel. Parts changing or parts maintenance may have been postponed some, it STILL needs to be done eventually. This includes the valve cleaning. No documented proof, only opinions on what the average time or milage would be.

I was going to to do the simple CRC intake cleaning at every oil change with my own vehicles until I found out the trade off with possible CAT replacement being done sooner. I would rather just let the valves get dirty vs. spending even more money for new CATs or sensors. The valve cleaning is much cheaper to do, even if it is possibly sooner?? I'm doing more frequent oil changes, so I'm hoping this alone will help push the valve cleaning further. Others may not agree. Just my opinion.

 

a previous mechanic told me that the only chemical that removes carbon the best is, water. Do not do this yourself!

 

That is really "ol'e school", think I did that once, forget which car it was, made a lot of steam.