How hard do dirty intake valves hurt mileage - Direction Injection - WRX

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How big an effect do dirty intake valves in a DI engine have on fuel economy? Does anyone have any good sources that quantify the impact? I can only find general and vague information.

My 2018 WRX is rated 18/24, 21 combined, and has been seeing a steady decrease in fuel economy over the past 6-9 months, as shown in the charts below. I'm not sure what's going on to cause the decrease.

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Granted, Q2 is a small sample size, but things have really been dropping since Q2-21. Q2-21 & Q3-21 are bit inflated due to more highway miles.

Whether or not you think it's DI deposit related, I'd love to hear your thoughts.
 
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Not saying it isn't mechanical in your situation, but the quality of fuel we are getting in the PNW is bad. Our household is getting less MPG across the board.
 
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dirty intake valves will reduce HP, but may improve MPG. Less air, and the ECU will reduce fuel, to keep the mix as close to 14.7 as possible.
 
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It hurts a bunch. Subaru has a TSB out on it. I've been using Berryman 2611 intake cleaner on our 2 2020 foresters. If you let it get too bad, possible burnt valves.
 
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Did DI come about for mostly emissions reasons, or to improve MPG? Seems annoying that mileage might go down with vehicle age..

I installed an air-oil separator in my WRX.... It's a brand-new car so I want it to stay clean inside as well. I realize some folks rank the AOS as barely a step up from snake oil, but I figure any improvement it gains me is good improvement. It's been a year already since I bought the car, and I've yet to crack 2000 miles, so we'll be waiting awhile to see if the AOS works, or not.
 
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Do you know if the Gas you are using is E15 now?

dirty intake valves will reduce HP, but may improve MPG. Less air, and the ECU will reduce fuel, to keep the mix as close to 14.7 as possible.

This, lots of people forget that engines are ran in a throttled position, you are not Wide open. So having an airflow restriction alone(dirty filter, reduced runner diameter due to deposits) will not really change your MPG on a Fuel injected motor. Power used not MPG.
 
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Clearly OP's mpg is trending down, but blaming valve deposits is just a WAG.
It could also be wheels going out of alignment, thermostat losing its mojo, etc.
 
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How big an effect do dirty intake valves in a DI engine have on fuel economy?

Whether or not you think it's DI deposit related, I'd love to hear your thoughts.
Gasoline quality is the biggest deal when I get a light mix of E10 the engine responds positively.

I would check your fuel trims and do a MAF cleaning with CRC MAF Cleaner.

if you have run octane boosters you may have metal fouled plug insulator which will cause a
higher misfire count.

Do you have an CAI on there too ? Or are you stock?
 
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At 84k miles I would suspect lazy O2 sensors first. As previously said long term fuel trims need a look at.
Don't forget lower winter temps and seasonal fuel blending having a big effect also.
 
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It’s gotta be Covid. 🤔

Did you gain 500 pounds?

Jk…. but I would check alignment, tire pressures, air filter, spark plugs. Could also be sticking brakes, slightly clogged fuel injector(s), wrong type of oil, bad quality gas.

But to answer your question, I think it depends if the carbon got to the valve seats. If the valve isn’t seating then it’s leaking some of the combustion and that’s bad for business…

If carbon builds up on the back of valves, and on the intake runners (causing less air flow and likely decreased air velocity), I’m not sure what that’ll do to fuel economy so someone else might have your answer for that.
 
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Apr 2, 2022
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It hurts a bunch. Subaru has a TSB out on it. I've been using Berryman 2611 intake cleaner on our 2 2020 foresters. If you let it get too bad, possible burnt valves.





Never used it but I've heard good things about the Berryman's. I've been thinking of trying it for a piston soak on my Camry.
 
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Did DI come about for mostly emissions reasons, or to improve MPG? Seems annoying that mileage might go down with vehicle age..

I installed an air-oil separator in my WRX.... It's a brand-new car so I want it to stay clean inside as well. I realize some folks rank the AOS as barely a step up from snake oil, but I figure any improvement it gains me is good improvement. It's been a year already since I bought the car, and I've yet to crack 2000 miles, so we'll be waiting awhile to see if the AOS works, or not.
Did you install an air oil separator or a cheesy catch can?
 
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Air oil separator..... The one from IAG. It's kind of neat because it circulates hot coolant through it which supposedly is to prevent water condensation in cold weather.
Very nice. Looks like a well thought out kit. I think the coolant circulation is to prevent freezing of any oil/water mix so you don't over pressurize the PCV system.

I've had a Mann Provent on the shelf since I bought my Touareg. Maybe I'll finally make a bracket
 
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