2011 Subaru Forester — accidentally added 15% ethanol gas

I accidentally put 88 octane gas with 15% ethanol in my tank, instead of 87%/10%. My dealer says I need to have the tank drained, removed, washed, then put back on. Is this just a way to make $250 or would one tank of this 88%/15% really hurt my engine?

Washed? No. That just seems silly to me. What would the wash be removing? All of the contents are the exact same contents the new gas will have.

The dealer is not going to tell anyone that 15 % Ethanol in a vehicle that is supposed to have only 10 % Ethanol . If it was mine I would start putting 10 % Ethanol fuel in every time it read 3/4 full for at least 4 times. Now if it is running poorly that makes a difference .

do you mean 88 octane E85 ?

The EPA says it won’t hurt anything. AAA disagrees. AAA warns E15 gasoline could cause car damage If you live in a state like Oklahoma where there non-ethanol gas is available you can top off with it and “dilute” what’s in the tank.

Good question. The answer should be informative.


Depending on how many gallons you actually put in the tank, you might be able to get by, by filling up with non-alcohol gas.


I have actually tested E85 mixed into the standard E10 we all buy to create as much as a 30% ethanol, 70% gas mix. The car didn’t care and it is not designed for E85.

If you put 10 gallons of E85 in a 16 gallon tank, the rest of which has E10, the mix is 50% ethanol and 50% gasoline. Pump E85 is actually more like E70 or E75. If you put in less, the % of ethanol will be less.

The car will likely run OK but may set a check engine light. As you run out the fuel, add 2-3 gallons of non-ethanol gas or even E10 regular fuel every time it runs out a 1/4 of a tank. You will get poor MPGs because of the E85 but the mixture will pretty quickly return to normal and the check engine light will go out.

I would just run it out.


Your dealer is IMO proposing and extreme and likely unnecessary solution. At the most extreme remedy I’d even consider is siphoning as much gas as possible (not easy to do with current tamper resistant fueling ports) and refill with regular fuel.

Somewhat similar to what someone else here suggested… Find a place that sells non-ethanol fuel and put as much in there as there’s room for. Drive a while, and as soon as there’s room for more non-alcohol fuel add more. Simply put, just dilute out the higher level of ethanol as much practical.

I agree with Mustangman. And I would be very suspicious of that service manager’s credibility in the future. I would have to either have to have doubts about his honesty or at least about his judgement.

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That makes sense. E-15 - I wouldn’t worry about it. E-85 is a different story.

It was 88 octane with 15% ethanol. Whatever the shorthand is for that.

If it was 15% ethanol (“E15”), then I wouldn’t worry, just fill up with E10 once you’re down to a 1/2 tank.


Then I wouldn’t be worried about it, E15 is not going to cause any problems on a modern car.

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That is only enough labor to drain the fuel tank and add fuel, to remove the fuel tank (unnecessary) would cost more than $600.

How did the dealer find out that you have that fuel in the tank?

If it really is E15 fuel, it won’'t matter at all. We have local E20 fuel available and it mixes freely with E10 in a modern car with no issues.

My area has 88 Octane E15. I wouldn’t worry about it for one tank

If you are concerned dilute it down with additional fuel. Me, I would not worry one second about it.

For what it’s worth, never rely too much (if at all) on what a dealer says. You’re generally speaking with service advisors and in some cases a service manager. Very few of them have much in the line of mechanical knowledge or expertise and they are not going to want to come across that way to you. Ergo, BS begins.

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Hey! Did anything happen to your car? I accidentally did the same thing to my Subaru Impreza only a half a tank of e15. Every Subaru in town told me I need to clean the lines and take out the engine. Told me I needed to tow it in and it would be around 700-1000 to fix.

They told you this for E15 (which is 85% gas)? Not E85 (15% gas), right?

I wouldn’t give it a second thought.

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Yeah they said that for getting E15. They said it was like “oil and water” :face_with_monocle: