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Yesterday I filled my 2009 Chevrolet Cobalt's tank with E85 gas. I saw it was the cheapest at the pump, so I used it (bad idea). I then used a little more today (like 1.5 gallons) until I just realized I shouldn't be doing that.

My car was acting funny this morning for about 5 minutes and then went back to normal. Example: I would try to accelerate, but it would remain in the lowest gear, and then after a bit all of a sudden it would upshift and jerk forward.

Should I be worried about any permanent damage? I don't know too much about cars. Thanks!

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Probably not, but you've confused the poor computer, because the fuel doesn't have nearly the energy content it is expecting.

Your best bet is probably to get some proper gasoline cans, siphon off most of the fuel in the tank into the cans, fill it up with E0 gas (no ethanol) immediately, then add some of the jerry-can gas every fillup, aiming for a final mix of E20 or so.

... By the way, a gallon of diesel has 15% more usable energy content (joules) than a gallon of gasoline. Now, compare the price of gas and diesel, notice how it's around 15%? ... The same thing applies to E85. It has less usable energy content than normal gas (E10). However when you factor for this and then compare prices, guess what. Not such a good deal, after all.

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  • The problem is that the computer knows the correct air-fuel ratio for E15 (normal gasoline), and E85 has less power-per-volume than E15, so the computer mixes the wrong amount. Diluting the E85 with regular gasoline, even if the ratio is only E40 or so would probably allow you to drive okay, assuming you don't mind the decrease power you'll have until you've burned all the E85 and do the next fill-up of normal gasoline.
    – the_storyteller
    Commented Sep 11, 2019 at 21:35
  • @the_storyteller The computer can cope with a certain Exx range due to feedback. The oxygen sensors are indicating lean and the computer can/is injecting more volume of fuel. But the software is saying "Whoa, these numbers don't make sense; they're outside reasonable range/gamut; it's most likely a faulty sensor". Obviously the software could be hacked to permit a larger gamut, and that would work great at low RPM/power... but you'd find at high RPM/power the fuel injectors are not large enough.
    – Harper - Reinstate Monica
    Commented Sep 11, 2019 at 22:15
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    Exactly my point.
    – the_storyteller
    Commented Sep 12, 2019 at 3:48
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t seems there is not a definitive answer to this as far as modern vehicles go (anything without a carburetor). The EPA says on their website, “The sale of 85 octane fuel was originally allowed in high-elevation regions—where the barometric pressure is lower—because it was cheaper and because most carbureted engines tolerated it fairly well. This is not true for modern gasoline engines. So, unless you have an older vehicle with a carbureted engine, you should use the manufacturer-recommended fuel for your vehicle, even where 85 octane fuel is available”.

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  • Welcome to mechanics.SE! Note that this question is about E85 fuel, which is 85% (bio-)ethanol and 15% gasoline. This is a very different property from the octane rating that you are talking about. As such, I'm afraid your answer doesn't really answer anything.
    – marcelm
    Commented Sep 13, 2019 at 18:38

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