Why is Ethanol Bad for Cars?

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Ethanol is a controversial fuel additive. But is it really that bad for cars? And if so, is it ever safe to use ethanol-infused gasoline?

Ethanol can damage many parts of your car at concentrations above 10%. Ethanol can destroy gas tanks, fuel pumps, gaskets, and attract moisture into your fuel. Engines can be completely ruined over time if they’re not designed for ethanol.

In this article, we’ll cover several of the reasons why ethanol is bad for your car. We’ll also go over when it’s safe to use ethanol in your car, and what types of vehicles can use this inexpensive fuel.

Table of Contents


What is Ethanol?

Ethanol (CH3CH2OH) is flammable alcohol distilled from plants, chiefly corn. According to the Alternative Fuels Data Center, about 98% of gasoline sold in the US contains some amount of ethanol.


Ethanol is essentially high-proof grain alcohol, but not the kind you should drink. It’s produced in a similar way and has the same clean-burning properties.

Why is Ethanol in Gasoline?

There are multiple reasons why manufacturers add ethanol to conventional gasoline. Ethanol is an octane booster. Companies can combine lower quality gasoline with up to 10% ethanol in still achieve an 87-octane rating.

Ethanol is more "environmentally friendly" than gasoline. It’s made from plants and theoretically unlimited, and its emissions are less harmful. The distillation and refining process of ethanol is also less toxic and ecologically detrimental.

You can make a strong economic argument for ethanol, too, as farmers in many parts of the United States benefit from growing “fuel crops.” Ethanol production uses less costly corn that couldn’t be used for human consumption. Plus, it brings valuable economic growth to rural areas.

True, there are many benefits to using ethanol and gasoline. So what’s all the fuss about? Is ethanol actually bad for your car?

But first, we should distinguish between high-ethanol gas and typical gasoline with ethanol in it. Gasoline with a few percent ethanol won't ruin your engine, but higher concentrations can cause serious problems if your vehicle isn't designed to use it.

Ethanol Gasoline Has a Lower Shelf Life

It’s common knowledge that gasoline has an expiration date. Fuel quality degrades over time, and it'll eventually turn into gel. Ethanol-free gasoline has a shelf life of around 4 to 6 months before it begins to oxidize and go bad. Ethanol has a much shorter shelf life of 1 to 3 months. So what’s with the discrepancy? Why doesn’t ethanol last as long?

Ethanol is a hygroscopic chemical, which means it attracts water. Do the math. Gasoline with a high ethanol content, if allowed to expire, will also have a very high moisture content. This is bad news for almost every component in your car. Contaminated fuel can cause anything from clogged fuel lines to cracked cylinder heads.

Does Ethanol Damage Engines?

So, is ethanol bad for cars, and what does it do to engines? Ethanol-infused gasoline can absolutely damage engines. However, some engines are more vulnerable than others. Here’s what ethanol does to a car and why there’s a good reason to be concerned about using this fuel additive.

Fuel Tank Damage

Ethanol isn’t a threat to most steel and aluminum fuel tanks. However, ethanol can dissolve resins in fiberglass and composite fuel tanks, which can cause leaks or total tank failure. It doesn’t take long either.

Fuel System Damage

Ethanol is a much stronger solvent than gasoline. It also attracts moisture. Can you think of a component in your fuel system that’s vulnerable to solvents and shouldn’t dry out? That’s right, your seals and gaskets.

Ethanol can rapidly dry out and destroy rubber fuel system components, including gaskets and hoses. Some are designed for ethanol, but most are not, and they can be quickly ruined by ethanol concentrations as low as 15%. That means no E15 in your conventional vehicle.

Moisture Problems

Ethanol’s hygroscopic properties are to blame for many of the problems it causes, including engine issues. Condensation that forms inside your fuel tank on cold days is normally inconsequential—but the presence of ethanol can draw all that water into the fuel.

This can cause all sorts of issues in your engine, including fouling, corrosion, and knocking as the damage intensifies.

Ethanol Works in Flex-Fuel Vehicles

Some vehicles are designed for ethanol. These vehicles are commonly referred to as “flex-fuel“ by manufacturers. Flex-fuel usually means one of two things. Either the vehicle is designed for E15 (15% ethanol) or more potent E85 (85% ethanol). These vehicles can take advantage of cheaper fuel without risking the same catastrophic damage that occurs on regular cars.

To clarify: Flex-fuel vehicles can safely use whatever percentage of ethanol that they’re certified for. Using low ethanol gasoline, such as E10 or less, probably won’t do them much harm either.

Can Old Cars Use Ethanol?

Generally, old cars should avoid ethanol gasoline as much as possible. However, the dose makes the poison, and low levels of ethanol usually don’t do an appreciable amount of damage. However, years of frequent ethanol use can wear out an old motor under the right circumstances.

If you drive a classic car, especially one designed for leaded gasoline, avoid ethanol like the plague. Running one of these old engines with unleaded fuel is hard enough on the valve seats—and dumping ethanol in the mix would be like striking them with a hammer.