What Ethanol Does to Small Engines—And how to Prevent it

That's why the mower that ran perfectly when you parked it in November may refuse to start in March.

Nothing necessarily happened to the engine while it was sitting. What changed was the fuel.

That's an important distinction, because ethanol gets blamed for almost everything that goes wrong with stored gasoline. Some of that reputation is deserved. Some of it oversimplifies what is actually happening.

E10 gasoline is approved for use in most modern small engines. The real problem is that a lawn mower, generator, trimmer, chainsaw or pressure washer uses gasoline very differently from your car. Your car might burn through a tank in a week or two. The gasoline in your mower may sit there for months.

And gasoline—whether it contains ethanol or not—doesn't stay the same while it sits.

Here's what ethanol changes, why small engines are particularly vulnerable to problems with stored fuel, and what you can do to keep those problems from showing up the next time you pull the starter cord.

Is ethanol gas bad for small engines?

Not exactly—and this is where it's useful to separate two different questions.

The first is whether a small engine can run on gasoline containing ethanol. For modern equipment designed for E10, the answer is generally yes. Briggs & Stratton, for example, approves gasoline containing up to 10% ethanol in its small engines.

The second question is whether ethanol-blended gasoline is ideal for fuel that may sit unused for long periods of time.

That's where the answer becomes more complicated.

Gasoline changes as it ages. Its more volatile components evaporate, oxidation reactions occur, and gums and deposits can form. Add ethanol to the fuel and you introduce another consideration: ethanol has an affinity for water.

A car that is driven regularly keeps turning over its fuel supply. A lawn mower that gets parked for the winter doesn't. Neither does the emergency generator that may go months without actually needing to run for an extended period.

So the problem isn't simply "ethanol is bad for engines."

It's that small engines combine ethanol-blended gasoline with exactly the kind of storage conditions that give fuel-related problems time to develop.

There's another distinction worth knowing. E10 and E15 are not interchangeable. E10 is commonly approved for modern small engines. E15 is not approved for ordinary gasoline-powered nonroad equipment such as lawn mowers and chainsaws.

So before worrying about anything else, make sure the fuel going into the equipment is fuel the manufacturer actually allows.

What does ethanol do inside a small engine's fuel system?

Ethanol changes gasoline in a couple of ways that matter when you're storing it.

The first involves water.

Ethanol has an affinity for water, which means ethanol-blended gasoline can hold a certain amount of water in solution. Small-engine fuel systems are also vented to the atmosphere, and fuel can be exposed to moisture through storage, handling, fuel containers and changing environmental conditions.

A small amount of water doesn't automatically mean the gasoline is going to phase-separate. That's important. Phase separation happens when enough water gets into ethanol-blended gasoline that the fuel can no longer keep the ethanol and water uniformly mixed with the gasoline.

When that point is reached, the problem changes dramatically.

The second issue is ethanol's solvent properties.

In modern equipment built with ethanol-compatible fuel-system materials, this is much less of a concern than it was with older equipment. But older fuel lines, seals, gaskets, diaphragms and some other materials may not tolerate prolonged exposure to ethanol as well.

Ethanol can also loosen deposits that accumulated during years of operation on straight gasoline. Moving the deposit doesn't make it disappear. It can simply move the problem downstream until the loosened material reaches a filter, jet or another small fuel passage.

And small carburetors contain some very small passages.

That's one reason a relatively modest amount of gum, corrosion or debris can cause such an outsized problem in a lawn mower or chainsaw. It doesn't take very much contamination to interfere with the amount of fuel passing through a carburetor jet.

What is phase separation, and why does it hit small engines hardest?

Phase separation is probably the most misunderstood part of the ethanol discussion.

Ethanol and gasoline normally remain blended together, and the ethanol can keep a certain amount of water suspended in that mixture.

But there is a limit.

If enough water enters the fuel, the ethanol can preferentially associate with that water and the resulting ethanol-water mixture can separate from the gasoline. Because that mixture is denser than gasoline, it settles to the bottom.

Now you've gone from having a small amount of water dispersed through the fuel to having two distinctly different layers in the tank.

That's a much bigger problem.

And this is where the design of many small-engine fuel systems works against you.

The equipment may sit unused for weeks or months. The carburetor bowl holds a small reservoir of fuel the entire time. Water and ethanol-rich material can collect in low areas of the fuel system, where it can contribute to corrosion and leave the engine trying to run on something very different from the gasoline it was designed to burn.

Meanwhile, the gasoline left above the separated layer has changed too. Because some of the ethanol has been pulled out of it, its composition and octane characteristics are no longer necessarily what they were when the fuel went into the tank.

That's why phase-separated gasoline isn't something you fix by shaking the can and hoping it mixes back together.

But phase separation isn't the only way stored gasoline causes problems.

Long before you have an obvious layer of separated ethanol and water, aging gasoline can lose volatility and form oxidation products, gum and varnish. Those deposits can restrict carburetor passages and make an engine difficult or impossible to start.

So when somebody says, "The ethanol ruined my carburetor," there may actually be several things happening at once.

How do you know if stored fuel has caused a small-engine problem?

Timing is one of the first clues.

The mower ran normally the last time you used it. Then it sat for several weeks or an entire season, and now it won't start—or it starts but doesn't run correctly.

That doesn't prove ethanol caused the problem. But it should put the fuel system high on the list of things to check.

Common fuel-related symptoms include hard starting, surging or hunting, stalling under load, an engine that will only run with the choke partially closed, or an engine that starts and then dies.

Old gasoline may have a noticeably stale or varnish-like odor. Fuel may appear darker than fresh gasoline, cloudy, or visibly separated into layers if significant water contamination and phase separation have occurred.

The carburetor can provide additional clues. Gum or varnish deposits can restrict small metering passages. Water can contribute to corrosion of aluminum and other metal components. Older seals, gaskets or fuel-system materials may also show deterioration.

But don't make the opposite mistake and assume every no-start after winter storage is automatically an ethanol problem. A fouled spark plug, ignition problem, clogged air filter or other mechanical issue can produce some of the same symptoms.

The point is to check the fuel before you start replacing parts.

If the fuel has been sitting for months, looks questionable or smells noticeably stale, you're looking at one of the easiest potential causes to eliminate first.

How do you prevent ethanol damage in small engines?

The best time to deal with stored-fuel problems is before the fuel goes into storage.

Start with fresh gasoline. Buy roughly the amount you expect to use rather than keeping a large supply sitting in a gas can for months. Briggs & Stratton recommends buying fuel in quantities that can be used within about 30 days because gasoline can begin deteriorating in that amount of time.

If the fuel is going to sit longer than that, treat it while it's still fresh.

That's an important point. A fuel stabilizer is much better at keeping good gasoline good than it is at turning badly degraded gasoline back into fresh fuel.

For ethanol-blended gasoline used in mowers, generators and other small equipment, Ethanol Defense is designed to address the storage problems associated with ethanol-containing fuel while also helping control oxidation and fuel degradation. The treatment rate is 1 ounce per 10 gallons.

But treating the fuel in the tank is only half the job.

Once you've added the treatment, run the engine for several minutes.

That pulls the treated gasoline through the fuel line and into the carburetor. Otherwise, you can have stabilized gasoline in the tank while the carburetor bowl—the part of the fuel system most likely to give you trouble after storage—is still holding untreated fuel.

For seasonal storage, follow the equipment manufacturer's storage procedure as well. Some manufacturers recommend storing equipment with a full tank of properly stabilized fuel; other equipment may have different instructions. The owner's manual gets the final say.

The larger principle is simple: don't put questionable gasoline away and expect it to become better while it sits.

Start with fresh fuel, use the correct ethanol blend, treat it before storage, circulate the treated fuel through the system, and keep the equipment stored in a dry location with as little unnecessary temperature variation as practical.

Here's how Bell's gasoline treatments map to small-engine use.

If you're weighing one treatment against another, Bell's guide to how to buy a fuel additive to treat ethanol walks through what the claims on the shelf actually mean.

Is ethanol-free gas worth it for small engines?

For equipment that spends a lot of time sitting, ethanol-free gasoline has a real advantage: it removes the ethanol-related water and phase-separation issue from the equation.

That doesn't make ethanol-free gasoline immune to aging.

Gasoline still oxidizes. Its lighter, more volatile components can still be lost during storage, and gums and deposits can still develop over time.

So ethanol-free doesn't mean "good forever."

What it does is remove one variable from a fuel-storage situation that already has plenty of them.

Whether that's worth the additional cost depends on the equipment and how you use it. If you burn through gasoline quickly during mowing season, fresh E10 that meets the manufacturer's specifications can work perfectly well.

If you're talking about a generator that may sit for months, a snowblower that gets used only part of the year, or a piece of equipment that's notoriously difficult to clean when the carburetor gums up, paying more for ethanol-free gasoline may make sense.

For most people, though, there is another perfectly reasonable option: use fresh E10, treat it appropriately when it's going to be stored, and don't leave untreated gasoline sitting in the equipment indefinitely.

What you should not do is substitute E15 because it's cheaper at the pump.

EPA specifically prohibits E15 in ordinary gasoline-powered nonroad engines and equipment, including lawn mowers and chainsaws. The same restriction applies to equipment such as boats and snowmobiles.

And don't assume the octane number tells you the ethanol content. Fuel marketed as Unleaded 88 is commonly E15. Read the ethanol label on the pump before filling a gas can for your small equipment.

Protect the fuel before it becomes a carburetor problem

Most small-engine fuel problems are easier to prevent than they are to repair.

That's especially true because the equipment most vulnerable to them is often the equipment we expect to sit. A mower waits through the winter. A snowblower waits through the summer. A portable generator may wait months for the one day you really need it.

During all that time, the fuel is still changing.

The answer isn't to be afraid of E10. It's to recognize what you're asking that gasoline to do.

Use the right fuel for the equipment. Start with fresh gasoline. If it's going to sit, treat it before storage and run the engine long enough to get the treated fuel all the way into the carburetor.

Ethanol Defense treats ethanol-blended gasoline at 1 ounce per 10 gallons and is designed to help control the fuel-storage problems that show up when gasoline sits.

Because the cheapest carburetor repair is still the one you never have to make.