Gas Additives Can Leave You Sputtering

Chris Cornell of Power Cruising says ethanol is bound to leave some gas-powered engines sputtering. A special report from our June 2007 issue

March 19, 2009

****| |Outboard engine owners need to pay attention to how much ethanol has been added to the gas they’re buying for their tenders. Any more than 10 percent can have a big effect on the performance of most small marine engines.| Because dinghies are almost certainly powered by two- or four-stroke outboards that live on a diet of gasoline, you should be concerned about the ethanol or MBTE (methyl tertiary-butyl ether, the chemical compound used in gasoline as an octane enhancer) that’s added to fuel these days-and the damage it can do to tanks and fuel systems. And if your boat’s still pushed by an old Atomic 4 or other gas engine, you’ll want to keep an eye on your fiberglass fuel tank (if you have one) and on hoses and seals that could fall prey to these fuel supplements.

Let’s first consider the implications of using blended fuels in your outboard engine. The performance of an outboard is critical when you’re facing long runs to and from a dock at the other end of a huge anchorage like, say, New Harbor, at Rhode Island’s Block Island. Do your chances of being stranded increase with so-called “gasohol”?

Here’s the good news: As you’ve probably already discovered, your outboard is likely to perform pretty well on the 10-percent ethanol blends that are available at most pumps. (Sailors in regions that still permit MBTE blends generally report similarly satisfactory performance.) Scientific evidence supporting this, though, is surprisingly hard to find, with most of the research coming from Orbital Engine Company of Australia.


Thanks to John McNight, director of environmental and safety compliance for the National Marine Manufacturers Association, I downloaded and printed several of the Orbital Engine reports and other ethanol/MBTE information that is available at NMMA’s website ( You can do the same by logging on, scrolling down to the folders, and clicking on “Ethanol-Based Fuels.”

In 2003, Orbital tested 10 two-stroke, 15-horsepower Mercury outboards, which are fairly typical of the outboards for the bigger inflatables currently popular among serious cruisers. The problems they found with carefully formulated 10-percent ethanol blends (more on this point later) were generally limited to intervals of low (trolling) speeds and to instances of wide-open-throttle acceleration after extended periods of low-speed use. Stalling and misfires were more common under these conditions than when nonblended unleaded gasoline was used. Orbital research found that adding ethanol to gasoline increases the available oxygen for the combustion process but that the engines will tend to run lean, with the fuel affecting “cold-starting, hot-operation, cold-operation, and wide-open-throttle performance.” Clearly, however, such problems can generally be corrected by adjusting the rich/lean mixture on the engine.

So other than experiencing some minor, if disconcerting, crankiness, you’re home free with 10-percent ethanol blends, right? Not quite. Orbital tested new, 2002 outboards, but a lot of us still have older motors. According to manufacturers such as Mercury Marine (www.mercury, older engines are a different story, and their fuel lines and gaskets can be attacked by additives like ethanol and MBTE.


“If an engine is a 1990 or older model,” suggest technical reports on the Mercury Marine website, “frequent inspections of all fuel-system components are advised to identify any signs of leakage, softening, hardening, swelling, or corrosion. If any sign of leakage or deterioration is observed, replacement of the affected components is required before further operation.”

There’s other bad news, too. Fuel blended with MBTE-and even gasoline without it-leaves a coating on the interiors of fuel tanks. Alcohol-based ethanol, though, is a particularly effective solvent for that “varnish,” so when you make the fuel switch, particulates (read “sludge”) can mix with the new gas and find their way into your outboard. There, such contaminants can clog fine jets and other mechanical components, causing rough running and even outright damage.

Cleaning out a portable tank that’s held an MBTE blend-by vigorously swishing around a small amount of ethanol blend (which must then be disposed of properly!)-before filling it with the new mix can help. Even more important is installing a 10-micron filter in the fuel line. Note to self: Manufacturers recommend carrying plenty of spare filters.


Also, remember that not all fuel filters are alike. Some will only intercept particulates from your boat’s fuel tank; others-called water-separating types-also trap water. You definitely want the more sophisticated filter, because ethanol readily attracts and absorbs moisture from the air, and water is highly corrosive to metal components inside your outboard.

Moreover, ethanol blends not only attract and absorb water; they’re also capable of spitting it back out, so to speak. Once the fuel’s saturation point has been reached, phase separation occurs: The ethanol and water separate from the gas and can collect at the bottom of the tank. If this mixture makes it to your engine, it can cause significant problems.

Phase separation is more likely to occur in tanks that sit idle over the winter or for long periods of time. Experts recommend that you top off or completely empty any portable tank before putting it to bed. And because ethanol has a shelf life of only 60 to 90 days, if you aren’t planning on using your dinghy frequently, try buying gas in smaller quantities so it gets replaced more often.


The owners of four-stroke outboards should pay particular attention to this. Says Lincoln Davis, a Maine-based certified outboard mechanic with 40 years of experience, “Most four-stroke engines are carbureted, and phase separation can actually occur in the carburetor, which is very susceptible to corrosion. And four-strokes have much smaller fuel jets that clog far more easily with the particulates so typical of tanks that are switched over to ethanol/gas blends.”

Challenging as all this may be, any of these protection strategies can be tossed out the window if the concentration of ethanol in the fuel increases beyond 10 percent, the maximum blend that outboard manufacturers say their engines can tolerate. Indeed, the trials conducted by Orbital Engine Company clearly demonstrated a sharp rise in the incidence of operational problems when 20-percent-ethanol gas was used in its 15-horsepower test engines.

But enough on outboards. Perhaps you have an older cruising boat still pushed along by a gas-powered iron genny. For advice on the impact of ethanol on these engines, we turned to the Internet’s most authoritative site on the Atomic 4 engine, Moyer Marine (www. moyer, which supplies parts, service, and advice to owners of Universal engines. Don Moyer has forgotten more about Atomic 4s than the rest of us will ever know.

On the company’s website, Moyer dutifully reports, “Other than a few occurrences, reported in carburetors during the last year or two, of a ‘mystery muck’ that might be attributed to ethanol fuel, we have no reports from any Atomic 4 owners to indicate that anything within the engine is at risk when a change is made to ethanol fuel, including rubber fuel lines and carburetor internals.”

But like the outboard mechanics and manufacturers with whom I spoke, Moyer notes that the tendency for ethanol to create problems is greatly increased in the presence of water and further heightened when ethanol fuel is added to a tank containing leftover MTBE blend.

Whereas fuel tanks for “portable” outboards are virtually always plastic, those aboard older cruising sailboats may be made of corrosion-prone aluminum, mild steel, or even fiberglass, which is particularly susceptible to degradation in the presence of ethanol. As Moyer notes, a 2005 study by BoatU.S. (see news/releases/2005/otctober/alerts.asp) has indicated that ethanol has a tendency to break down some fiberglass fuel tanks to the point of creating leaks.

Moyer offers some good news, however: A poll of his customers reveals that only approximately 4 percent of Atomic 4-powered sailboats have fiberglass fuel tanks and that fewer than 1 percent of owners reported problems.
He recommends that owners continue to check fiberglass fuel tanks regularly for any sign of leakage or fuel odor. If the tank is accessible, a leaking fiberglass tank can be lined with ethanol-resistant epoxy; otherwise it should be replaced. And owners should install a high-quality water-separating fuel filter.

Moyer also urges sailors to monitor the gas they buy and to avoid ever mixing ethanol and MTBE fuels.
If you take these steps, perhaps you’ll avoid gas pains this sailing season.

Chris Cornell, the managing editor of Power Cruising, has a boatbuilding background.


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