Storage Tips for Gasoline and Diesel

Posted by at 05:36AM - Comments: (2)

The test lineup (from left): Sea Foam, ValvTect, PRI-G, Mercury Quickstore, Sta-Bil, Star brite Star Tron, Biobor EB, and CRC Phase Guard 4.

If you're putting your boat into storage this winter, one of the simplest jobs you can do to save you headaches next spring is to make sure you’ve treated your fuel system for storage.

For owners of diesel engines, you want to protect your tank against biological growth—primarily fungus and bacteria. For owners of gasoline powered boats—the bad side-effects of ethanol fuel are your chief enemy.

Our test of biocide treatments safe for diesel fuel appeared in July of 2009, with Biobor, Racor, Starbrite, and Valvtect all rating well.

And for those with gasoline engines, the problems with ethanol (see below) require a specialized product. The August 2012 has an update on our test of ethanol fuel additives for marine engines. BioborEB again proved its superior ability to fight corrosion. Sta-Bil Marine Formula, Star Tron, and Mercury QuikStor (the most expensive treatment, by far) picked up recommendations. Sea Foam came in as the low-budget contender.

Ethanol has added a whole new twist to the fuel storage game, and since many of our readers are still rely on good ol’ Atomic Four gas engines, and most of us have outboards to care for, I’ve included some relevant background information on the problems with E-10 fuel and why we need to take fuel storage seriously.

Phase Separation

For many years, methyl tert-butyl ether (MTBE) was the oxygenate of choice for reformulated gasoline. MTBE was very soluble in gasoline, forming a stable blend with physical properties consistent with conventional gasoline. MTBE was also found to contaminate groundwater. For this reason, it was removed from the nation’s reformulated gasoline pool.

Switching to E-10 brought some unintended consequences, not the least of which was the process called cooling-induced phase separation, a process by which typically 1 to 3 percent and potentially 8 percent of the fuel can precipitate to the bottom of the tank. Here’s how it works:

Ethanol is soluble in gasoline only if the alcohol is very dry. Unfortunately, the mixture is hygroscopic, drawing water from the air. In addition, the blend can become unstable when the temperature drops with the change of seasons. During the day, as the temperature rises, ethanol’s vapor pressure increases markedly. In fact, E-10 begins to boil at a mere 95 to 105 degrees Fahrenheit. (MTBE gasoline, by comparison has a far more stable vapor pressure.) These rapid changes in vapor pressure effectively act as a pump, drawing more humid air into the tank every night.

In addition, E-10 is an active absorbent. A half-full tank will become moisture saturated within 30 to 90 days, depending upon the temperature range and humidity. (Unless there is a leak, MTBE can collect water only from condensation, which is usually caught by a fuel/water separator.)

In E-10, the solubility of water decreases with temperature—0.6 percent is soluble at 85 degrees, but only 0.4 percent at 32 degrees—and when the solubility drops due to cooling, a portion of the water drops out of suspension, along with six times its volume in ethanol.

Tips for Storage of Ethanol Fuel

  • • Run only the highest-octane gas for your last tankful of the season, burning as much E10 out of the tank as safely possible to negate ethanol's affinity for water absorption.
  • • If completely emptying the tank is not practical (empty tanks can increase fire risk), then fill the tank with non-E10 gas, leaving 10 percent room for expansion.
  • • Carefully stabilize the remaining fuel (check engine manufacturer for recommended stabilizer) and run the engine long enough so the stabilized fuel can circulate through the entire fuel system.
  • • When commissioning, fill the tank with high-octane fuel. Check fuel- water separators consistently. Have spare separators on board.

Comments (2)

Since I performed the testing, a few corrections:

* High octane fuel makes no difference unless the engine requires it due to high compression ratios. Not something that is relevant to sailboats. The ethanol content is the same.

* Water separators do not work with e-10. I have had Raycors on several boats and I know this for a fact. If the tank separates, it will always overwhelm the separator, if it does not separate, is is disolved and will pass through. Unlike diesel, if the gas does not separate, the separator will never, in the life of the boat, collect any water. It just re-dissolves.I do like the extra filtration.

* A silica gel vent filter will eliminate water absorption due to breathing. (PS January 2013)

*Closing the vent on portable tanks and integral small outboard tanks when not in active use prevents breathing and is very effective.

* Vent filters preserve volitiles. While this is good for smog, it also has two very practical benefits: gum formation is reduced (more solvent, less oxygen) and starting is made easier (more volitiles).

A few months ago I opened my 6-year old outboard carbs for inspection. Typically there would be some pitting and some graying of the aluminum alloy. In fact, they were as clean and shiny as new foil. I credit the vent filter and Biobor EB for stopping corrosion dead. With proper fuel management, there is no need to avoid e-10.

Posted by: Drew Frye | December 15, 2016 11:26 AM    Report this comment

If you can find gasoline without ethanol use it instead. Of course only a very few have gasoline without ethanol and the price around here is much more. Learn to raise your sails and maneuver in closer quarters.

Posted by: donnymac | October 17, 2012 11:11 AM    Report this comment

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