Features November 2008 Issue

Bench Tests of Five Ethanol Gas Additives Yield Surprising Results

None of the five products tested were capable of preventing phase separation, one of the most troublesome problems associated with ethanol gas.

Since the introduction of ethanol fuels at dockside pumps, we’ve had a slew of reader inquires about the effects of E-10 on fuel lines, fuel tanks, two-stroke engines, and four-stroke engines. We recently tested some products designed to address these issues, specifically those that claim to combat the problems associated with phase separation in E-10. Practical Sailor tested: E-Zorb from Marine Development Research Corp. (MDR), Sta-bil Marine Formula Ethanol Treatment from Gold Eagle, PRI-G from Power Research Inc., Star Tron from Star brite, and Techron from Chevron. The claims of each product varied, but the test products fell into one of these general categories: conventional fuel storage additives, ethanol fuel storage additives, and restorative gas additives. Our tests looked at whether the additive itself would leave ash deposits, gum deposits, or residue that might cause contaminated lubrication oil; whether an additive had a tendency to emulsify, suspend, or absorb free water; and whether the additives could delay the onset of cooling-induced phase separation.


Ethanol Gas Additives
Photos by Drew Frye

All five of the products Practical Sailor tested claimed limited protection from the potential ill effects of using ethanol fuels. In the foreground are the additives mixed with water. All separated the water quickly, except for E-Zorb, which is heavier than water and makes a lumpy mixture. All of the additives dissolved in gasoline, though E-Zorb required vigorous mixing.
In the January 2007 issue, Practical Sailor looked at some of the problems caused by the introduction of 10-percent ethanol (E-10) blends at dockside pumps. Two of the most serious concerns were: The fuel acts as a powerful solvent, even dissolving some fiberglass tanks; and phase separation, when water in the fuel separates out of the fuel mix and drops to the bottom of the tank, where it can be sucked into the pickup feed or promote corrosion within the tank.

Not surprisingly, a variety of additives claiming to cure or prevent the E-10 ailments are popping up in marine stores. As with many of the existing fuel additives, the claims of these newer product are often difficult to prove. In fact, Practical Sailor was not able to confirm one of the key claims (either inferred or explicit) in most ethanol additives: They will prevent phase separation. But before we delve into these E-10 "miracle cures" and Practical Sailor’s recent tests, a bit of background is in order.

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.

In places where daily temperatures do not vary sharply (Florida, for example) this may not be a serious problem, but around the Great Lakes and in the Northeast, a fall cold front can result in phase separation in as little as a week.

A side effect of phase separation is that it lowers octane by removing ethanol. However, because the octane stripping is incomplete, the fuel’s octane level has been documented to drop only 1-1.5 units. (About 2.8 units of the E-10 octane rating is due to 10-percent ethanol addition.) While not desirable, this octane stripping alone, over the short term, should not lead to any serious running problems in most marine engines. (Some additive makers contend the octane stripping is more complete, and thus its effects are more severe.) Apart from this octane stripping, there is no other effect on combustion. Storage stability with regard to oxidation and any gum buildup is the same as MTBE gasoline.

What We Tested

This Practical Sailor comparison focuses primarily on products claiming to combat the problems associated with phase separation in E-10. Aiming to include a cross section of what’s on the market, Practical Sailor tested five products from five manufacturers: E-Zorb from Marine Development Research Corp. (MDR), Sta-bil Marine Formula Ethanol Treatment from Gold Eagle, PRI-G from Power Research Inc., Star Tron from Star brite, and Techron from Chevron. The claims of each product varied, but the tested products fell into one of four general categories:

Conventional fuel storage additives. These classic stability additives are designed to extend storage time beyond what the minimal additive doses in standard gasoline can deliver. They do not explicitly claim to prevent separation. Sta-bil Marine Formula Ethanol and Techron are examples of this type of product.

Ethanol fuel storage additives. These products claim the above, plus limited improvement to phase stability of E-10. They do not claim the ability to restore separated fuel. PRI-G and Star Tron from Star brite are examples, but it should be pointed out that neither product claims to deal with the extreme — though not unrealistic, in our opinion — case of cooling induced phase separation simulated by our test.

Restorative additives. These are not storage additives; they are designed to prevent separation and to restore separated fuel. E-Zorb is the only product we tested that claims to restore separated fuel. (MDR also makes a separate storage additive,
X-TEND, which was not tested.)

Testing Challenges

Considering the range of standard tests available, the Practical Sailor evaluation is limited, focusing principally on ash residue, emulsion breaking, and phase separation stability. A full explanation of the testing appears above, and tables documenting the results appear on the following pages. The protocols were modeled on standard tests recognized by the American Society of Testing and Materials (ASTM).

Some makers worried that the results could be misleading. Star Tron chemists suggested that continuously vibrating the test samples (as a running engine would cause a tank to vibrate) would better demonstrate how the enzymes in Star Tron work. They also contended that our 0.6 percent concentration of water in samples used to simulate overnight phase separation was excessive. Our testers did not believe the protocol was unrealistic, although it is arguably strenuous.

Practical Sailor is researching the possibility of an onboard field test that would produce reliable data. This is a nut that even the fuel specialists have yet to crack. Here are some of the other fuel properties that Practical Sailor did NOT test:

Gas line freezing. Since phase separated material contains too much ethanol (40 to 85 percent) to freeze, this function is obsolete.

Fuel oxidation and detergency to prevent gum and varnish formation. Unlike lube oil, refined gasoline is inherently unstable and wants to polymerize, so the refiner applies more additives to stabilize gasoline. These additives also add detergency, reducing engine and fuel tank deposits. They are not intended to clean tanks or emulsify water, but can help prevent dirt and water from attaching in the first place.

Many additives at the retail level mimic the same chemistry used by refiners; polyisobytlyene amine and polyether amine are common active ingredients. The Sta-bil, Techron, and PRI-G test samples contained this or related chemistry. Star Tron uses an enzyme chemistry.

Effect on two-stroke oil. Many products claim to improve two-stroke performance. We repeated the emulsion breaking tests with a 50:1 dose of TCW 3 Pennzoil two-stroke oil; emulsion breaking was 20 percent slower, but the difference probably would not be noticed in practice.

Bacteria and algae growth prevention. While micro-organisms can thrive in diesel fuel tanks, they’re not known to grow in gasoline tanks.

Certainly, many more questions need to be answered, but the data collected highlighted the differences among the various products tested, and isolated a few that stood out. Here is a closer look at the individual products and how they fared:


MDR says E-Zorb is the only product designed to react with water and ethanol at the bottom of a fuel tank and absorb it back into fuel. Thick and amber in color, E-Zorb stands out because it is slightly heavier than gas and can seek out the phase-separated water at the bottom of the tank. For this reason, it did not mix as easily with the fuel as the others in our test.

The label suggests one ounce of E-Zorb for every 20 gallons of gasoline at each fill-up to help the fuel better tolerate water absorption. If free water contamination is found in the tank, a 1:1 ratio of E-Zorb-to-
water is recommended to totally emulsify the free water back into the fuel. At the recommended maintenance dose, E-Zorb and all other products tested contributed only insignificant amounts of non-volatile material (material that would not burn during combustion).

However, when E-Zorb is dosed high enough to absorb free water (which it does, just as advertised), the non-volatile contribution
becomes the dominant non-volatile component in the fuel stream. This, in our opinion, could effect lubrication in two-stroke engines over the long haul. This would cause less concern in a four-stroke engine. There is also a chance that salt water could be passed through the fuel system instead of being intercepted by a fuel/water separator.

E-Zorb, like the other tested products, did not measurably inhibit phase separation in our test. E-Zorb is available in 16-ounce bottles.

Bottom line: Practical Sailor does not recommend routinely using the higher dose of E-Zorb to remove phase-separated water, particularly in a two-stroke engine, which could be more vulnerable to any non-volatile residue. Carefully draining or filtering out the water is preferable.


Power Research Inc. develops a range of additives for industrial and marine purposes. PRI-G is a relatively thin, brown liquid that mixes easily with gasoline. It claims only limited protection against phase separation, and is primarily an oxidation stability additive. Users are instructed to mix 1 ounce per 16 gallons of fuel. It comes in 16- or 32-ounce bottles with a built-in measuring dispenser. A 1-gallon bulk jug is also available. PRI-G yielded the worst results of all the additives tested, requiring the longest separation times and yielding cloudy water with an unresolved ragged layer.

Bottom line: Based on the test results, Practical Sailor does not recommend the PRI-G over the other products.

Sta-bil Marine Ethanol

A well-recognized name in the fuel-treatment field, Sta-bil offers a wide range of fuel storage and treatment products. Its Marine Ethanol treatment is a medium-green liquid that mixes easily with gasoline. It recommends 1 ounce per 5 gallons of fuel for storage protection, or 1 ounce per 10 gallons of fuel during the season. The high levels of additives in this product suggest that Sta-bil’s recommended doses are conservative. The product comes in 16- or 32-ounce bottles with built-in measuring dispensers. This product delivered exceptionally clean fuel/water separation.

Bottom line: It doesn’t prevent phase separation (nor does it claim to), but this economical product performed well in testing and comes with a long-standing reputation. It is our Best Choice.

Star Tron

Marketed by Star brite, maker of a wide range of marine maintenance products, Star Tron stands out because it does not use conventional chemical oxidation inhibitors, relying instead on a proprietary enzyme technology. Because it works differently, the makers say, it is difficult to judge using conventional testing methods. This thin, light blue liquid mixes easily with gasoline. The recommended dose is 1 ounce per 8 gallons of fuel for storage protection, or 1 ounce per 16 gallons during the season. It comes in 16- or 32-ounce bottles with narrow necks for easy pouring. Testers found this product to have a remarkably low non-volatile content and exceptionally clean fuel/water separation.

Bottom line: Economical, Star Tron is a safe choice for those concerned about residue. Contrary to claims, Star Tron, like the others, did not prevent phase separation in our test. It is our Budget Buy.


Developed by energy giant Chevron, this thin, clear liquid mixes easily with gasoline. Chevron recommends 20 ounces per 20 gallons of fuel for system cleanup. The dosages for continuous usage or winter storage are not stated, but they would logically be much lower. Adding 1 ounce per 5-10 gallons would seem correct based on our non-volatile test results and product specs. It comes in a 16-ounce bottle with a narrow neck for easy pouring, but has no dispenser.

Bottom line: Techron gave exceptionally clean fuel/water separation. It is expensive as a cleanup dosage, but at a lower maintenance dosage, it would be economical.


In our opinion, the makers and retailers of these additives aren’t doing a good job telling consumers what these products do and don’t do. None of the additives had any measurable effect on the amount of water that separates due to overnight cooling. It is Practical Sailor’s view that any products claiming even limited ability to prevent or delay phase separation should pass ASTM test D6422 using 85-degree, saturated E-10 cooled to 32 degrees—a reasonable worst-case scenario.

So, don’t expect miracles. These blends may reduce maintenance, but they can’t fix material problems.

If you are interested only in oxidation stability and detergency, choose from Sta-bil, Star Tron, Techron, or a similar formula from another respected maker. Testers saw no substantial performance differences.

With its fair price and good separation, Sta-bil Marine Formula Ethanol Treatment is our top choice. Avoid alcohol-based treatments; they are not for E-10.

If you have free water in your tank or phase-separated material, remove it. Practical Sailor strongly recommends fuel/water separators on any boat, even outboards with portable tanks. Practical Sailor does not recommend using an emulsifier to re-suspend free water.Although water emulsions have been burned in controlled settings, the fuel tank of a boat is different.

Practical Sailor does not recommend using E-Zorb to dry a wet tank. (MDR contends years of experience with a similar product, Water Zorb, do not support our testers concerns regarding non-volatile residue.) Drain the free water or use filters and separators instead. This is not a concern at the recommended maintenance doses of 1 ounce to 20 gallons. Testers did not like PRI-G’s tendency to form discolored water emulsions, hazy fuel, and a ragged interface.

Whenever possible, refill your tank after each use, particularly in the latter half of the season when temperatures are dropping.

For winter storage, the logical approach is a full tank and treatment with an oxidation prevention additive; this minimizes evaporation and exposure to oxygen and humidity. A full tank of E-10 should not absorb enough water to phase separate within nine months, nor admit enough oxygen for refinery-treated fuel to oxidize or form gum. If you drain the tank, it must be completely dry. A near-empty tank in a cold climate will surely phase separate.

Some yards have reported success storing partial tanks and then topping off with high test in the spring to compensate for any octane stripping. This approach might work in warm climates with little temperature change, but in cooler climes, the half-full tank could promote phase separation. Either way, take it easy the first day and top-off with a few gallons of high-test if a small loss in octane is a concern.

One final note: If you want to check the stability of your existing fuel, place a small glass jar of the fuel in a bucket of ice water for one hour. Though minor haze is not grounds for rejection, any free water hints of trouble in the spring.

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