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Burning Biodiesel Fuel: Biodiesel Quality

Noticeable Differences Between Diesel and High Quality Biodiesel

Aside from the noticeable benefits (smoother running engine, and less noxious exhaust) from using high quality biodiesel, there are a few issues to be aware of when running biodiesel instead of petrol-diesel. The most common one that people notice is the difference in gel points of the fuels.

Standard diesel tends to gel at around zero degrees F. Biodiesel fuel can gel at temperatures up to 75 degrees F, depending on the feedstock from which it was made. As a very general rule of thumb, biodiesel tends to gel about 30 degrees F lower than the feedstock from which it was made. Therefore, biodiesel made from highly saturated fats such as beef tallow will have a much higher gel point than biodiesel made from unsaturated oils such as canola oil.

A problem associated with gel point is the cold filter plugging point of the fuel. This problem is quite common among those using waste oil from restaurants. The majority of the feedstock is usually nice vegetable oil but there is most often some saturated fats mixed in. After this mixture processes into biodiesel, the biodiesel made from the saturated fats will often solidify into very small crystals as the fuel cools off. These little crystals will give the fuel a cloudy appearance. As the fuel filter of your vehicle strains out these small pieces of solidified biodiesel, it can quickly clog, which will result in a dramatic loss of power.

On many engines, the fuel filter is located very close to or even on top of the engine block. As a result the filter will stay warmer and the solidified biodiesel will melt as it gets caught in the filter. Consequently vehicles with this filter arrangement often have little or no trouble burning this cloudy biodiesel. In other vehicles, the fuel filter is located further from the heat of the engine, allowing it to plug with even slightly clouded fuel.

What can you do?

One strategy for dealing with the cloudy biodiesel is to let it sit still in a vessel for about a week. The solidified biodiesel is slightly heavier than the liquid, and will slowly settle to the bottom. The clear biodiesel may be drawn off of the top and utilized while the gelled fuel at the bottom may be burned during warmer weather when it melts.

Another strategy is to devise a way of preheating the biodiesel before it reaches your fuel filter. Arctic Fox and Racor are companies that provide a number of such cold weather products. Please ask your dealer about fuel preheaters that they may carry.

Other biodiesel users simply prefer to blend their biodiesel with diesel fuel #2 to thin it down to allow it to more easily pass through their fuel filter. This may be done in any ratio the customer desires and with no ill effects. In the industry, the percentage of biodiesel in a fuel is often designated by a B, followed by the percentage. For example, a mix of 35% biodiesel and 65% standard diesel would be called B35.

Finally, there are a number of fuel additives such as Lubrizol and Wintron that claim to reduce the gel point and cloud point of biodiesel. We cannot verify their effectiveness, as we have not yet tested them. Their effectiveness is discussed in several of the resources mentioned in the Insightful Links section.

Storing Biodiesel

Standard diesel #2 will often build up fungus or algae growth when stored for a period of months. Because biodiesel is nontoxic, it is naturally more susceptible to such growths. Fortunately, there are numerous biocides on the market to help preserve the life of stored fuel. A good way to distinguish if fuel is cloudy from fungus or cloudy due to saturated fats is to heat up a sample. If it stays cloudy even after being warmed, it is likely that there is some contaminant such as fungus or water.

It is important to note that even high quality biodiesel has some solvent properties. As such, it is often able to scour out deposits that have formed in engine components that have been using #2 diesel for a number of years. These deposits will end up in the fuel filter of the vehicle. It is a wise practice to change out a fuel filter after running the first couple tanks of biodiesel.

Another result of the solvent properties of biodiesel is the slow degradation of natural rubber in fuel lines. After months (or often years) of carrying high percentages of biodiesel, some fuel lines may begin to swell and weep. These are easily replaced. If they are replaced with Viton or another synthetic polymer used in modern diesel vehicles, (1993 or later) they will be permanently chemically resistant to biodiesel.


Most auto manufacturers only warranty very low blends of biodiesel in their vehicles (5-20% blends). This has caused some consumers to mistakenly assume that using biodiesel will void their vehicle’s warranty. This is not the case due to the Magnuson-Moss Warranty Act which was passed by Congress in 1975.

This act is applicable to burning biodiesel in that an auto maker cannot, by law, refuse to honor a warranty if their product has a problem not directly caused by burning biodiesel. If a person burns dirty biodiesel, and this directly causes a problem with their car, then the auto manufacturer may refuse to honor the warranty in this instance.

Similarly, if a person burns poor quality diesel fuel #2, directly from a fueling station pump, and it directly causes a problem with their vehicle, then the auto manufacturer may refuse to honor the warranty in this instance as well.