Most Diesel Engine Failures
Originate In The Fuel Tank

The Real Story of Bad Diesel Fuel

Fuel FiltersFrequent diesel fuel filter changes and the expensive and time consuming task of cleaning diesel fuel tanks have become acceptable periodic maintenance instead of a warning signal for diesel engine failure. Diesel fuel filter elements should last a thousand hours or more, and injectors some 15,000 hours. However, since diesel fuel is inherently unstable, solids begin to form and the accumulating tank sludge will eventually clog your diesel fuel filters, ruin your injectors and cause diesel engines to smoke.


  Clogged and slimy filters
  Dark, hazy fuel
  Floating debris in tanks
  Sludge build up in tanks
  Loss of power and RPM
  Excessive smoke
  Corroded, pitted injectors
  Foul odor

The solids that form as the result of the inherent instability of the diesel fuel and the debris formed in the natural process of fuel degradation will accumulate in the bottom of your fuel tank. The sludge will form a coating or "bio-film" on the walls and baffles of the fuel tank, plug your fuel filters, adversely impact combustion efficiency, produce dark smoke from the exhaust, form acids that degrade injectors and fuel pumps, and impact performance. Eventually fouled diesel fuel will clog fuel lines and ruin your equipment.

Filter plugging, often the first symptom of a problem, can have several causes and often critical consequences. For example, low temperatures can cause wax crystallization, which can lead to fuel filter plugging. An example would be using untreated summer diesel fuel in cold weather. Wax or paraffin is part of the diesel fuel. This can be quickly be treated by changing to winter grade fuels or by using an additive made to lower the fuel pour-point and improve cold flow filtration properties.

Contaminant build up resulting from excessive microbial growth and bio-degradation of diesel fuel can cause fuel filter plugging without regard to temperature. Micro-organisms, bacteria and enzyme activity, fungus, yeast and mold cause diesel fuel degradation and the formation of waste products. The process is similar to milk turning into cottage cheese, a different form of milk. (To understand the "Rest of the Story about microbial and bacteria in fuel, CLICK HERE.) Of all the microbial debris and waste products in the diesel fuel tank only about .01% is "bugs". Even though microbes may cause and accelerate the process of fuel degradation, it should be clear that the waste products clogging your filter are not the microbes but fuel components which have formed solids. These waste products can float in the fuel, being visible when looking into a fuel tank, but also coat the sides, bottom, baffles, and even top of the tank - wherever the fuel touches. Most service personnel are quick to recommend the application of a biocide product to the fuel to address the problem. Unfortunately, by the time you realize you have a problem, the effectiveness of biocides are limited. If a heavy bio-film has accumulated on the inside surface of the tank or other fuel system components, the biocide may not be able to penetrate to the organisms living deep within the film. You may see short term relief, but the problem quickly returns as the remaining organisms further reproduce.

Frequently, the application of a biocide aggravates the situation and turns bio-film into solids, creating a real fuel filter nightmare. As a further complication in the use of biocides, the removal of water from a tank bottom that was treated with biocides must be disposed of appropriately because biocides are toxic.

Bio-film develops throughout the entire diesel fuel system. As most organisms need water to grow, bio-growth is usually concentrated at the fuel-water interface in the bottom of the tank. The organism colony feeds from nutrients in the fuel additives. It takes time for the organisms to grow to produce enough acidic byproducts to accelerate tank corrosion or biomass sufficient to plug filters, so a problem may not show itself for months. If you have a problem now, its genesis was likely 6 or months ago!

Keeping the water out of the tank is the first and most important step. Water often enters the tank through badly fitting or missing fuel caps. But even with good housekeeping, water still gets into tanks on a regular basis. Fuel used is replaced by air drawn in through vents. This air often brings moisture with it. Temperature fluctuations cause droplets to condense on the inside walls of fuel tanks and accumulation of water over time provides the habitat for the organisms to reproduce. Also, an unlucky end user may be filling up his fuel tank and getting water, organisms and this debris delivered as a part of the process of taking on diesel fuel, for the same price as the diesel fuel, This is less common than many folks think, but it does happen.

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A separate but equally perplexing problem is the natural degradation of diesel fuel. A diesel engine uses only some of the fuel it pulls from the tank. All of that fuel goes through the high-pressure fuel pump and to the injectors operating under enormous pressure and high temperatures. The surplus fuel the engine is not using goes back to the tank. This fuel is continuously re-circulated and exposed to extreme pressure and heat, which results in the agglomeration of asphaltenes, the high carbon content, heavy end fuel molecules. It leads to the formation of larger and larger clusters and solids, which are very difficult to completely combust. These solids may grow so large that they will not pass through the filter element. They become part of the polymer and sludge build up plugging the filter.

In addition, the hot fuel coming back to the tank will raise the fuel temperature in the tank, cause condensation (water accumulation in the bottom of the tank) and contribute to microbial contamination, fuel break down, bio fouling and the build up of sludge and acid.

Large fuel droplets and high asphaltene concentrations require more time, more energy and higher temperatures to combust than is available in engines during the combustion cycle and before the exhaust valve opens. Any device in the fuel system exposing the fuel to stress (heat and pressure) such as pumps, heaters, or centrifuges will increase the formation of asphaltenes. If you have seen fuel that has turned dark, or almost black, in comparison to clear, bright fuel, then you have witnessed the results of this process. This degraded, dark fuel negatively impacts combustion.

Note that fuel in storage for long periods show this same darkening of the fuel. Most diesel fuel is consumed within a couple months of leaving the refinery, but fuel in storage, such as for back up power generator service or companies or farms that have fuel tanks that receive refills only a couple times per year (or less) suffer from the darkening of the fuel. Over time, depending on the quality of the fuel, and even with vigilance in keeping water out of the tank, the fuel will degrade and turn dark. This happens naturally as the asphaltene components in the fuel that is in solution attracts other asphaltenes and grows in size until the fuel is dark and these components drop to the bottom of the tank. If you have ever seen the inside bottom of an old fuel tank that looks like it was painted with roofing tar, you have witnessed the results of this process.

The diesel fuel of today is not the same as what was available years ago. Prior to about 2006, much of crude oil was refined utilizing a process called distillation. This process provided a fuel that remained stable and fuel degradation was not a common problem. As the use of ultra low sulfur diesel fuel (ULSD) was mandated and phased in in years since 2006, different processes were used to produce fuels, including diesel fuel, referred to as hydrocracking and hydrodesulfurization (check out Wikipedia for additional information). Since this change and the subsequent expansion of mandates to use ULSD in other applications (like off-road fuel and locomotives) the problems with fuel degradation have become much more common in equipment that never experienced these problems before.

Previously, refineries used only about 50% of a barrel of crude oil to make distillates such as gasoline, jet fuel and diesel fuel. The remainder of the barrel of crude oil went to "residual oil" such as lubricating oils and heavy oils. Today, according to the U.S. Energy Information Administration, in 2014 there was 19 gallons of gasoline, 12 gallons of diesel fuel, and 4 gallons of jet fuel produced from 42 gallons (one barrel) of crude oil (83 percent of the barrel of crude).

According to the U.S. Energy Information Administration: "A refinery's ability to upgrade low-value products into high-value products and convert high-sulfur material to low-sulfur material with a secondary unit like a hydrocracker plays a key role in determining its economic fate."

Some believe that the use of cracking units, alkylation and reformer processes are having consequences for fuel stability. Further, some regions of the U.S. also use a percentage of bio-diesel blended into fuels. These changes have made diesel fuel more unstable (higher susceptibility to bread-down) and more apt to separate, contributing to the accumulation of water in the tank (microbial habitat).

Today's advanced engine designs also contribute to diesel fuel reduced stability. Poor thermal fuel stability can plug filters. Fuel will form particulates (solids) when exposed to pumps and the hot surfaces and pressure of the fuel injection system. This will result in an increase in asphaltene agglomerations, polymerization and a dramatic loss of combustion efficiency.

Fuel systems, in general, are designed to return a significant proportion of the fuel not used for combustion back to the tank. This return fuel is very hot and will promote polymerization and fuel breakdown. Eventually, more and more solids from the tank will reach the filter and over time, plug the filter. These problems continuously occur in commercially operated engines, such as trucks, heavy equipment, shipping, and power generation, but will also appear in recreational boats, RV's and all types of fuel storage tanks.

Truck engines are used continuously and, in most cases, the tanks "appear to be clean". However, a 2-micron filter element does not last very long, in general 15,000 miles or less. It should be 30,000 miles or more. In the marine industry filter changes of 400 hours is in many instances standard operating procedure, while filters should easily last 1,000 hours or more.

Short filter life is quite remarkable realizing how "thin" diesel fuel actually is and knowing how clean the tanks on most trucks "appear" to be.

Short filter life is symptomatic of polymerization, increase in the size of the fuel droplet, agglomeration of asphaltenes and the formation of solids in fuel systems. The consequences are carbon build up in engines and exhaust systems, higher fuel consumption and excessive smoke.

The stuff that clogs your filters is actually fuel in some way, shape or form. In excess of 90% of this organic debris are fuel breakdown products. It is not sand, dust, stones, rust or in-organic matter that blocks your filter.

The inorganic material like sand, dust and other particles will not cause your filters to clog. In fact, a lot of sand in a fuel filter would act as extra filtration. The pores between the sand particles are much larger than the pores in a standard fuel filter element. Sand filters are commonly used to filter water. A hair is approximately 40 micron and fuel filter elements range all the way from 30 micron for a pre-filter to 2 micron in a fine filter. (Engine manufacturers typically call for fuel to be filtered to 4 micron to meet their standards due to clearances in the injector components.)

Fuel is an unstable, organic liquid that goes "bad". Your vendor will always sell you the highest fuel quality possible. However, due to a variety of circumstances fuel may have "aged", oxidized and/or may contain water. It may have been contaminated before it was delivered to you or to your vendor.

Fuel has to travel from the refinery to the end user destination. It is pumped through pipelines, barged, trucked and stored in tank farms. Changes in temperature throughout any given day and exposure to the atmosphere will cause condensation and water in storage systems. As a result, your fuel quality diminishes.

When your fuel is finally used, it is exposed to the heat and pressure of engine injection systems, centrifuges, pumps, and heaters causing an increase in asphaltene agglomerations, which negatively impacts combustion efficiency and emissions.

Fuel is made to certain ASTM specifications and diesel engines are designed and built to operate on fuels that consistently meet these specifications. When it does not meet these specifications, we could refer to it as "bad fuel". However, we tend to refer to fuel as "bad fuel" when we see symptoms such as: •dark hazy fuel, •filter plugging, •sludge build up in tanks, •poor engine performance, •excessive smoke & •emissions, •etc. We refer to fuel as "good fuel", when it is clear and bright. Or rather in that case, no reference is made at all to our fuel. We simply use it and take fuel quality and peak engine performance for granted.

Dark fuel is symptomatic of poor quality and even though, in most cases, it can be used, fuel in this condition will provide poor combustion and filtration problems.

"Dark fuel" is in general indicative of oxidation and that the process of fuel degradation is in a far advanced stage. Hazy fuel is indicative of water emulsified in the fuel. In general, dark hazy fuel will not damage your engine. However, it does indicate poor fuel quality which will definitely not provide you with peak engine performance.

As long as fuel meets the (ASTM) specifications, it will perform in your engine. Using fuel that is less than optimal quality negatively impacts engine efficiency and accelerates the process that degrades fuel injectors, causes fuel pump failure, and makes new engines old.

Diesel fuel can range from colorless to amber or light brown in color depending on the crude oil and the refinery process used to produce it. In addition, dyes may be added to change the fuel color for tax identification purposes.

In time, stored fuel will darken due to oxidation, repolymerization and agglomeration of certain components. The darkening is accompanied by the formation of sediment that plugs filters and causes poor combustion. Fuel & Oil vendors know what time can do to fuel quality and suggest that if diesel fuel is stored for emergency use, it should be replaced with fresh fuel within a year, unless special precautions or remedial actions are taken. Some ULSD fuels and fuels containing bio-diesel will not perform acceptably before a year has passed.

The implementation of the AXI Technology (formerly known as Algae-X) eliminates these costly, wasteful, and environmentally unfriendly dumping practices. AXI equipment and systems will preserve fuel integrity almost indefinitely and can help you put in place good housekeeping measures along with a quality fuel-monitoring program.

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