The Real Story of
Bad Diesel Fuel

Most Diesel Engine Failures
Originate In The Fuel Tank

What Are The Signs?

Frequent 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 filters 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 (for more on how diesel injectors can be impacted by degraded and contaminated diesel fuel, see the “Diesel Fuel and Injector Failures” discussion on this web site).


  • Clogged and Slimy Filters
  • Dark, Hazy Fuel
  • Floating Debris in Tanks
  • Sludge Build-up in Tanks
  • Loss of Power and RPM
  • Excessive Exhaust Smoke
  • Corroded, Pitted Fuel Injectors
  • Foul Odor Emitting from Fuel Tank

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 adversely impact performance. Eventually fouled diesel fuel will clog fuel lines and ruin your equipment.

Clogged Fuel Filters

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 (See the Diesel Recovery page for additional information). An example would be using untreated summer diesel fuel in cold weather.  Wax or paraffin is part of the diesel fuel and cold temperature crystallize these fuel components. This can be quickly treated by changing to winter grade fuels or by using an additive made to lower the fuel pour-point and improve cold flow properties (See the AFC Series Fuel Additive AFC-805).

But 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. (For a more in-depth look into what microbial and bacteria contamination is, why it is more of a problem now than in the past, and how to deal with it, see the article: “Why Bacteria Hate Magnets“.) Of all the microbial debris and waste products in your 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.

The Great Downside of Biocides

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. The introduction of biocide into a fuel tank results in the organic materials turning into a grit that settles to the tank bottom. Then, each time you re-fuel, these materials are stirred up and find their way into your fuel line. 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.

Although bio-film develops throughout the entire diesel fuel system, most organisms need water to grow, and the 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 more months ago!

Water is the Key

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 process can be addressed with Desiccant Breathers mounted on the fuel tank vent that captures moisture and prevents the infiltration of the microbial, fungal and particulate contamination.

Without a mechanism to arrest the moisture and contaminants in the vent air flow, this air brings with it these problem-causing issues. 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, although less likely, 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. Consider that the storage tanks that held the fuel before it came to you also pulled air in when the contents were removed, and many of those tanks have accumulated years of debris, bio-film and water.

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Diesel Fuel Degrades During Engine Operation

Black - slimy fuel filter

Degraded Fuel Shortens Filter
Life and Hurts Engine

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 present in the fuel.  Asphaltenes are the high carbon content heavy end fuel molecules that are in solution when the fuel exits the refining process. Asphaltene agglomeration leads to the formation of larger and larger clusters and solids, which are very difficult to completely combust. These solids 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 and are clearly visible on the filter medium surfaces.

In addition, the hot fuel returning to the tank will raise the fuel temperature in the tank.  This temperature change causes the condenses the water that is in the air inside the tank into droplets or water.  The water droplets accumulate in the bottom of the tank.  This water becomes the habitat for microbial and fungal contamination, leads to fuel break down, bio fouling and the build up of sludge and acids.

Fuel droplets and high asphaltene concentrations in the fuel require more time, more energy and higher temperatures to result in combustion.  This higher temperature is not available in engines during the normal combustion cycle before the exhaust valves open.  The result is incomplete combustion.  A truck going down the highway with black smoke billowing from the exhaust is a clear sign that incomplete combustion is occuring in the combustion chambers and the exhaust contains unspent diesel fuel.

Any device in the fuel system exposing the fuel to stress (heat and pressure) such as pumps, heaters, or centrifuges will increase the agglomeration of the asphaltene component of the fuel. 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 is what negatively impacts combustion.

Degradation of Stored Diesel Fuel

Note that fuel in storage, even for relatively short 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 stored by companies or farms for use in equipment that have fuel tanks that receive refills only every few months (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 clusters 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.

Ultra Low Sulfur Diesel

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 since 2007, 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, heavy oils and petrochemicals. According to the U.S. Energy Information Administration, in 2016 there was 20 gallons of gasoline, 11 gallons of ultra-low sulfur diesel fuel, and 4 gallons of jet fuel produced from each 42 gallons (one barrel) of crude oil (83 percent of the barrel of crude). This has contributed greatly to the increase in the heavy end asphaltene that previously did not make it into diesel fuel to become a common component in today’s fuel.

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.”

Fuel Lubrication and Microbe Deterrent Lost

Some believe that the use of cracking units, alkylation and reformer processes are having consequences for fuel stability. The sulfur component of diesel fuel prior to 2006 acted as a lubricant of components and seals, and the loss of that lubricant when ULSD was introduced caused considerable difficulties to older diesel engines. What is less recognized is the sulfur in diesel fuel also was a deterrent to microbial and fungal growth that was lost when ULSD was mandated. 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’s reduced stability. Gums and resins, the result of poor thermal fuel stability, cause plugged filters. Fuel will form particulates (solids) when exposed to pumps and the hot surfaces (fuel is used to cool injector components) 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, RVs and all types of equipment fuel storage tanks.

Diesel Fuel Shelf Life

Even when all fuel storage maintenance practices are followed, Caterpillar recommends a maximum of 1 year from production for distillate ULSD diesel fuel storage, and a maximum of 6 months from production for bio-diesel and blended bio-diesel storage. Storage life for bio-diesel and bio-diesel blends that are greater than B20 may be much shorter than 6 months. (Caterpillar® Commercial Diesel Engine Fluids Recommendations – Fuel Specifications – page 45 – SEBU6251-17 September 2014)  Google this document number for a complete copy.

Under normal storage conditions, today’s diesel fuel when stored at temperatures no higher that 68 deg. F (20 deg. C) can be expected to stay in usable condition for 12 months. Stored at temperatures higher than 86 deg. F (30 deg. C), Fuel can degrade to unusable condition in as little as 6 months. Increased use of above-ground storage tanks have made these problems even more wide-spread.

Diesel Fuel Cleanliness

Truck engines are used continuously and, in most cases, the tanks “appear to be clean”. However, a 4-micron filter element provided for by the engine manufacturer 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. Unfortunately, many equipment operators and maintenance personnel do not recognize fuel filter life to be a problem until the filters are only lasting 40 or 50 hours.

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 fuel you depend on should always be in optimal condition, and the ultimate costs paid for fuel degradation and contamination is paid for by the equipment owner. All major diesel engine manufacturers have set minimum fuel standards for the diesel fuel to be used in their engines, and failing to maintain the fuel that you use in a manner so that it meets those standards could lead to engine component failures and denied warranty claims.

As an example, Caterpillar has published “Cat Commercial Diesel Engine Fluids Recommendations” (SEBU6251-17 September 2014 [a more recent version may be available]) that states “In order to meet expected fuel system component life, 4 micron(c) absolute or less secondary fuel filtration is required for all Cat diesel engines that are equipped with common-rail fuel systems.” (Page 34)

You should not assume that fuel being delivered to you meets the manufacturers requirements, and certainly fuel that has been in your storage tank for an extended period of time may not meet requirements of diesel engine manufacturers. Additional cleanliness standards apply to diesel fuel. Caterpillar recommends that the fuel dispensed into the machine tank meets “ISO 18/16/13 cleanliness level.” (Page 49) For more information on fuel cleanliness specifications and requirements, see the “Diesel Fuel and Injector Failures” discussion on this web site).

If you are depending solely on the engine manufacturer’s 4-miron filter to keep your diesel fuel in optimal condition and protect your warranty and engine, you are looking to the manufacturer’s last line of defense as your only defense.

It should be no surprise when there is a failure in engine components on equipment that is under a manufacturer’s warranty that the dealer representative would take a sample of the fuel in the supply tank. Although much of what has happened to your fuel before you took possession of it is out of your control, your investment is at risk if the cause of component failure is the result of bad fuel.


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 and other specifications set by the engine and component manufacturers. 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 exhaust 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, filtration problems, and, eventually, engine component failure.

“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 directly damage your engine, but using sub-par fuel will eventually cause serious problems. However, it does indicate poor fuel quality which will definitely not provide you with peak engine performance, and further degradation is problematical.

As long as fuel meets the (ASTM) specifications and engine manufacturer’ stated requirements, 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.

Whether you have one diesel powered motor home, or are responsible for dozens, or even hundreds of diesel powered equipment, the need for a Fuel Management Program that addresses your specific needs is critical. Not only to the dependable operation of your enterprise, but also to it’s financial well-being. Implementation of AXI Fuel Conditioning and Tank Cleaning Technology should be the centerpiece of a program that eliminates these costly, wasteful, and environmentally unfriendly 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.