**The ABC's of Fuel System Maintenance

The fuel system supplies, transfers, cleans and delivers fuel to the engines' cylinders to facilitate combustion, thereby producing power. Although fuel systems vary from engine to engine, all systems are the same in that they must supply fuel to the combustion chamber and control the amount of fuel supplied in relation to the amount of air.

The fuel system should meter the exact amount of fuel and deliver the fuel to the injector assembly with precise timing. As the fuel is delivered, the final conditions for providing complete combustion are atomization and the spray pattern of the fuel. Atomization is accomplished as a result of the injection pressure, due in part to the diameter of the holes in the injector. The spacing, angle and number of holes in the injector tip determine the spray pattern.

In modern diesel engines, fuel system pressures are extremely high, ranging in the 20,000 to 30,000 psi range. These high pressures are necessary to ensure optimum fuel economy, as well as providing exhaust emissions that comply with more stringent Environmental Protection Agency regulations.

Supply, timing and the precision of fuel delivery are all critical elements in the efficient operation of a diesel engine. High system pressures relate directly to the strict tolerances found in the fuel system components. Contaminants can have various adverse effects on these components, affecting overall efficiency. These contaminants are generally introduced through refining, mixing, storage or through transferring of the diesel fuel. Regardless of where the fuel is purchased, or how high the quality, if you don't do what is necessary to prevent contamination, trouble will result. It is the function of fuel filters, as well as proper maintenance practices, to ensure the most economical and efficient fuel system life.

The major components of the fuel system involved in the fuel delivery process are the fuel tank, the fuel transfer pump, the injection pump assembly and injectors, and the fuel system filtration products.

Fuel system contamination is a fact of life. The most common contaminants found in diesel fuel are organic elements, micro-organisms such as algae, water and inorganic elements.

Organic contaminants such as aspheltenes are natural occurring contaminants in fuels that form from oxidation or degradation of the fuel, thermal stability (ability to handle heat), cold temperature reactions (waxing), and unstable reaction to additive mixing (lube additive mixed with fuel additive).

If you cut open a used fuel filter and discover a shiny black material coating the media or a gel buildup, this is evidence of organic contamination. Typically, 80 percent of the material in a plugged fuel filter will be organic contaminant, which greatly affects filter life. Fuel additives, along with increased filter capacity and proper fuel sourcing, can minimize the negative effects of organic contamination.

Algae growth can occur when a mixture of water and diesel fuel sits idle for an extended period of time. These microorganisms live in the water, feed off the diesel fuel, and can thrive at any fuel/water/air interface. Diesel fuel with algae will have a definite sour odor, and this contamination can lead to premature fuel filter plugging. A slimy, smelly, greenish-brown material coating the media of a cut open fuel filter is evidence of algae contamination.

Water in fuel comes in two forms: free and emulsified. Free water, or, coarse water, is water that is not entrained in the fuel and will settle out over a short period of time. Emulsified water is actually entrained or bonded in diesel fuel.

Now, more importantly, how does this water get into the fuel? Water is introduced into the fuel system through the vented fuel filler cap. The vented cap intentionally allows air to enter the tank to equalize pressure, as fuel is removed. This air naturally contains moisture. As a machine sits, and is subjected to changes in temperature, this moisture in the air condenses into water inside the tank. This is how free water gets into the fuel. As this free water and fuel mixture goes through the injection pump, it is vigorously churned, and the water becomes thoroughly mixed with the fuel, resulting in emulsified water. It then flows to the injectors, or returns to the tank, ready to cause problems later.

In addition to water, inorganic contaminants can cause the most damage to fuel system components because of their hard and abrasive nature. This type of contamination typically comprises 10 to 20 percent of the material trapped in the fuel filter. Inorganic materials include component wear-metals, rust, scale and dirt.

Of these, wear metals are the toughest to deal with because they occur through normal system operation and quite often occur downstream of the filter. In the diesel industry, small holes in injector tips, tight spaces and precision fitting components are described in terms of strict tolerances. To best meet the varying demands of the combustion process, fuel system components are designed and built to very strict tolerances. As a result, contaminants can have highly adverse effects on these components, and it doesn't take very large or very many particles to damage strict tolerance components such as injector tips, thus reducing overall engine efficiency.

Other effects of contamination include wear and corrosion in the transfer pump and injection pump causing reduced pressures and flows, usually indicated by hard starting and unnecessary wear of the injector tips causing inefficient spray patterns and poor fuel economy. The main point is this: trouble results when contamination is neither prevented nor controlled.


It is the goal of proper maintenance practices, which includes effective filtration, to keep the system clean and help ensure the most economical and efficient fuel system life. Some of the ways contaminants can be kept out of the system from the onset are by keeping fuel transfer equipment like nozzles, tanks and hoses free from surface dirt and out of the rain, and by using filtration at the pump. Proper storage techniques, such as keeping storage tanks covered to minimize temperature fluctuations, also reduce the impact of contamination.

The keys to maintaining the fuel tank are periodically check for free water in the tank, visually inspect the tank exterior and connections for rust and inspect the interior for signs of corrosion or algae growth. If contamination is present, the tank must be drained and cleaned. If algae is found, a commercially available biocide can be used to kill it, and then the tank must be drained, rinsed and refilled with clean fuel.


Suitable filtration includes using a fuel filter and/or fuel/water separator and changing them at recommended intervals. The purpose of the fuel filter is to remove unwanted contamination before it reaches the other components of the fuel system.

There are two types of fuel filters, primary and secondary. Typically, the primary filter is considered the suction side filter and the secondary filter, the pressure side. The primary filter is upstream of the fuel pump and the fuel is pulled (suction) through the filter. Its function is to catch contaminant before the fuel flows through the fuel pump. The secondary filter is downstream of the fuel pump and fuel is pushed (pressure) through the filter. Its main function is to capture contaminant in the fuel before it flows into the injection pump assembly.

Some engines require a primary filter, some require only a secondary filter and some require both. In some cases, in place of a primary and secondary filter setup, a more efficient fuel/water separator is used, as is the case on Cummins engines. The primary filter, or fuel/water separator, will be found on the suction side or before the fuel pump.

As mentioned before, water gets into the fuel as a result of natural condensation, leaking fuel storage tanks and so on. It can be in the form of large droplets, or it can be emulsified in the fuel. The water must be removed to achieve maximum performance and maximum life from the engine. Fuel/water separators are designed to provide both filtration and water removal. Use a fuel/water separator in place of a fuel filter or in conjunction with a fuel filter to remove water as well as contaminants from the fuel system.

Fuel additives are intended to disperse contaminants, clean components, increase fuel lubricity and decrease extreme temperature effects — all impacting filter and component life. Competitive products have varying degrees of success in relation to their claims, depending on the formulation.

Some products provide a gauge that tells you when to change the primary or suction side fuel filter. Service intervals of fuel filters vary due to factors such as fuel quality, temperature and filter size. The fuel filter restriction indicator gives the operator an easy-to-read gauge that gives feedback on the remaining life of the fuel filter.

Finally, use diesel fuel analysis for troubleshooting a fuel-related problem. Fleetguard offers different test packages that include testing for calculating the cetane number, gravity, distillation, viscosity, cloud point, flash point, sulfur, carbon residue, water & sediment, fuel stability and more, and that give specific recommendations based on the results. Remember that the fuel system is the heart of a diesel engine. Keep it functioning properly through proper maintenance practices, and you will be assured of long, trouble-free life.

Clevenger is manager of the product management group for Fleetguard, Nashville, Tenn.

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