Motorhome Diesel Fuel Problems

Diesel Engine Fuel
Polishing for Motor Homes

Motorhome Living

It used to be that the biggest problem that owners of motorhomes, travel trailers, other RVs, or pop up tents had with diesel fuel was the relationship of miles-per-gallon to dollars per gallon. Although this situation is likely to remain somewhat out of the control of the equipment owner responsible for the bills, the problem of diesel fuel condition is not. The quality of diesel fuel from the pump is fairly steady, but most owners of diesel equipment know that the quality can vary. Most owners of motorhomes or diesel tow vehicles are judicious as to the source of their fuel, whenever possible. However, the discerning motorhome owner is still the responsible party if the fuel going into the tank does not meet the recommended requirements of the engine manufacturer.

Although fuel quality going into the tank is important and the risk that the fuel does not meet the engine manufacturers recommendation is fairly low, what happens to the fuel while it is in the tank often escapes the imagination. Problems in the performance and longevity of the engine and its various components is predicated on what the cleanliness of the diesel fuel is when it comes out of the tank. There are those that recognize that there are benefits to using a quality diesel fuel additive that will give back some of what has been lost since the introduction of Ultra Low Sulfur Diesel fuel (ULSD) a decade ago, such as lubricity. But, unfortunately, most additives on the shelves of truck stops or the discount or auto parts stores will only go so far.

According to Caterpillar, ULSD has a shelf life, even when all basic fuel storage maintenance practices are followed by the distributor, of one year beginning when the diesel fuel leaves the producer, and six months for bio-diesel and blended bio-diesel. At temperatures above 86 deg. (30 deg. C), diesel and bio-diesel fuel storage life is cut in half.

However, Cat contends that diesel fuels can deteriorate rapidly when fuel is “stressed”. The high pressure (in excess of 30,000 psi) and high temperatures (fuel is used as a coolant for high-pressure fuel injection systems) that the fuel endures as it is repeatedly recirculated back to the fuel tank is responsible for much of fuel degradation. Gums and resins that occur in diesel fuel under these stressful conditions are the result of dissolved oxidation products in the fuel that do not evaporate easily and do not burn cleanly. Excessive gum in the fuel will coat the inside of fuel lines, pumps and injectors and interferes with the close tolerances of the moving parts of fuel systems (Caterpillar's publication SEBU6251-17 "Cat Commercial Diesel Engine Fluids Recommendations", Page 45).

In addition, water commonly accumulates in stored fuel and is the most damaging contaminant to diesel fuel and the primary catalyst to additional fuel breakdown. Although some water can be added with a fuel fill, much more common is moisture being introduced through a tank vent as air is drawn in to replace used fuel. Temperature changes (see heated fuel above) cause droplets to form on fuel tank walls and accumulate in the bottom of the tank. Water is the required component to microbial habitat and growth, and as air is drawn into the tank, so is fungal and other microbial growth. These little critters live in the water, and feed off the components of, and additives in, the fuel. The reproduction process of these microbes produce a black, slimy organic material sometimes seen floating in diesel fuel and on fuel filters. The microbes are too small to filter out, but this material shortens filter life, sometimes to hours instead of thousands of miles. Uncontrolled microbial growth creates acids that can corrode the fuel tank, fuel systems and injectors.

This situation does not happen overnight. Many motorhome owner will blame a fuel fill at a shady convenience store a few days ago, but this situation can take six and even nine months to develop to where the slime becomes problematic.

Another problem less recognized by the equipment operator is the process involving the agglomeration of asphaltenes, a component of diesel fuel, in clogging fuel filters. Asphaltenes are present in all diesel fuel as a function of the process of refining crude oil, and are less than 2 microns in size in diesel fuel. But stored fuel allows the asphaltene time to form clusters, and as these clusters grow in size, and achieve sizes of 100-200 microns, the fuel begins to appear dark, sometimes black, and filters become black and clogged with these clusters of asphaltenes. In storage tanks, they can gain size and weight and drop to the tank bottom. Inspections of tank bottoms often disclose an appearance of being painted with roofing tar. This is the accumulated asphaltenes.

Fuel injector nozzle holes generally have two failure conditions, which result in a partial functional failure of a fuel injector. These two conditions are blockage, caused by the contaminants in the fuel, and erosion, from the acids formed by the microbial contamination, which lead to fuel non-atomization and leakage of fuel into the cylinder head. The degraded fuel spray pattern results in carbon buildup on injectors and the piston. Leaked fuel results in an increase in piston crown temperature, which can cause the crown to deform or melt, resulting in engine failure.

Diesel fuel that is maintained in a condition as close to "refined" as possible will out perform fuel that is under, or poorly, maintained. In order to achieve optimal fuel quality, the contaminants in fuel must be filtered out, water must be separated and removed, and the fuel must be conditioned. This process of filtration, separation, and conditioning (commonly referred to as the Fuel Polishing process) is the foundation for achieving, and maintaining, the quality of fuel necessary for ultimate reliability.

Although filtration and de-watering is critical, fuel conditioning is paramount. You cannot filter microbes out of fuel, and the asphaltene agglomeration will not be reversed without the use of the Fuel Conditioner. The process of returning the fuel to a clear and bright condition is key to returning the combustibility of the fuel that was lost to degradation.

Diesel fuel cleanliness is the ultimate goal. For a more complete understanding of fuel cleanliness and how that term is defined, see the Diesel Fuel and Injector Failure page on this web site. Caterpillar, like most engine makers, recommends a final filter rated at 4 microns. This level of filtration is assumed to allow injectors and other fuel system components to live a happy, long life. But the reality is this final filter should not be your first line of defense against nasty fuel. Even with a primary filter rated at 10 microns that may double as a water separator, it is not wise to put a lot of faith in a belief that the fuel is not-so-bad so as to overpower these safeguards. Keep in mind that filters do not stop the acids, and the gums and resins can still get through a 4 micron filter you wreak havoc on your pocketbook.

Caterpillar, as will all engine manufacturers, set standards and make recommendations for a reason. If there should be a failure of components in the fuel system within the warranty period, it is not unheard of for the servicing dealer to check the fuel. If fuel being used in the motorhome is not within recommended standards, warranty claims could be denied.

We recommend you establish a Fuel Management Plan that allows the manufacturer's filters to be the last line of defense. Place protections in front of those filters by establishing a Fuel Management Plan that keeps your diesel fuel water free, clear, bright, and providing optimal performance.

TK 240 XT Tank Cleaning
Fuel Polishing System

TK-240 XT Portable Tank Cleaning System

Click on Graphic to Enlarge

Motorhome owners have a couple choices to address all of this. If you use your motorhome 12 months a year and are using a tank of fuel a couple times a month, than you should not have degradation of your fuel. But you do need to be prepared for that inevitable “bad load of fuel”. To address the ongoing risk of microbial contamination, take steps no less than every 60 days to drain water from the bottom of your tank. Installation of an in-line Fuel Conditioner on your motorhome or tow vehicle will assure microbial contamination is controlled, asphaltene clusters are dispersed, and, with the use of either AFC-705 (pre-2007 motors) or AFC-710 (Tier 4 compliant) tank cleaning fuel additive every few months, your tank will stay clean and your fuel filters will remain serviceable for 10,000 miles or more. If your particular equipment can accommodate it, use the Water Eliminator to capture any water that gets into your tank. Check it every time you fuel up.

If your life style involves your motorhome sitting idle for more than two months in a row, you will need to consider other steps. The in-line Fuel Conditioner only works when the equipment is operating, and the occasional running the engine for twenty minutes to circulate the motor oil will not maintain the fuel. Periodic circulation of the fuel is needed. The TK-240 XT Portable Fuel Maintenance System, which incorporates a The Fuel Conditioner, is cost effective, easily manageable for the non-mechanic DIY motorhome owner, and does an excellent job of cleaning the fuel and fuel tank. You may wish to incorporate the in-line Fuel Conditioner to operate during the portion of the year that you are active with your motorhome, or use the TK-240 XT year around. Maintain the use of the AFC-705 or AFC-710 tank cleaning fuel additive every few months as above. More information on the operation of the TK-240 XT is available at the Portable Tank Cleaning web page.

With only moderate effort, you can be assured that your diesel fuel will provide optimal performance and enjoy the expected life of your fuel filters, diesel engine and engine components.