Foul Fuel

A gummed-up fuel filter is a happy filter. It’s done its job, after all: a fuel filter is supposed to gradually trap contaminants that can damage your diesel engine’s pumps, lines, and injectors.

Unlike oil, fuel has no way to bypass a restricted filter, so when the fuel can no longer reach the injection pump, the pump can’t produce the fuel pressure and fuel volume you and your engine demand.

A fuel filter that becomes restricted prematurely is a symptom of a problem with the fuel, not the filter. Somewhere between the refinery and your engine, your fuel has picked up tiny contaminants that can spell big trouble. To keep emissions in check, your new diesel engine’s fuel system meters out just the right amount of fuel at precisely the right time. Even a minuscule amount of water or grime can wreak havoc on the ten-thousandths-of-an-inch tolerances within today’s fuel pumps and injectors.

The most common and potentially serious contaminant in your fuel is water, often the result of warm, moisture-laden air condensing on cold metal walls of fuel-storage and saddle tanks, and it can harbour other filter-plugging contaminants like rust, fungus, and bacteria.

If you trace the fuel path through the injector nozzle, where injection pressures can reach 30,000 psi, it’s easy to see why diesels don’t like water. First, water is incompressible, unlike diesel fuel. Adding a volume of incompressible liquid to the injector is like putting a rock in there: something has to give. Secondly, as a glob of water in your fuel pulses into the injector nozzle, it approaches the hot cylinder head and turns to steam — in effect, creating a small explosion at the tip of the nozzle, blowing away bits of the nozzle tip’s needle valve. Eventually, the valve no longer seats properly and fails. When someone tells you that water in your system can cause a tip to blow off an injector, this is what’s going on.

Water and steam hit other engine surfaces and can lead to rust, scale, or debris that restrict fuel flow through the filter. Water also provides the necessary medium for fungus and bacteria to live, breed, and feed on hydrocarbons in your fuel.

Cold weather adds another wrinkle: low temperatures cause moisture in the fuel to freeze. In fact, ice generally forms before wax, which occurs when the temperature drops below a fuel’s cloud point and paraffin in the fuel starts to crystallize.

The point is you need clean fuel. It will be even more important later this year with the implementation of rules requiring ultra-low-sulphur diesel. The new ULSD blend will be the standard made by most refiners in fall 2006. Regulators believe the lower sulfur content used in conjunction with 2007 and later heavy-duty diesel engines will further reduce particulate matter and nitrous oxide (NOx) emissions to the atmosphere.


1. Stay on spec. Filter manufacturers design products according to the level of filtration required by each engine OEM. A more “open” substitute may prolong replacement intervals, but it also will let contaminants pass downstream toward more expensive fuel system components. A tighter filter, on the other hand, can trap more contaminants but could require more frequent service.

Check the blend of fuel you’re buying. Some fuel suppliers
will thin out No. 2 diesel with No. 1 or kerosene

Use filters that meet the engine maker’s minimum performance requirements. Check the specs: Caterpillar, for example, uses a 2-micron secondary filter, while Cummins uses a 10- or 15-micron filter, depending on the engine.

And then pack extra filters in the truck and train your drivers to change them.

2. Cures for the cold. Cold, thick fuel strains the fuel pump, especially when the fuel has to move through a tight filter. An onboard fuel pre-heater will help the pump do its job, as well as strategic placement of the filter: the longer the distance between your tank and your filter, the harder the pump has to work to move the fuel.

Other steps can help prevent fuel from gelling. Anti-gel agents can’t “de-gel” a tank of gummy fuel. They’re more effective when added to a tank that’s partially full of fuel that’s been warmed by fuel heaters or the fuel-return. Even then, pouring 150 gallons of cold fuel on top of warm fuel may not stop the fuel from gelling. You may be best off to add 50 gallons of cold fuel to the warm fuel, head down the road, add 50 gallons more, and so on until the cold is less extreme.

3. Drain your water separator.

In-line fuel filters/water separators are a key to removing water that can lead to emulsion problems, injection system corrosion, and microbial growth. Talk to your separator’s sales rep about the ideal drainage schedule. Once a week, once a day, at every fill — it may change depending on your operating conditions or fuel supplier. If you let the media become saturated, you’ll pump water-contaminated fuel right through it. With Mack’s recent introduction of its MP7 engine comes a water warning light advising operators to drain the water separator.

4. Good housekeeping. Most sources of contamination are at the fuelling site. Dirty tank caps, dispensing nozzles, hoses, and storage tanks promote contamination. If you store fuel on-site, test it for water, sediments, and microbes as it comes in. Check the tank bottoms once a month. Inside the tank, diesel fuel slowly reacts with oxygen in the air to form gums and varnishes. As temperature increases, the reaction accelerates.

Also check the blend of fuel you’re buying. Depending on the weather and your geographic location, some fuel suppliers will thin out No. 2 diesel with No. 1 diesel, or kerosene in the winter. The mix ratio will vary, and you won’t know that you’re getting what you’re paying for unless you check.
Another reason to inspect the blend: kerosene is lighter than No. 2 diesel. If you have No. 2 in your tank, and you’re adding a “winter blend” that includes kerosene, ideally the lighter kerosene blend should be added beneath the heavier No. 2, allowing for a more rapid, thorough mixture.

5. It’s what’s inside that counts. Your filter media can provide diagnostic clues about your fuel. Green, brown, or black slime on the surface of the media indicates microbial activity you’ll have to treat with a biocide. A dark, sticky coating indicates a high amount of asphaltenes, a naturally occurring molecule in diesel fuel that can plug your filter and drastically shorten its life. The cleanliness of your fuel reinforces the point that, where you can, buy fuel from a trusted source. If you suspect something wrong, dispense some into a jar and take it to a laboratory for testing (many oil analysis labs also test fuel).

As you sit there contemplating whether to hurl invectives at your plugged filter, remember that fuel is a four-letter word, too. The filter was just doing its job.

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