What Kind of Contamination in the Fuel System Is the Typical Cause of Diesel Engine Failure?

There are only a few contaminants that can lead to diesel engine failure on a boat. Of the handful, most are described as sludge or algae, a few come from human enterprise, some come from human stupidity and one, water, doesn't come from a leaky fuel cap, but from a source of consternation in other parts of the boat -- condensation. Once you understand the problem, you've gone a long way to preventing it in your diesel.

  1. Water

    • As the fuel level in the tank drops, more of the interior wall of the fuel tank is exposed. Water begins to appear on the inner walls of a fuel tank as condensation. The specific gravity of diesel fuel -- its weight when compared to the weight of water -- is between 0.81 and 0.88 at 60 degrees Fahrenheit. This means that when fuel is added to the tank, it washes the heavier water to the bottom. The water will collect on the bottom of the tank along with other contaminants.

      Water in the tank is, like the fuel, a liquid. When the fuel pump sucks the water up into the engine, not only does the water not burn, it causes a condition similar to vapor lock -- an interruption of the fuel flow by a bubble of liquid or air. Marine engineers often say that a diesel engine is "air-locked," when in fact, the engine was "water-locked."

    Trash

    • As the diesel fuel is used, the fuel that floats on top of the water in the tank becomes a smaller percentage of the total liquid present in the tank. The bottom 10 or even 20 percent of the tank will be home to water and a nasty collection of chemicals such as paint thinner, paint and and objects. These substances, along with fuel-absorbent pads that have been inadvertently shoved into the filler, cigarettes thrown into the tank to prove diesel fuel doesn't explode, along with the occasional wrench that ends up there through some unknown agency are collectively known as "trash." Trash is a contaminant because it can impede the flow of fuel.

    Varnish

    • When diesel fuel is left unattended and unused for more than two weeks, it begins to break down. One of the chemicals formed in this breakdown is a varnish-like substance called, oddly enough, varnish. It's consistency is something like that of a weak plastic film.

      Particularly when the vessel is kept in constant or near-constant operation, new fuel is constantly being piled into the fuel tanks for the main engine or the generators long before the tanks are empty. This means that the fuel that has remained in the tank for more than a few weeks has a chance to break down, losing some of its propulsive potential and creating a contaminant that can kill the engine stone dead.

    Diesel Algae

    • While outboard mechanics and ship's engineers talk about "algae in the fuel," there's no such thing. What they're referring to is the sludge that appears on the fuel filters they have to change every 100-or-so hours. It's nasty, it's often green, but it has no kinship to the aquatic life form known as algae. Instead, it's a sludge drawn from the bottom of the fuel tank by a fuel pump starved for fuel. Like the septic tanks found in rural areas, the fuel tank gives all of the contaminants previously listed a chance to break down -- with the possible exception of the wrenches -- into the unrecognizable mass known throughout the world of marine engineering as "diesel algae."

      There are several species of bacteria that live on the surface of diesel fuel. Their excretions may contribute to this sludge, as well. The use of biocides may ease your mind -- and eliminate some of the problem.