There have been two significant changes in the world of marine diesel engines over the past few years: size and price.
Diesels were once the huge iron behemoths that powered workboats, tugs and landing craft. They were used in larger yachts, of course, but only ones that were capable of handling both the sheer bulk and the considerable weight.

Today, however, the trend toward lighter and more fuel-efficient automobiles and trucks has resulted in an array of marine diesel engines that can rival conventional gasoline engines on a weight-to-horsepower basis. No longer are diesels limited just to large yachts, and you'll find diesels offered as optional equipment on everything from center-console fishing boats to 100-mph sportboats.

At the same time, this increased demand for diesels has led to more engines being produced, which in turn has lowered prices. Today, you can have diesel power in your 30-footer without it costing an arm and a leg.
The seemingly bulletproof nature of diesels, which run without overhaul for thousands of hours longer than gas engines, and which have a well-deserved reputation for absolute reliability, has brought the diesel engine to owners previously experienced only with gas engines.

And while it's true that diesels have many strengths, they also have an Achilles heel ' fuel. Diesels require good, clean fuel, which is why fuel filters are standard equipment. Diesels don't like water in their fuel (it can literally destroy the injector tips), but they particularly don't like gunk, and that's often a surprise to new diesel owners.

The bottom line is diesel fuel grows stuff. There are organisms in the air that begin to grow in the interface area between your diesel fuel and the layer of water at the bottom of the fuel tank.
How can you tell if your diesel fuel is alive? Your diesel engine may seem to be down on power, not able to reach full rpm, or stumble occasionally. It may smoke more than normal. It may be hard to start, or it might stall when cold. You might notice the particularly pungent smell of rotten eggs. And your fuel filters will certainly need frequent cleaning to remove clumpy or stringy 'slime.' The diagnosis is clear: You have microorganisms growing in your fuel system.

If you want to hear something that sounds like a B-grade sci-fi movie, con-sider that given the right conditions, these organisms can double in number every 20 minutes! As they grow and multiply, they chemically alter the fuel to produce water, sludge, acids and other byproducts of their metabolism. When they cling to the side of your fuel tank or fuel system, they hold both water and waste products against the metal, which starts to corrode. Steel, iron, aluminum and copper are all affected by these critters, and these hungry microbes will consume rubber hoses and gaskets as well as plastic linings. As they grow, they form mats of stringy and slimy goo that can get into fuel lines. If they get far enough, they can damage your high-pressure pumps and injector nozzles.

Three conditions contribute to the growth of this biological contamination: darkness, water and warmth. There's not much you can do about the fuel tank being dark, and the warmth comes both from the return of hot injector fuel from your engine and from normal summer temperatures.

But, while these microbes can live in a dormant state for long periods of time in anhydrous (free of water) fuel, there is a sudden explosive growth when water appears. So water will be your worst enemy in the battle against the micro-organisms living in your fuel tank.
It's easy to blame this goo on bad fuel, but in most cases, it's created right inside your own boat.

Nevertheless, try to buy clean, 'dry' fuel from a reliable dealer. Water, however, will inevitably appear in your fuel tank, either from condensation in a partially filled tank, from moist air entering the system through the vents, or from water delivered along with your fuel.

Your first line of defense both to prevent the growth of microorganisms and to get rid of them is to add a powerful biocide to your fuel. Star brite Bio Diesel, for example, requires one ounce of biocide for 60 gallons of diesel fuel. Be sure to follow the directions carefully, and don't get any of this chemical on your skin, either. Other popular brands of biocide are Biobor, Racor, CRC and Valvtect.

The initial treatment of your fuel will cause the dead micro-critters to clog your fuel filters, so you'll need to clean or change them much more often than with clean fuel.

At the same time, you need to remove the water from your tank, and unless you have unusually large amounts of water present, a water absorber (Star brite Diesel Fuel Water Absorber, for example) will cause the water to be mixed with the fuel and safely burned away. Add the proper amount to your tank, allow it to mix thoroughly (you can rock your boat to slosh the fuel around), and then burn off the water along with the microbes.

Once your tank is free of contamination, you should institute an ongoing preventative maintenance program. Use a biocide on a regular basis, but how often will be determined both by temperature and humidity. Use more biocide in the warm summer months, or if you keep your boat in a tropical climate.
To monitor your fuel tank for water, use a water probe indicator such as the one from Marine Development and Research. You apply this liquid to a dipstick, insert it into your fuel tank, and if water is present, the liquid changes color to give you an indication not only of whether water is present, but the depth of the layer as well.

You should have a fuel/water separator filter for your fuel system, and you should drain off any water on a daily basis. If you find that you seem to have water on a regular basis, you might want to change fuel dealers. Try to use a dealer whose fueling equipment includes a fuel/water filtration unit.

Equally important is to regularly give your fuel tank a dose of biocide, even if you have no symptoms of contamination. If you're a typical boater who uses your engines less than 25 hours a month, add a mild concentration each time you add diesel fuel. If you use your boat more often, you can add the biocide at every third or fourth refueling. The biocide will have no effect on the performance of your engine, but it's a good insurance policy.

When you store your boat for the winter, be sure to use a fuel stabilizer additive, as well as a water absorber and a protective dose of biocide to keep your fuel fresh and clean until spring.
Fuel contamination is a problem that just about diesel boatowner will have to face at one time or another. Happily, the cures are 100 percent effective, and with an ongoing prevention program, you'll never have the problem again.

May 2003