I'm not sure how long the diesel fuel has been in the tank of the used sailboat I just bought. What should I do about it?
Diesel engines are extremely reliable if treated with regular maintenance. Clean air, fuel, and compression are all they need to get going and stay going. You’re right to be concerned about the fuel in the tanks, especially if it is a new boat to you and you’re not sure of the engine's history. The best cure is always prevention. Diesel fuel has a finite shelf life that varies with heat and moisture. It can break down into different types of gum, resin, and varnish, and cause your powerplant to shut down—usually at the most inopportune moments. Condensation is also an ongoing problem, as temperature differences between the inside and outside of the tank can allow water to contaminate the fuel. Algae and bacteria can then grow in the fuel if it sits long enough, but only if water is present.
But even with a full tank, diesel fuel has a shelf life and is essentially unstable, breaking down over time. The best way to find out what kind of fuel you’re dealing with is to take a sample from the bottom of the tank. Even if the sample looks clear and void of particles, we'd still recommend that you change all the filters in the system. If you see any suspicious grime, gunk, or if you have water in your tank or water separator, you may want to consider pumping out the tanks. There are stabilizers and anti-algae biocides you can add, but a load of truly contaminated fuel needs to be pumped out into jerry cans and delivered to a hazardous waste site at your local dump, or other fuel reclamation site. Sure, it might not be the most pleasant way to spend the afternoon, but it beats hunkering over a recalcitrant engine trying to coax it to life only to have it die again.
Even if the engine ran fine the last time you fired it up, you should be aware that sailing conditions can also play a part in whether you’re having fuel problems. In choppy seas, the fuel tank and its sediments can be shaken up and in bad cases this can clog the fuel line. So take the mystery out of this equation by way of a close inspection. Hopefully your tank has an inspection port that will allow you access to cleaning the tank and inspecting it.
After that, the best way to prevent fuel contamination is to filter the fuel before it even goes into the boat, using what's known as a Baja-filter. While this may take you a little longer at the fuel dock, it's certainly worth the time. If you look at your fuel filters as the last line of the defense, not the first, that outlook will go a long way to preventing fuel-related problems.