The short answer is that modern diesel fuel is less “stable” than it was a few years ago. That has led builders of boats with larger tanks to install fuel-polishing systems to filter out the precipitates in fuel that have sat in the tanks for more than 90 days. It used to be, before the “clean air” requirements, that the only thing a boater had to worry about was the bacterial buildup at the fuel/air interface in the tanks. Diesel fuel could sit for at least a year before any other problem occurred. Now, however, the refineries are using a larger percentage of a barrel of crude for gasoline and diesel than they used to, and that has led to more suspended solids in the fuel. Over time, these precipitate out. That window can be as short as 90 days.

The good thing is that fuel additives can help solve the problem, as can simple and relatively inexpensive fuel-polishing systems that can be installed on board. A good filter in the fuel return line can also help.