Fuel and its filtration are hot topics when talk turns to diesel-engine maintenance, and they should be, because they're often the source of motor trouble. In fact, taking into account statistics as well as my own anecdotal evidence gleaned as a mechanic and boatyard operator, I'd say fuel-either the lack of it or its contamination-accounts for more engine problems than any other single source.
The good news is that nearly all fuel-related problems are well understood, and as long as you haven't run out of fuel, most can be avoided by the judicious use of filtration. In any boat with a diesel engine, a comparatively inexpensive fuel-filter element is all that stands between reliability and potentially serious engine problems. The key to proper fuel filtration is to approach it in a sequential fashion, a process known within the industry as multistep filtration. Industries whose manufacturing processes call for large quantities of ultra-pure water-pharmaceuticals and electronics, for example-routinely use such an approach, where the fluid moves through progressively finer filters rather than passing through the finest filter first.
At the risk of being branded a heretic and incurring the wrath of cruisers everywhere, I suggest that you should use two filters of sequentially finer filtration, too. The first, or primary, filter should have a 10-micron rating. The specification of the secondary filter, the one that's on the engine, is determined by the engine manufacturer; this filter typically carries a 2-micron rating. Sure, you've spent your entire cruising life using a 2-micron primary filter, but there's method to my madness, and it's endorsed, and in many cases mandated, by many fuel-filter and engine manufacturers.
There are two reasons for multistage filters. First, fine filters-those with a 2-micron rating-are much better at filtering out fine contaminants (often asphaltine, a naturally occurring tarlike substance found in diesel fuel) if they're called upon to filter only fine contaminants and not larger particles, too. (A micron, by the way, is a millionth of a meter. By comparison, a grain of salt is 100 microns wide, a human hair is 70 microns thick, and bacteria measure about 2 microns in diameter.)
Here's a useful analogy: If you took a bucket, punched a few small holes in the bottom, filled it with fine sand, and then poured water into the bucket, some sand would leak out of the holes at first, but then the sand would stop and eventually only water would flow out. If, however, you filled the bucket with a mixture of gravel and sand, chances are that the sand would continue to flow out of the holes because the gravel would keep some paths open. Multistep filtration works much the same way: Contamination is segregated between coarse and fine filters, and both filters work more effectively.
The second reason for a multistage system is that if you capture all the debris, coarse and fine, in the first filter, the second filter really ends up doing very little. As a result, you've effectively halved your filter surface area, and you've limited the ability of your filtration system as a whole to capture and hold contamination while still allowing fuel to flow to the engine.
Arguments against this approach often have more to do with convenience than science. Granted, some on-engine filters are a bear to service. But here's a thought: Install another filter, one that's more easily serviced, after your 10-micron primary but before the on-engine secondary and lift pump. Within this filter install a 2-micron element, and presto! You now have an easily serviced multistep filtration system. As an added measure of precaution, replace the on-engine-and now tertiary-filter seasonally either before your boat's winter layup or spring launch.
Steve D'Antonio is a regular Cruising World contributor who offers services for vessel owners, boatbuilders, and others in the marine industry through Steve D'Antonio Marine Consulting (www.steved