Dealing with Diesel Fuel
Date Posted: May 29, 2023
Source: Steve D’Antonio

In my experience as a boat building and marine systems consultant, the majority of engine-related problems encountered by cruisers and marine industry professionals alike are the result of fuel "issues."

Contaminated fuel—be it by water, asphaltene, biological masses, or just plain dirt—is the nemesis of any internal combustion engine.  Because of the mind-bogglingly close tolerances to which diesel fuel system components are machined (measured to ten-thousandths of an inch), diesel engines, in particular, are susceptible to fuel that is anything less than scrupulously clean, and this is even more so for those utilizing high-pressure, common-rail fuel injection systems.  

Taking on clean fuel from the start, often easier said than done, is the best defense against fuel-related problems.  Once the tank and fuel system are contaminated, significant labor will almost certainly be committed to reversing the process.  Better to prevent it from the start.  Sampling fuel before filling up is a worthwhile approach, especially when the fuel is suspected of being less than pristine.  A glass quart jar is fine, however, better options exist.  

A one-gallon white plastic pail with a removable lid (a mini version of the familiar spackle bucket) will often highlight gross contamination such as water and solids.  Before filling up, pump a quart or so of fuel into the bucket and then wait a minute or two for settling to occur.  Any water or sediment should be clearly visible against the pail's white bottom.  Good quality diesel fuel (40 cetane or higher is preferable and often required by engine manufacturers) should look like honey (if it's destined for on-road use) or dilute cranberry juice (off-road diesel is dyed red).  If it looks like coffee, pass on it.  You should certainly be able to see through a quart's worth in the bottom of the gallon container.  

If it passes the bucket test, you may want to go ahead and fuel without any further delay, especially if you purchase fuel from the vendor regularly.  However, if the sample is borderline, or you just have to get fuel, you could use one of the purpose-made filtering funnels.  These will filter out the coarsest sediment, but don't expect miracles.  Large amounts of water or ultra-fine dirt will not be stopped by its rudimentary filter system, and you can forget using one if you are in a hurry, as they're slow to drain.  

If nasty fuel should make it into your tank, the best secondary lines of defense are two-fold: a high-quality primary fuel filter and inspection ports that allow access into each baffled chamber of the fuel tank(s).  The former will alert you to the problem before it goes super critical, and hopefully will prevent damage to your engine.  The latter is really the only solution to a seriously contaminated tank—a hands-on clean out.  

Additives are fine if used judiciously; however, they cannot dissolve years of accumulated asphaltene or shag carpet-like accumulations of biological growth (which are, contrary to popular belief, bacterial based and not algae).  Using a biocide may treat the symptom but not the cause, which is water.  The long-term solution to the problem is removal of the water via a drain, stripper tube or polishing system.  Without water, biocides are unnecessary.  It's worth remembering, bacteria lives in water, and only eats at the fuel—water interface, the diesel fuel diner of sorts.  Take the water away, and the microbes die off.

Comment Submitted by Gary Haring - June 1, 2023
For those that use hundreds of gallons of diesel a good start is to buy diesel from a professional retail source. I try and use one that pumps a lot of diesel and has a 10 micron filter installed after the pump. This is a good first step but not always possible so the next step is to have plenty of replacement fuel filters onboard because when bad fuel shows up it is usually unexpected and at the worst of times. Think, middle of nowhere, heavy seas and deteriorating conditions. Since no sane person wants to shut down an engine to do a filter change during such conditions you might consider an additional step of installing a double parallel filter array. These systems allow you to bypass the clogged filter to pass fuel through the other parallel filter without the need to shut down the engine by simply throwing a valve. Dependant on how bad the fuel is and the sea conditions you can then do a filter change out just in case the backup filter clogs. These systems are expensive and they take up room but they are well worth it if only for peace of mind. Nothing worse or more dangerous than changing a fuel filter in a hot cramped engine room in a rolling, pitching sea. If the engine has a built in fuel filter of a finer micron then the parallel system it will still reduce the frequency of clogged filters. IF, and it is a big if, you can install a like same micron parallel system in theory this could eliminate the built in engine filter but I wouldn't suggest doing this without first consulting the engine manufacturer. You might also consider installing an electric primer pump upstream of all the filters so that with the press of a button you can bleed the system all the way to the engine. Safe Boating
Comment Submitted by bill - May 31, 2023
The overall quality of marina diesel fuel has become so poor over the last 5+ years (for a host of reasons) I have taken personal measures to totally avoid contaminants in my only diesel fuel tank. First: I open my tank inspection port once per year and pump five to ten gallons (from the tanks lowest area) into plastic cans and inspect for any irregularities. Second, add a small quantity of biocide with each fill-up. And, lastly, but most importantly: I only pump diesel into my 5 gallon plastic containers....I then inspect the yellow containers visually and also with a 300 lumen light for any water or sediment. I have been known to return a 5 gallon can of crappy fuel back to the marina for a refund, and subsequently acquire fuel from another source. This process has avoided condiments in my fuel tank, alleviating those oh-sh!p moments in heavy seas, fuel being sloshed around, you know, those unique moments when filters become clogged and your main propulsion system starts to die at the most inopportune time. Old Navy saying, believe no one, trust nothing unless you personally verify yourself!
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