Chances are, there’s water in your fuel right now. If it’s left there in cold weather, we all know that your fuel system will freeze and stop your truck dead. But in any weather, water also diminishes the lubrication qualities of diesel fuel, so engine internals will wear faster. You might get rust inside your engine and fuel-system components (especially the pump). And if that’s not bad enough, you’ll have microbial growth and eventually sludge.

One way or another, water in your fuel is going to cost you money

The simple solution is to spend a little dough up front and have a fuel/water separator installed in your fuel line. It’s standard on many new trucks, but you’d best make sure. If you’ve bought a used truck, check it out.

Water can actually be dissolved in the fuel, in which case no separator will help. More likely it will be in relatively large droplets (about 100 microns) called ‘free’ water, or tiny droplets (2 or 3 microns) that stay suspended in the fuel and give it a milky appearance – a tell-tale sign of trouble.

Some separators work better than others, but all will dump the water – or most of it – into a small reservoir. You must drain it regularly, a simple job, especially if your separator has a sight bowl that makes monitoring easy. If you make it part of a routine, you’ll be safe.

Choosing Well

There are many fuel/water separators to choose from, so start by consulting your engine manufacturer. You need to know your engine’s fuel-flow rate, measured in gallons per hour or gallons per minute, and then match it to separator capacity. That rate can be restricted by the mismatching of separator to fuel line, creating changes in fuel pressure and potentially significant performance reductions.

There are three separator configurations: some simply separate water from fuel; some add a fuel heater; and others incorporate filtration that replaces or works with your engine’s standard fuel filter(s).

Obviously, you want the fuel/water separator that removes the most water, bearing in mind potential restriction hassles, so look for the manufacturer’s statement of efficiency. If you don’t see one, look elsewhere.

This is where we talk about ‘microns’. One micron is a millionth of a meter, or 39 millionths of an inch. We noted earlier that larger droplets of ‘free’ water are as big as 100 microns in diameter, while suspended water may be just two or three microns. A human hair is about 50 or 60 microns thick, incidentally.

Filter and separator manufacturers will talk about ‘micron ratings’, but beware. You may come across the phrase ‘nominal micron rating’, but experts warn that this means nothing on its own. It refers to the filter’s ability to snag particles of a given size at anything under 100% efficiency. But that rating is only useful when combined with its percentage efficiency at removing contaminants. For example, if a filter is rated at 10 microns and 90% efficiency, it supposedly removes 90% of particles that are 10 microns or larger in size. If the manufacturer just talks about being able to catch particles of 10 microns, it might well be only 20% efficient – meaning it removes only one in five particles.

You may also see the phrase ‘absolute micron rating’, which should be a little more comforting. It tells you how big or small a particle the filter will retain at 100% efficiency.

Once again, remember that fuel-flow rates are critically important here, so don’t judge a separator’s worth solely by its micron or efficiency ratings. You absolutely don’t want a separator that alters your fuel pressure, no matter how good its on-paper ratings may seem.

Figuring out where a fuel/water separator should go is not difficult. It has to be away from ram air and weather factors (and probably should be insulated), at or below the level of the engine’s fuel pump to avoid air problems. Avoid elbows in fuel lines, which act as freeze points, and if your fuel lines have to be long, consider going one size up to avoid fuel-flow problems.

Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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