Time for a New Filter?

Avoid too-frequent replacements by accurately measuring the flow of air and fuel to your engine. "Hands-On Sailor" from our December 2009 issue

January 14, 2010

368 fuel filter

This simple-to-read air-filter gauge has an indicator that drops into a red zone to tell you when a replacement is needed. Steve D Antonio

It’s an accepted truism: You can’t change your fuel filter too often. The reasoning goes that while filters cost money and old ones simply end up in rubbish heaps or landfills, there’s no harm in making sure that the diesel fuel coursing through your engine’s fuel-injection system is as fresh and clean as it can be.

But is that really true? It turns out that filters, especially those used for air and fuel, can in fact suffer from being replaced too often. Indeed, as difficult as that concept may be to believe, most filters are least efficient when they’re new. As a filter begins to capture contaminants, its ability to capture more contaminants increases. The bits of belt dust and acoustic insulation that become lodged in paper air-filter elements, for instance, improve the paper’s ability to capture new debris and contaminants as they pass through the filter media. And the same is true for fuel filters: As they begin to capture the first bits of asphaltene (a common diesel contaminant), rubber hose particles, and rust from steel tanks, they become more efficient at capturing still more of this and other contaminants.

The key, then, is to determine just when a filter needs replacement, and to do so requires anything but seat-of-the-pants engineering. Instead of simply guessing at how contaminated or impacted your fuel and air filters are, you can use a measuring device or tool to gauge their effectiveness.


For air filters, a vacuum-measuring device can easily be installed on the clean-air side of the filter element, that is, between the filter and the engine’s intake manifold. Many larger diesel engines supply these as standard equipment; however, they’re frequently excluded from smaller engines, such as those found in many sailboats. I’ve used air-filter monitors from a company called Engineered Products, the manufacturers of the Filter Minder (

The Filter Minder is designed for aftermarket installation using nothing more than a stepped drill bit. The graduated scale on the monitor measures and retains the highest vacuum reading, typically in inches of H20, enabling the user to determine, at a glance, with the engine either running or not, the condition of the air-filter element. Air-intake vacuum thresholds vary from engine manufacturer to engine manufacturer, however; typically, anything measuring more than about 23 inches of H20 is considered ample reason to replace the element. With the Filter Minder, when the airflow restriction reaches the critical point, the indicator in the clear cylinder goes into a red zone to alert you that a change in filters is due.

Monitors for primary fuel filters work in much the same way. Installed between the filter and the engine or its lift pump, they monitor the vacuum created on the “clean” side of the filter element. If the vacuum is low, measuring less than about 5 inches of mercury, there’s no need to replace the filter. Higher vacuum readings, on the other hand, indicate that a filter has done its job and is in need of relief. It’s important to remember that fuel-filter vacuum readings must either be taken while the engine is at cruising rpm or by using a recording vacuum gauge. A recording gauge uses a resettable drag needle that maintains the highest vacuum reading for later observation, even when the engine isn’t running.


Ideally, the vacuum gauges installed on primary filters shouldn’t be the kind that replaces the T-handle often found on filter elements. Instead, they should be permanently installed on the filter outlet hose or pipe so the filter can be easily serviced. This also prevents damaging the gauge when replacing the filter element.

Steve D’Antonio offers services for boat owners and buyers through Steve D’Antonio Marine Consulting (


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