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Diesel Fuel Filters: Which Should You Choose?

If you want your diesel engine to run properly, you need the right fuel filters.
Racor's Marine Duplex 900 Turbine Series diesel fuel filter and water separator is a popular choice as a primary filter.

Racor's Marine Duplex 900 Turbine Series diesel fuel filter and water separator is a popular choice as a primary filter.

One key to keeping your diesel engine running smoothly is to have the proper micron rating for your primary and secondary fuel filters. A micron rating indicates the ability of a filter to remove contaminants by the size of the particle.

How it works

Diesel engines are equipped with a fuel transfer pump that sucks fuel from the fuel tank and supplies it under pressure to the fuel injection system. Between the fuel tank and the fuel transfer pump (the vacuum side), boatbuilders or boaters fit a fuel filter/water separator (the primary filter). Between the fuel transfer pump and the fuel injection system (the pressure side) the engine manufacturer fits a fuel filter (the secondary filter).

A popular primary filter is Racor’s combination turbine series unit, which is both a fuel filter and a water separator. For these units, Racor offers a choice of three filtration efficiencies: 2, 10 and 30 microns. This article will advise you on which filtration efficiency you should choose for your engine. The engine-mounted secondary fuel filter has a filtration efficiency appropriate for the engine, which ranges from 15 microns down to 2 microns.

Any analysis begins with the engine manufacturer’s recommendations. For the legions of Yanmar owners, the choice is clear. Yanmar recommends 30-micron primary filters, since using a 20-micron or smaller as a prefilter can cause fuel flow starvation on some models, according to the company. The engine-mounted secondary filter supplied by Yanmar for its mechanically governed engines is 10 microns. You should never deviate from the engine manufacturer’s recommendations with respect to the secondary filter. The filtration efficiency of the primary filter, on the other hand, may be somewhat increased if oversized filters are used to compensate for the loss of flow occasioned by an increase in captured contaminants.

The new electronically controlled diesel engines by Yanmar, Caterpillar and others have a 2-micron secondary filter. Racor recommends that a 2-micron filter only be used in final or secondary filters where the fuel is first filtered by a primary filter. Further, the company says a 30-micron filter should be used as a primary to filter raw or poor-quality fuel before it is further filtered by finer media, such as a 10- or 2-micron. Racor says a 10-micron can be used as a primary filter for fuel that is known to be of good quality. Yanmar recommends a 30-micron primary filter regardless of fuel quality for use with the 2-micron secondary filter on its new electronic engines.

Conceptually, think of the primary filter/water separator as providing filtration of raw fuel to extend the life of the secondary fuel filter, with the ultimate objective of protecting the fuel injection system while keeping the engine running when there are contaminants in the fuel. Many boaters assume that using a 2-micron filter in the primary has to be better than using a 10- or 30-micron. But that conclusion ignores the fact that the 2-micron filter will clog quickly when there is contamination, causing fuel flow starvation and possible engine shutdown. It also ignores the fact that the manufacturer-supplied secondary filter on the engine is completely up to the task of protecting the engine, and will rarely need to be changed between mandatory maintenance intervals if protected by an appropriate primary filter.

Doing the math

To analyze what is best for your boat you need to know how many gallons per hour your engine’s fuel transfer pump has to move at full-rated rpm. On Maramor, my Grand Banks 42, the fuel transfer pump’s maximum flow at full-rated rpm is 65.5 gph. The actual fuel burned at full-rated rpm is 23.2 gph, the rest being returned to the fuel tank through the fuel cooler. A primary filter/water separator should have a rated capacity of more than 1-1/2 times the delivery volume of the fuel transfer pump at full-rated rpm, which in Maramor’s case is 98.25 gph.

The yellow Caterpillar fuel filter on this engine is the secondary filter intended to capture the particles the Racon primary filter on the far right did not catch.

The yellow Caterpillar fuel filter on this engine is the secondary filter intended to capture the particles the Racon primary filter on the far right did not catch.

Maramor has a single Caterpillar 3126B 420-bhp engine with hydraulically activated, electronically controlled unit injectors. The fuel pressure generated by the HEUI injectors is about 23,500 psi, and Caterpillar supplies a 2-micron secondary filter to protect this system. The primary installed by Grand Banks is a Racor 75/900MAX turbine series fuel filter/water separator that has a flow rate of 180 gph. For the reasons discussed here, suitable filter media for Maramor’s primary Racors are either the 10- or 30-micron filters.

Caterpillar mandates that the secondary 2-micron filter be changed after 200 service hours or 1,500 gallons of fuel. Maramor’s engine is operated at a load that consumes 6 gph, so the limit of 200 service hours applies. Since the primary filter has such a large capacity relative to actual fuel flow of about 18 gph, clogging to the extent of causing fuel starvation would have to be major. That means that with fuel known to be of good quality, the 10-micron Racor filter can be used on Maramor. However, the 30-micron filter also is adequate for the task, and best from the point of view of keeping the engine running when there is contamination, and necessary to deal with fuel that isn’t pristine.

When to change filters

Fit a vacuum restriction gauge on your primary filter. Used fuel filters are toxic waste, so be a responsible boater and only change your filters before the limits specified by the manufacturer when the vacuum gauge shows they are clogged, not because they appear discolored or dirty. Among other criteria, Racor recommends that the primary fuel filter be changed after one year or 500 hours, which is considerably beyond a typical one year’s running hours for a pleasure boat. So for the typical boat, unless the vacuum gauge on the primary filter indicates a restriction, the 30-micron Racor filter in the primary normally needs to be changed only once a season.

The Racor MAX primary filter/water separator is two complete filter/water separator units connected by a manifold with a valve control allowing the isolation of one filter at a time for servicing, even during engine operation. The MAX can be used in two ways. With both filters in use the maximum flow rate is 180 gph. With only one filter in use the flow rate is 90 gph, and the other is a “spare,” ready for use in an instant should the on-line filter clog or its separator bowl fill with water. For Maramor, the latter is the preferred method. Maramor is never run anywhere near full-rated rpm and, therefore, the 90-gph flow rate is in practice five times the delivery volume. The MAX is readily accessible so the filter manifold’s inlet valve handle can be quickly and safely reached in an emergency.

The Racor 75/900MAX fits nicely in Maramor’s spacious engine room but is unsuitably large for many boats. Remember, you also need room above the filter unit so you can change the filters, and below so you can drain off water and sediment from the bowls. If the primary filter/water separator that is fitted in your boat is in the minimum recommended capacity range of 1-1/2 times the fuel transfer pump’s maximum delivery volume at full-rated rpm, a 30-micron filter is the best choice.

Preventing diesel fuel contamination

Diesel engines are more efficient and safer than gasoline engines. Diesel fuel has a higher density than gasoline and contains more energy per gallon. Most engines are designed to operate on ASTM No. 2-D grade, which specifies such requirements as cetane number, cloud point, flash point, gravity, pour point, lubricity, sulfur and the like. Kerosene and home heating oil are similar to diesel fuel, but they aren’t formulated to meet the needs of your engine.

During the transportation and distribution process — not to mention on your boat — there are many opportunities for the fuel to become contaminated with water and particulates that can plug filters. Once in the fuel injection system, these will cause severe damage because of the close tolerances within fuel pumps and injectors. I have observed that many boaters automatically add chemicals to their diesel fuel to deal with the possibility of degradation during storage, microbial growth, water contamination and so forth. However, diesel fuel can be stored in a clean, full tank for six months to a year without significant quality degradation; these chemicals may or may not be benign to your fuel and your engine, and should be used with caution.

If the cause of a fuel problem isn’t patently obvious, the best course of action is to take a fuel sample and test it yourself with commercially available kits or, better, have it tested by a lab. If filtration isn’t a viable solution to the problem, it is a very good idea to ask the engine manufacturer for additive recommendations to make sure you aren’t solving one problem while causing another. The bottom line is: Use additives only when you have to, but always filter your diesel fuel through a water separator, and high-quality primary and secondary filters.

This Racor fuel filter and water separator system is the first line of defense against fuel contamination.

This Racor fuel filter and water separator system is the first line of defense against fuel contamination.

A word about filter quality. Fuel filters are rated by filtration efficiency. Racor’s 2-, 10- and 30-micron filter elements have an efficiency rating of 98 percent, 95 percent and 90 percent, respectively, meaning that when tested to SAE or ISO test methods the filter will retain that percentage of the micron size and larger particles. The new high-pressure common-rail and hydraulically activated fuel injection systems require that the filter remove 95 percent of 3-micron particles. The secondary fuel filter fitted by Caterpillar on Maramor’s electronic engine is rated at 98 percent of 2-micron particles. This filter is fitted on the front of the engine, is cool to the touch even when the engine is running, and can be changed quickly with a strap wrench.

A simple precaution would prevent one of the most common causes of water-contaminated fuel: Inspect the gasket on the fill-pipe cover every time you fuel, and replace it as necessary.

In summary

Higher filtration efficiency in the primary filter isn’t necessarily the best strategy. 

• Fuel filtration should be staged first through a coarse filter medium before the fuel transfer pump, then through a finer filter medium after the transfer pump. 

• The filtration efficiency appropriate for your engine’s primary filter is the one that achieves both adequate protection of the secondary filter and the fuel injection system, while minimizing the possibility of fuel starvation.



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