How To Change Diesel Fuel Filters

Many diesel engine breakdowns are due to dirty fuel. Regular filter changes can help prevent stalls and other problems that could ruin your day.

Many boat owners think nothing of major DIY projects but shy away from even the most basic of diesel engine maintenance jobs, thinking they’re too complicated. In many ways, however, a diesel is more straightforward than a comparable gas engine. While there are some differences, these aren’t of great concern when it comes to changing out fuel filters.

To work effectively, all engines need a supply of clean fuel that’s devoid of particulate matter and water. Many diesel engine breakdowns are directly related to fuel issues, so regular fuel filter changes at the engine manufacturer’s specified intervals (and sometimes more often) will go a long way to prevent failures. Most manufacturers will suggest filter changes after a certain number of engine running hours, but they should be changed at least once a year even if the boat has seen little use.

Technical Support

Difficulty: Moderate


  • Fuel filters
  • Oil absorbent pads
  • Disposable gloves
  • Paper towels
  • Plastic zipper bags


  • Wrenches
  • Screwdrivers
  • Small funnel

Time: 30 minutes to 1 hour

Cost: Approx. $25, depending on the number and size of filters

Changing fuel filters, even on a large engine, is well within the capabilities of the average DIYer who uses a methodical approach. It may take a couple of hours to complete the first time, but it will get easier and faster after that. There are lots of different types and styles of filters, and the photos shown are from my particular boat. Even if your engine requires different filters, the principles are likely to be similar. Here’s how.

1. (Not shown.) Turn off the fuel petcock (if fitted) to prevent fuel from draining or siphoning out of the tank, then place a suitable receptacle under the primary filter to collect any spilled fuel. Lay down some oil-absorbent pads to capture inevitable spills.

2. Unscrew the T-handle on top of the filter and remove the cap. I have a Parker Racor filter, which is probably the most popular type of primary filter. If you have a different type or design, refer to the manufacturer’s instructions, often contained within the box that the new filter comes in.

3. After removing the top cover and T-handle, lift out the old filter element and place it into a plastic zippered bag for proper disposal. Open the petcock (see arrow) on the bottom of the filter to remove any water and dirt in the bottom of the bowl into a suitable receptacle. This receptacle should be clear so you can inspect the fuel to see if there is trash or water in it. Sometimes the bowl on the actual filter body will not be very clear. With sediment drained, retighten the petcock. If the fuel is particularly dirty, simply draining the fuel may not be not enough; the bowl must be cleaned more thoroughly, which can typically only be accomplished after disassembly and is beyond the scope of this article.

4. Remove and dispose of the rubber O-ring from the base of the threads on the T-handle. Use a thin-blade screwdriver to unseat and remove the rubber gasket from the groove on the underside of the lid (see photo 6). The gasket is often quite well stuck in and difficult to remove and should be pried out with a screwdriver or the end of a very blunt knife, taking care to not damage the seat. With the seals removed, clean the mating surfaces and seal groove with a clean, lint-free rag.

5. Drop the new filter into the housing, and fill with fresh clean fuel.

6. Smear some clean fuel onto the surfaces of the new gasket and O-rings before carefully reseating them into their respective grooves.

7. Replace the cap, making sure the filter housing is correctly seated in the groove. Then screw down the T-handle, complete with new O-ring, making it hand tight only.

8. With the primary filter completed, turn your attention to the secondary filter(s); usually mounted on the engine and close to, and immediately after, the lift pump. The engine-mounted secondary filters are on the pressure side of the lift pump, while the primary filter(s) are on the suction side. (See illustration below) Your engine filters may be different, so consult your manual.

You’ll likely have to bleed the fuel system after you replace the filters. It’s not hard to do, and we’ve created a video to walk you through the process. Go to Bleeding A Marine Diesel Engine to learn how.

9. Place oil-absorbent sheets or pads under the filters. Loosen the central bolt and remove the bottom bowl, then drop the filter. This type of filter may have a bowl that separates from beneath the filter element. The wall of that element may also serve as part of the containing wall for the filter. There may be a lot of fuel spilled; be sure you’re prepared. If you have more than one secondary filter (a common scenario on larger engines), repeat the process.

10. Remove the O-ring seals from the grooves in the top carrier and the bottom bowl, then wipe the seating surfaces with clean paper towels to remove any dirt and debris.

11. Install new O-ring gaskets after smearing fresh clean fuel on their surfaces, making sure they’re correctly seated.

12. Mount the new filter and screw the assembly together, making sure not to overtighten the bolts.

Carefully check your work, then reopen the fuel petcock (if fitted). Bleed air from the fuel system. (The process of bleeding varies with the particular engine. Consult your manual.) Start the engine and check for leaks. If the engine fails to start, you may have an air leak at the primary filter or on the suction side of the lift pump. In this case, check your work again. Often an air leak is due to incorrectly assembled filter components, or you may need to bleed the engine again.

A Word On Microns

All fuel filters will have a micron rating on the side, typically 30, 10, or 2 microns. These numbers refer to the size of particulates that the filter will trap. To put this in perspective, 30 microns is just slightly larger than the diameter of human hair. Both 10- and 2-micron particles are so small that they are invisible to the naked eye.

It’s essential to follow the manufacturer’s recommendations. Yanmar, for example, recommends using 30-micron primary filters and 10-micron engine-mounted secondary filters for most of their engines. There are exceptions, so always consult the owner’s manual for specific requirements.

Common-rail diesel engines generate extraordinarily high pressures (in the tens of thousands of pounds per square inch) and are thus very susceptible to poor fuel quality. Consequently, many common-rail diesels will use a 2-micron secondary engine-mounted filter and involve other considerations.

Mark Corke

Contributing Editor, BoatUS Magazine

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