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Keeping water out of your fuel

Another article in our series of maintenance topics suggested by readers of Boaters Update. This one is about keeping water out of your fuel by draining sedimentors and agglomerators. By now, it won’t surprise you to learn that we've turned to the experts at River Canal Rescue for their help.

Debbi changing the fuel filter on her boat engine

When storing fuels in any environment, a water build-up is highly likely; particularly in the leisure marine industry where a low fuel turnover rate coupled with the realities of cruising almost guarantees this happening. The only question is how much water is in there?

Luckily, most vessels have installed a ‘last line of defence'- this makes sure water does not reach vital components and warns of the issue before any damage is caused. These ‘last line of defence' components have multiple names such as sedimentor, agglomerator, cav-filter and pre-filter, however the one common denominator is that they all remove water from the fuel before it reaches the engine.

Due to the nature of their role, they need regular maintenance in the form of draining and checking the water content. Note - if water is present every time they are checked, it is a sure sign the tank needs maintenance.

How to check your filters

Check your water ‘filters' by unscrewing the drain plug and seeing what escapes. All water separating filters have a drain plug on the bottom which is detailed in the following images.

Draining water from the filter will not affect the engine and once you've done this, you do not need to ‘bleed' the fuel system. You will need a container such as an old margarine tub or small plastic tray to catch the drained fluid, simply place it underneath the filter and loosen the drain plug. Don't remove it completely as things will get messy.

Allow fuel to flow/drip into the tray until there is a small puddle in the tray. At this point you should close/tighten the drain plug and inspect the liquid. If it is clean red diesel (blue or green if from an automotive source) the check is complete. If there's water, slime or debris, loosen the plug again and wait until clean fuel flows into the tray. If water/debris is present, re-check in three to seven days.

Cav filter

This image is a CAV filter - the most common filter used on leisure marine diesel systems. It's been adapted multiple times so may have different ‘bowls' on the bottom, ranging from a very shallow aluminum cup to a large glass adapter or even a complete glass lower section like the one above.

Pinch screw

The drain plug will have one or other of the following fastening types:

  • simple
  • bolt
  • or a pinch screw (illustrated right)

Other types of filter

Fuel guard

Fuel guard - also common, but a newer filter design, works very well in multiple circumstances. Some are fitted with a sensor that warns if any water is present, greatly reducing the amount of maintenance needed.


Wasp - this filter is a sedimentor and has a steel mesh, cleanable filter element. Fairly common on the waterways, it is often a forgotten element of a service. Note - if the filter ‘bowl' is removed, the O-ring is damaged, so make sure you order one before any planned removal.


Agglomerator - again fairly common but with no filter element. The only maintenance needs are the periodical draining of water/debris that has built up. This filter is designed to deal with water only and does not remove other contaminants.

Thanks again to the experts at River Canal Rescue.

Last Edited: 11 February 2019

photo of a location on the canals
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