Short answer
Close the fuel valve, place absorbent pads, then change the primary filter and water separator first and the secondary last. Fill new filters with clean fuel before fitting, seal them by hand to the specified tightness, bleed the system, then run and re-check every joint for leaks.
You will need: Filter wrench, spanners, absorbent pads, clean fuel, waste container, spare seals
Filter changes are routine, and they are also the single most common cause of an engine that will not restart — because of air, not the filters.
Order and why it matters
The primary filter and water separator comes first: it does the coarse work and holds water. The secondary or engine-mounted filter is finer and comes last. Changing the secondary first pushes whatever the primary was holding straight into the clean side.
The sequence
- Close the fuel supply valve. Lay absorbent pads under the work.
- Drain the primary bowl into a container and note what comes out — water, sludge and growth are findings, not just waste. See diesel bug and water in fuel.
- Remove the old element, clean the sealing face, and fit a new seal where the design uses one.
- Fill the new element with clean fuel from a clean container to reduce the air you have to bleed.
- Fit by hand to the tightness the maker specifies. Overtightening distorts seals and causes leaks.
- Repeat for the secondary filter.
- Open the fuel valve and bleed the system.
- Start, run, and check every joint under running pressure. Check again after an hour.
Record it
Note date, hours, part numbers and what you found in the bowl. That record turns the next repeat blockage into a diagnosis rather than a mystery — see service records.