By the system's own priming method - a hand pump, an electric pump run on the ignition, or cranking in a specified sequence. Establish which before removing the filter.

Many older systems have a hand priming pump on the filter head and a bleed screw, and the sequence is pump until firm, crack the bleed screw, close it when fuel runs clear.

Others run an electric lift pump when the ignition is switched on, and simply cycling the ignition several times purges the filter.

Modern common-rail systems often prime themselves but can be difficult if air reaches the high-pressure pump, which some manufacturers explicitly warn against.

Extended cranking to prime is hard on the starter and, on some systems, on the high-pressure pump itself, which relies on fuel for lubrication.

Check the procedure before starting the job, not after the vehicle will not start.

Where it mounts vertically, yes - with scrupulously clean fuel, because anything poured in goes straight to the injection equipment.

Pre-filling removes most of the air and makes priming quick or unnecessary.

The fuel must be clean and drawn from a clean container, ideally through a funnel with a fine screen - the filter is being bypassed by definition.

Pour into the outer holes rather than the centre outlet, so what goes in still has to pass through the media.

Some manufacturers specifically prohibit pre-filling on high-pressure systems for exactly this contamination reason, and their instruction governs.

On horizontally mounted filters it is impractical, and priming is the only route.

Because water is what destroys injection equipment, and diesel always carries some - the drain is a two-minute job that prevents a very expensive failure.

Water enters through condensation in tanks, through poor-quality fuel and through storage.

It causes corrosion and cavitation damage in pumps and injectors, and it destroys the lubricating film those components rely on.

Most diesel spin-on filters have a drain tap or plug at the base, and many have a water-in-fuel sensor that warns before the water reaches the outlet.

Draining is done with a container ready, opening the tap until clean fuel runs, and it belongs on the service schedule as well as being done when the warning appears.

Water drained from a filter is contaminated waste and is not poured down a drain.

Substantially. Modern common-rail injection requires far finer filtration than older systems, and a coarser filter that threads on will destroy pumps and injectors.

Older indirect-injection diesels tolerated relatively coarse filtration because clearances were larger and pressures much lower.

Common-rail systems operate at extremely high pressures with very fine clearances, and they specify correspondingly fine filtration.

A filter with the correct thread but a coarser rating will fit and run, and the damage accumulates invisibly over months.

Efficiency at the rated size matters as much as the number itself, and reputable manufacturers publish both.

Water separation efficiency is a separate specification again, and modern low-sulphur and biodiesel-blended fuels make it harder rather than easier.

It is, and it is often the actual fault - the priming pump, the drain tap, the sensor and the head's own seals all fail.

The head carries the priming pump on many systems, and its diaphragm or seals perish so it will no longer build pressure.

Drain taps seize or leak, and a leaking tap draws air in on the suction side without leaking fuel out.

Water-in-fuel sensors corrode and either fail to warn or warn constantly.

The head's internal seals and any bleed screw washer are consumables and are available as kits on most systems.

A vehicle that repeatedly loses prime overnight usually has a head or a check valve problem rather than a filter problem.

Air being drawn into the suction side - a leaking drain tap, a perished seal, a failed check valve, or a cracked pipe.

A leak on the suction side lets air in without letting fuel out, so there is nothing to see.

Fuel drains back to the tank overnight and the system has to be primed each morning.

The drain tap is the commonest single culprit, followed by the filter seal itself if it was not seated properly.

A failed non-return valve in the head or the lift pump allows the same drain-back.

Clear plastic sections in the line make the air visible while cranking, which is the quickest way to locate the source.

No, and the resemblance is dangerous - the media, ratings, pressure and seal compounds are all different.

They look similar and sometimes share thread sizes, which is exactly why the mistake happens.

Fuel filter media is designed for fuel and often for water separation; oil filter media is not and will disintegrate in fuel.

Seal compounds are chosen for the fluid, and an oil filter's gasket can swell or harden in diesel.

Micron ratings are entirely different - an oil filter is far too coarse to protect injection equipment.

Oil filters also contain bypass valves that would deliberately pass unfiltered fuel, which is the last thing an injection system needs.