How to bleed a marine diesel fuel system

Fill the filters with clean fuel, then work air out of the system in the direction the fuel flows — lift pump, primary filter, secondary filter, injection pump, and injectors last if needed — opening each bleed point in turn until fuel runs without bubbles. A diesel will not run with air in the fuel line, which is why an engine that ran perfectly before a filter change will not start after one.

Many modern engines self-bleed and need nothing more than cranking. Older mechanical engines usually need the full sequence.

Before you start

Engine cold, batteries good. Bleeding often involves cranking, and a marginal battery will fail halfway through.
Plenty of rags and a container. This is a messy job and spilled diesel in the bilge is both a pollution problem and a slip hazard.
Know your engine’s bleed points. They vary. The manual is the authority.
Never bleed injectors on a common rail engine. Those systems run at pressures high enough to inject fuel through skin, and they are not owner bleed points.

The procedure

Establish that you actually have fuel. An empty tank, a closed fuel cock or a blocked pickup will defeat any amount of bleeding.
Fill the new filters with clean fuel from a container before fitting them. This removes most of the air before you start.
Open the bleed screw on the primary filter or water separator.
Operate the manual lever on the lift pump until fuel runs from the bleed screw without bubbles, then close it. If the lever feels solid with no resistance, turn the engine over one revolution by hand or on the starter — the pump cam may be at the top of its stroke.
Open the bleed screw on the secondary or engine-mounted filter and repeat until the fuel runs clear of bubbles. Close it.
Open the bleed screw on the injection pump, if your engine has one, and repeat. Close it.
Attempt to start. Many engines will run from here.
If it will not start on a mechanical engine with individual injector lines, slacken the union nuts at the injectors by about half a turn, crank until fuel appears at each, then tighten them while cranking. Do this only on a conventional mechanical system, never on common rail.
Once running, check every joint you opened for leaks, and run the engine long enough to confirm it does not falter.

Why it failed in the first place

Air in the fuel is a symptom. The common causes are worth checking before you congratulate yourself on the bleed:

A filter change, which is the benign explanation
Running the tank dry, which is the second most common
A leaking joint on the suction side, which draws air in without leaking fuel out — look for a weeping union or a perished hose
A blocked filter pulling hard enough to draw air past a seal
Contaminated fuel blocking the pickup, which is a tank problem rather than a fuel line one — see marine diesel fuel quality and storage

If you bleed successfully and the engine loses power again in a seaway, you have a tank problem, not a bleeding problem.

When to stop

Stop if the engine has common rail injection and will not start after filling the filters and cranking. Stop if fuel will not come through at all, which points at supply rather than air. Stop if you find diesel in the bilge.

Frequently asked questions

Why won’t my diesel start after changing the filters? Air entered the system when the filters were opened. It needs bleeding, and filling the new filters with clean fuel before fitting removes most of it.

Do modern marine diesels need bleeding? Many self-bleed and need only cranking. Older mechanical engines usually need the full sequence.

Can I crack the injectors to bleed the system? On a conventional mechanical engine with individual injector lines, yes. Never on common rail — the pressures can inject fuel through skin.

What if fuel won’t come through at all? That is a supply problem, not an air problem. Check the tank level, the fuel cock and the pickup before continuing to bleed.

The engine bled fine but loses power in rough water. Why? Motion is stirring contamination off the tank bottom into the pickup. That is a tank problem.