Definition
Bleeding is the process of removing air from a diesel fuel system by working through bleed points in sequence from the tank towards the engine, and is required after a filter change, a fuel run-out or any repair that opened the fuel circuit.
A diesel injection system pumps liquid, and air is compressible. With air in the lines, the pump cannot deliver the precise volume at the precise moment injection requires, and the engine will not run.
The principle
Air is pushed forward and out through bleed points in order, working from the tank towards the engine. Skipping a point leaves a pocket that stops the engine minutes after it starts — which is why an engine that runs briefly then dies after a filter change is almost always an incomplete bleed. Full procedure: how to bleed air from a marine diesel.
The common-rail rule
Never crack a high-pressure line or an injector on a common-rail engine to bleed it. Those pressures can inject through skin and cause life-threatening injury. Most self-prime or have a specified procedure — follow the manual exactly.
If it will not stay bled
Suspect an air leak on the suction side: a filter lid seal, a cracked hose, a loose fitting, or a tank pickup lifting clear of the fuel in a seaway.