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Air entering the fuel circuit and reaching the high pressure pump, which cannot compress it. A diesel injection system depends on fuel being incompressible; air in the circuit means the pump strokes without building pressure, and the result is a long crank, a rough start or a stall that clears once the system purges itself.

It differs from a suction-side leak in scope rather than in kind. A leak draws air continuously under vacuum; this entry covers the broader case, including air left after a filter change that was never bled, a tank run genuinely empty, or a system opened for repair and closed without priming. The timing is the strongest clue — a fault that begins immediately after fuel work is air until proven otherwise.

Bleeding is the repair and finding the entry point is the diagnosis, and skipping the second means doing the first repeatedly. Many systems self-bleed given enough cranking, which is exactly why a genuine leak hides: the vehicle recovers each time and the fault reads as intermittent. Check whether it returns after standing — air that reaccumulates overnight is coming in somewhere, and air that never returns was simply left behind.