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Air trapped in the high pressure oil circuit that actuates the injectors, which cannot be compressed usefully. The circuit relies on oil being incompressible — air in it absorbs the pump's work as compression rather than transmitting it, so pressure builds slowly or not at all.

Its signature is a hard start that improves with cranking. Each stroke purges a little air, so the engine catches after a long crank and then runs acceptably — and refuses again after standing. That pattern distinguishes it from a worn pump, which fails consistently rather than improving as it works.

Air arrives through the same paths that oil leaves. A failed injector seal, a loose standpipe, a leaking fitting or a low reservoir will let air in as the circuit bleeds down when the engine stops. So air and a leak are usually the same fault seen from two sides, and purging without finding the entry point means meeting it again the following morning. Check the reservoir level and look for a joint that has been disturbed before pursuing the pump.