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A restriction in the line feeding fuel to the engine, so flow falls short of demand while pressure may look acceptable at rest. Pressure and flow are different measurements and this cause is where that distinction earns its keep — a restricted line can hold static pressure perfectly and still starve the engine the moment it is asked for volume.

So the fault appears under load and vanishes at idle, which sends people towards the turbocharger, the filter or the injectors. Rail pressure drops away at full throttle, the module compensates and then stores a pressure or lean code, and everything tests fine on the ramp at idle.

The causes are mostly mechanical and mostly outside the engine bay. A crushed or kinked line after work under the vehicle, a collapsed flexible section that has softened internally, a filter beyond its interval, or debris lodged at a fitting. Measure delivered volume over a timed period at the rail rather than reading pressure — that is the test that distinguishes a restriction from a weak pump, and the two are otherwise indistinguishable.