Ensemble de dosage de la pompe d'injection usé

The high pressure pump's metering elements — pistons, bores and the control valve — worn to the point where the pump can no longer reach or hold commanded rail pressure. It is the failure at the end of the fuel pressure diagnosis rather than the start of it, and the order matters because a restricted filter and a weak lift pump imitate it exactly while costing a fraction as much.

These pumps are lubricated by the fuel they pump, which is what makes their wear self-accelerating and what connects it to everything upstream. Water displaces that lubricating film; debris scores the bores; running the rail short of pressure worsens both. A pump that failed early was usually failing because of something that is still in the tank.

Wear shows under demand rather than at idle. The symptom is a shortfall that appears under load or at high engine speed and disappears at light throttle, because a worn pump can still meet a small demand.

Check the low-pressure feed while the fault is present before condemning the pump. A pump starved by a blocked filter behaves identically from the module's point of view, and debris from a genuinely failed pump circulates through the whole system — so a replacement fitted without flushing the rail, lines and injectors is fed its own predecessor's wreckage.