Unlike older mechanical diesels, the OM646 uses an electric lift pump in the fuel tank or a mechanical gear pump on the front of the cylinder head. The workshop manual details the bleeding procedure after replacing the fuel filter to prevent dry-running the high-pressure pump, which causes internal metal flaking. 3. Core Engine Mechanical Overhaul
To service the valves, head gasket, or camshafts, the engine must be set to Top Dead Center (TDC) on cylinder 1. Mercedes Workshop Manual Om646 Engine