Low-pressure fuel circuit in engine leaky (in case of pressure drop)
O-rings on unit pumps leaky, defective or a second O-ring not installed or incorrectly installed:
Visually inspect oil dipstick for fuel in engine oil and visually inspect unit pumps (O-rings); if necessary, remove unit pump concerned and replace O-rings.
Filter housing leaking at fuel filter or fuel prefilter:
Clean or replace sealing ring/parts and retighten filter housing.
Fuel lines and connectors leaky:
Replace sealing ring and tighten threaded connections
Seal between fuel pump and crankcase is leaky:
Replace seal.
Seal between fuel filter and crankcase is leaky:
Replace seal.
The fuel return quantity at the fuel tank is too low
Fuel return lines downstream of fuel filter are kinked or clogged:
Replace corresponding fuel return line.
Check valve in plug-on valve on fuel filter does not open or does not open enough (opening crosspiece on fitting bent) or is clogged:
Replace plug-on valve.
Overflow valve is stuck in closed position:
Replace overflow valve.
Permanent purging bore of filter mount is clogged due to fouling:
Check permanent purging bore for unrestricted flow and clean; replace filter mount if necessary.
Bypass hole from fuel feed duct to fuel return duct of unit pumps in crankcase is clogged due to fouling:
Remove the unit pumps on the last cylinder and check diameter of the bypass hole; clean if necessary.
The fuel return quantity at the fuel tank is too high
Overflow valve is stuck in open position:
Replace overflow valve.
Leakage from fuel filter element to return line (foot seal defective or missing):
Replace fuel filter element or foot seal.
Filter mount in fuel filter is loose, seal is defective, missing or incorrectly installed:
Seal and securely screw on filter mount.
Connection fitting on one or more nozzle holder combinations is leaky, incorrectly installed or cracked:
Correctly install or replace connection fitting.
Nozzle holder combinations are leaky or have cracks:
Replace nozzle holder combinations.
Fuel suction vacuum upstream of the fuel pump too low
Filter housing on fuel prefilter is leaky (draws air):
Clean or replace sealing ring/parts and retighten filter housing.
On vehicles with heated water separator, filter housing on fuel prefilter is leaky (draws air):
Clean parts, replace if necessary and retighten filter housing.
Fuel line and connectors are leaky (draw air):
Replace sealing ring and tighten threaded connections
If the fuel intake pressure is still too low even after working through the remedies mentioned, the fuel pump is worn or the protection valve in the fuel pump is stuck in the open position:
Replace fuel pump
Fuel suction vacuum upstream of the fuel pump too high
Fuel line is kinked or clogged:
Replace fuel line concerned.
Intake manifold on fuel fill level sensor is on floor of fuel tank:
Check intake manifold by the fuel filling opening on the fuel tank; remove fuel fill level sensor if necessary and shorten intake manifold.
Filter screen on intake manifold of fuel fill level sensor is clogged:
Remove fuel fill level sensor and check filter screen; clean or replace if necessary.
Vent valve on fuel tank is defective:
Replace vent valve
Shrink hole or crack in fuel filter housing:
Replace fuel filter housing