The following lists match the various results obtained from the low-pressure fuel circuit test to possible sources of fault and remedial measures.
Fuel return quantity too low
Fuel return lines downstream of fuel filter housing are kinked or clogged:
Replace fuel return lines.
Check valve in plug-on valve on fuel filter housing does not open or does not open enough (opening crosspiece on fitting bent) or is clogged:
Replace plug-on valve.
Permanent purging bore in filter mount is clogged due to fouling:
Clean filter mount; replace if necessary.
Permanent purging bore in banjo union (under overflow valve) is blocked by dirt:
Clean banjo union; replace if necessary.
Fuel return quantity too high
Overflow valve is stuck in closed position:
Replace overflow valve.
Fuel intake pressure too low
Hand pump leaky:
Replace hand pump.
On vehicles with fuel prefilter with heated water separator, filter housing on fuel prefilter leaks (draws air):
Clean fuel prefilter, replace if necessary and retighten screws on filter housing.
Fuel heat exchanger leaky (draws air):
Replace fuel heat exchanger.
Fuel line (feed) and connectors leaky (draw air):
Replace sealing ring and retighten 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 intake pressure too high
Fuel line (feed) kinked or clogged:
Replace fuel line.
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.
Sieve on intake manifold of fuel fill level sensor is clogged:
Remove fuel fill level sensor and check sieve; clean or replace if necessary.
Check valve in connection of fuel return line does not open or does not open enough (opening crosspiece on fitting bent) or is clogged:
Repair or replace check valve.
Vent valve on fuel tank is defective:
Replace vent valve.
Leak located between filter insert and return line (foot seal on filter element defective):
Replace filter insert or foot seal.
Filter mount in fuel filter housing loose:
Seal and securely screw on filter mount.
Shrink hole or crack in fuel filter housing:
Replace fuel filter housing.
Permanent purging bore in filter mount has incorrect nominal width (bore too large):
Replace filter mount.
Permanent purging bore in banjo union (under overflow valve) has incorrect nominal width (bore too large):
Replace banjo union.
Low-pressure fuel circuit in engine leaky (in event of pressure drop)
O-rings on unit pumps leaky or defective, a second O-ring not installed:
Visually inspect oil dipstick for fuel in engine oil, remove oil pan if necessary and visually inspect unit pumps (O-rings); if necessary, remove unit pumps concerned and replace O-rings.
Hand pump leaky:
Replace hand pump.
Fuel heat exchanger leaky:
Replace fuel heat exchanger.
Fuel lines and connectors leaky:
Replace sealing ring and retighten threaded connections.
Check valve in plug-on valves leaky:
Replace plug-on valve.