Page Number - 39
Appendix C
Returnless (Mechanically Regulated)
Fuel System Diagnostic Guide
Scenario 2
Diagnosis:
A restriction to the supply of fuel to the engine such as a
clogged inlet screen, inline filter, or pinched fuel supply line will
reduce the flow of fuel. This will be evident by the reduction in
peak flow. Idle pressure will remain normal or drop only 1 or 2
psi below spec. Capability pressure will be low, depending on
the amount of restriction and the type of return/filtration method
used.
Additional Action:
Inspect the fuel supply line for damage. If the return/filtration
method is type 1 or 2, connect FST between fuel tank and inline
filter, and retest. If peak flow is normal, replace inline filter. If flow
is still low replace inlet screen/sock.
Scenario 3
Diagnosis:
Low idle and capability pressure, combined with normal peak
flow is a sure indication of an under-restricting pressure
regulator.
Scenario 4
Diagnosis:
High idle and capability pressure, combined with normal peak
flow is a sure indication of an over-restricting pressure regulator.
Scenario 5
Diagnosis:
Low values for all three indicators are an indication that the
output of the fuel pump is not adequate.
Additional Action:
Verify that the vehicle is not using a multiple speed
pump. If so, a scanner must be used to operate the
pump at its highest speed. Before replacing pump, follow
the vehicle manufacturer’s proper procedure to check for
low voltage or a bad connection or ground.
