Form 824262
Page Number - 29
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 strainer, 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 .05 or .15
bar (1 or 2 psi) (5 or 15 kpa) below spec. Peak demand
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 strainer/sock.
Scenario 3
Diagnosis:
Low idle and peak demand pressure, combined with normal peak
flow is a sure indication of an under-restricting pressure
regulator.
Scenario 4
Diagnosis:
High idle and peak demand 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.
