Appendix B
Return 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. However, pressure will remain normal or drop
only .05 or .15 bar (1 or 2 psi) (5 or 15 kpa) below spec. If the
flow at idle is less than the calculated maximum fuel volume re-
quirement, then the engine will be starved of fuel in a heavy-load
condition such as wide-open throttle.
Additional Action:
Inspect the fuel supply line for damage. Connect FST between
fuel tank and inline filter, and retest. If flow is normal, replace
inline filter. If flow is still low replace inlet strainer/sock.
Scenario 3
Diagnosis:
High flow and low pressure at idle are a sure indication of an
under-restricting pressure regulator. If values for peak pressure
and flow remain normal, replace the pressure regulator.
Scenario 4
Diagnosis:
High pressure and low flow at idle indicates too much restriction
in the path of the fuel as it returns from the fuel rail to the fuel
tank. The cause can be an over-restricting pressure regulator or
pinched return line.
Additional Action:
Inspect the fuel return line for damage. If the regulator is vacuum
modulated, check the level of vacuum at the regulator. If it is low,
repair the cause. Connect the FST after the pressure
regulator and retest. If pressure remains high, the return line
is restricted. If it drops off to near zero, replace the pressure
regulator.
Scenario 5
Diagnosis:
Low values for idle and peak pressure and flow 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.
Form 824262
Page Number - 27
