procedures for the inspection and repair of these components.
If the FST test results indicate a fuel delivery system malfunction, and
the diagnostic chart clearly pinpoints the cause, follow the vehicle
manufacturer’s repair procedures to correct the malfunction. After per-
forming the repairs, re-test the fuel delivery system with the FST
to ensure it is operating normally.
If the FST test results are inconclusive as to whether there is a fuel
delivery system malfunction, or if a malfunction is evident but the
diagnostic chart does not clearly indicate the cause, additional testing
may be performed to provide more insight into the performance of the
system. Extended test procedures are outlined below, and provide a
more in-depth understanding of how the components of the fuel
system affect its performance.
Additional Testing and Diagnostics
To perform more in-depth diagnostics, consider the fuel delivery
system split into three zones as shown in figure 21. The initial test
procedures outlined above are performed in Zone 2 because the fuel
pressure and flow within this zone, most accurately represent the con-
ditions present in the fuel rail. Depending on the suspected
component malfunction, connecting the FST in either Zone 1 or
Zone 3 and comparing the test results to those in Zone 2 can help
pinpoint a malfunction.
Blocked Inline Fuel Filter
If the inline filter is clean, pressure and flow in Zones 1 and 2 should
be equal. A clogged filter will cause the pressure to increase in Zone 1
and the flow to decrease in Zone 2.
If the results of testing in Zone 2 indicate a blocked fuel filter, but are
not entirely conclusive, retest the fuel system with the FST connected
in Zone 1, preferably at the inlet of the fuel filter. If the idle pressure and
the peak flow in Zone 1 are higher than Zone 2, replace the inline fuel
filter and retest.
If the idle pressure and peak flow in Zone 1 remain the same as Zone
2, this would indicate a clogged inlet strainer/sock.
Clogged Inlet Strainer/Sock
The test results for pressure and flow of a clogged inlet strainer will
closely match those of a clogged inline filter. The peak pressure of a
clogged inlet strainer will be slightly lower than the peak pressure of a
clogged inline filter, and the current draw will also be lower.
A clogged inlet strainer can cause the fuel pump to cavitate because it
is starved for fuel. Cavitation will create rapid changes in fuel density,
causing the float in the FST flowmeter to bounce up and down. It can
also cause the needle on the pressure gauge to bounce. Also, as the
pump tries to pull gas through the clogged strainer, it creates a pressure
drop that may cause air bubbles to form and become visible.
These indications of a clogged inlet strainer may or may not be
evident during testing. If the results are inconclusive, retest the fuel
system with the FST connected in Zone 1, preferably at the inlet of the
fuel filter.
If the idle pressure and peak flow in Zone 1 remain the same as
Zone 2, this would indicate a clogged inlet strainer/sock. Replace
the strainer or the pump module, and retest.
Form 824262
Page Number - 15
