Mityvac MV5545 FST PRO FUEL SYSTEM TESTER - Manual (Page 30)

Fig. 55
Fig. 54
9. With the car idling, rotate the flow control valve past the 90°
CLOSED position towards BYPASS (Fig. 54). Adjust the valve until
the flowmeter indicates an amount of fuel flow equivalent to the
value for the maximum engine fuel volume taken from the table
(Fig. 55).
10. Note the pressure reading on the gauge.
11. After noting the capability pressure, return the flow control valve to
the OPEN position. Testing is complete.
Diagnosing the Results
The values for four critical fuel system performance indicators should
have been noted while following the procedures and performing the
tests outlined above:
• Idle Pressure
• Peak Demand Pressure
• Peak (Dead-head) Pressure
• Peak (Bypass) Flow
These indicators are the key to properly diagnosing a malfunctioning
fuel delivery system, and pinpointing the cause.
Compare the four values to the Returnless (electronically regulated)
Fuel System Diagnostic Chart (Appendix D). If according to the chart,
the FST test values indicate a normal operating fuel delivery system,
then the engine is receiving the proper pressure and flow of fuel, even
under maximum load conditions. If the FST indicates a normal oper-
ating fuel delivery system, yet the vehicle continues to experience
symptoms of a fuel delivery malfunction, it could be caused by con-
taminated fuel, faulty fuel injector(s) or electronic control system, or an
intermittent component malfunction such as a loose electrical con-
nection.
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
performing 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 sys-
tem affect its performance.
Additional Testing and Diagnostics
Blocked Inline Fuel Filter
If the vehicle has an accessible fuel filter, and initial test results indi-
cate it may be partially blocked, retest the fuel system with the FST
connected between the fuel tank and the filter, preferably at the filter
inlet. If the peak flow and peak demand pressure are higher then in
the initial test, replace the fuel filter and retest. If the peak flow and
peak demand pressure remain the same, 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 demand pres-
sure of a clogged inlet strainer will be slightly lower than that 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 densi-
Page Number - 30 Form 824127
Maximum fuel
volume
requirement
(Appendix A)