Mityvac MV5545 User Manual download pdf (Page 27)

Fig. 49

Fig. 48

Form 824127 Page Number - 27

OPEN position.

eak Demand Test

7. Note the vehicle’s engine size and maximum engine speed (RPM).

Refer to the Maximum Engine Fuel Volume Requirements table

(Appendix A), and use the size and speed values to determine the

maximum fuel volume requirement of the engine.

8. With the car idling, rotate the flow control valve past the 90°

CLOSED position towards BYPASS (Fig. 48). Adjust the valve until

the flowmeter indicates an amount of fuel flow equivalent to the

alue for the maximum engine fuel volume taken from the table

(Fig. 49).

9. Note the pressure reading on the gauge.

10. After noting the capability pressure, return the flow control valve to

the OPEN position. Testing is complete.

Diagnosing the Results

The values for three critical fuel system performance indicators should

have been noted while following the procedures and performing the

tests outlined above:

• Idle Pressure

• Peak (Bypass) Flow

• Peak Demand Pressure

These indicators are the key to properly diagnosing a malfunctioning

fuel delivery system, and pinpointing the cause.

Compare the three values to the Returnless (mechanically regulated)

Fuel System Diagnostics Chart (Appendix C). 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 nor-

mal operating fuel delivery system, yet the vehicle continues to expe-

rience symptoms of a fuel delivery malfunction, it could be caused by

contaminated fuel, faulty fuel injector(s), or an intermittent component

malfunction such as a sticking pressure regulator or loose electrical

connection.

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

Not all mechanically regulated returnless fuel systems utilize an exter-

nally mounted fuel filter (see Types of Fuel Delivery

Systems/Mechanically Regulated). If the vehicle has an accessible

fuel filter, and initial test results indicate it may be partially blocked,

retest the fuel system with the FST connected at the inlet of the fuel

filter. 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.

Maximum fuel

volume

requirement

(Appendix A)

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