Mityvac MV5545 User Manual download pdf (Page 25)

Form 824127 Page Number - 25

Under-restricting Fuel Pressure Regulator

An under-restricting pressure regulator will cause the idle pressure in

Zones 1 and 2 to drop below spec, and the idle flow to be higher than

normal. If this is what the FST is indicating, and the peak pressure

and flow are normal, then the cause is most certainly the pressure

egulator.

If the fuel pressure regulator is vacuum modulated (see Fuel System

Components/Pressure Regulator), it is possible to check if it is stick-

ing by disconnecting the vacuum line. If the regulator is functioning

properly, disconnecting the vacuum line should cause the fuel pres-

sure to increase.

Over-restricting Fuel Pressure Regulator

If Zone 2 testing indicates idle pressure is high and flow is low, yet

peak pressure and flow are normal, this is a clear indication of an

over-restriction in Zone 3. Over-restriction can be caused by a mal-

functioning fuel pressure regulator or blockage in the return fuel line.

Perform a careful visual inspection of the fuel return line to ensure it’s

not crushed, dented or kinked. If damage or restriction is noted, repair

the line and retest.

Inspect the fuel pressure regulator for physical damage. Also look for

dampness that could indicate a ruptured diaphragm that is leaking

fuel. Replace and retest if necessary.

If the pressure regulator is vacuum modulated, use a vacuum gauge

to verify the vacuum reading at the regulator. A below normal vacuum

reading would cause the regulator to over-restrict the flow. Follow the

vehicle manufacturer’s procedure for locating and repairing the cause

of the vacuum loss, and retest.

If vacuum is normal, connect the FST in Zone 3, preferably right after

the pressure regulator. Test the fuel system at this point, and compare

the results to Zone 2. The pressure in Zone 3 should be very low. If it

remains high, there is a restriction in the fuel return line. If the pressure

drops off to near zero, then replace the fuel pressure regulator.

Fuel Pump Malfunction

A malfunction at the fuel pump is typically indicated by a reduction in

both pressure and flow. At idle, the regulator may be able to maintain

pressure in Zones 1 and 2, but doing so will severely reduce the flow.

Performing pressure and flow demand tests should clearly indicate a

pump problem.

If the FST indicates a pump is under performing, be sure to verify that

the vehicle does not use a multiple speed pump (see Fuel System

Components/Fuel Pump). To properly test a fuel system with a multi-

ple speed pump, a scanner must be used to operate the pump at

high speed while the vehicle idles.

NOTE: Insufficient pressure and flow are an indication that a fuel

pump is under-performing, but do not necessarily mean that it is fail-

ing. Low voltage or a bad connection or ground will cause a pump to

under-perform, producing the same test results as if the pump were

bad. Before replacing any fuel pump based on the FST test results,

always follow the vehicle manufacturer’s recommended procedure for

testing all electrical connections and the electrical system charge. A

minor issue such as a loose ground can cause many problems that

resemble more serious malfunctions.

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Mityvac MV5545 User Manual Page 25

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