Mityvac MV8500 silverline elite aUtoMotive test kit - Manual (Page 10)

fueL pressure reGuLator
SYSTEM OPERATION
Fuel pressure regulators are used to maintain
fuel pressure at a steady level, consistent with the
requirements of the engine . Automotive manufac-
turers employ different methods for controlling
fuel pressure . Most utilize a mechanical spring and
diaphragm style pressure regulator such as the one
shown in figure 6 . Mechanical regulators most often
utilize engine vacuum and/or pressure to vary fuel
pressure in response to immediate engine require-
ments . A vacuum or vacuum/pressure pump with
appropriate gauge is required to properly diagnose
these types of regulators .
Vacuum/pressure controlled fuel pressure regula-
tors are commonly mounted on or immediately
after the fuel rail, and inline with the flow of fuel .
When the fuel pump is not operating, the spring
causes the diaphragm to close so that no fuel can
pass . Once the fuel pump is activated, the pressure
it produces begins to overcome the spring force
and the diaphragm opens to allow fuel to flow . The
spring and diaphragm maintain a constant pressure
on the flow of fuel . This creates back-pressure in the
fuel system, which is commonly referred to as “fuel
pressure” . The job of the fuel pressure regulator is
to maintain the “fuel pressure” at a level specified
by the vehicle manufacturer .
A vacuum/pressure-modulated fuel pressure regu-
lator is connected to the intake manifold by a small
hose . Vacuum from the manifold assists the spring
in opening the diaphragm . When a load is placed
on the powertrain, engine vacuum drops . This drop
in vacuum causes the diaphragm to increase the
resistance to fuel flow . The additional resistance
increases the fuel pressure to the injectors to com-
pensate for the higher fuel demand of the engine .
On forced-air induction systems (turbochargers/
superchargers), the boost pressure created in the
manifold functions the opposite of the vacuum pro-
duced in normally aspirated systems . Under boost
conditions, the pressure in the manifold causes the
regulator to increase fuel pressure, resulting in a
richer fuel mixture .
SERVICE PROCEDURES
Visual Inspection
1) Consult the vehicle’s service manual to determine
if the pressure regulator is vacuum/pressure modu-
lated, and to identify its location .
) Inspect the exterior of the pressure regulator for
fuel leaks, and the vacuum hose for visible damage
or cracks . Replace if necessary .
3) Disconnect the vacuum hose from the fuel pres-
sure regulator .
4) Check inside the hose for liquid fuel . If present,
replace the regulator . If not present, reconnect the
vacuum hose .
Diagnostics
5) Install a fuel pressure tester .
6) Start the engine and allow it to idle .
7) Disconnect the vacuum line from the fuel pres-
sure regulator .
8) Fuel pressure should increase 8 to 1 psi when
the line is disconnected . No change would indicate a
faulty regulator, or leaky or plugged vacuum line .
9) Connect the vacuum pump to the vacuum hose
extending from the manifold .
FIGURE 6:
PRESSURE
REGULATOR
Vacuum/Pressure
Connection
Spring
Diaphragm
Fuel In
Fuel Out
Page Number - 10 Form 8433