BMW 5251 Service Manual page 484

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I
Fuel Tank and Fuel Pump
I
4
Relieve system pressure and attach afuel pressure gauge as
described in Fuel pressure gauge, installing (6-cylinder
models) or Fuel pressure gauge, installing
(V-8
models).
WARNING
-
Fuel in fuel line is underpressure (approx. 3
-
5 bar or45
-
75psi) and may be expelled underpressure. Do not smoke
or work near heaters or other fire hazards. Keep a fire ex-
tinguisher handy. Before disconnecting fuel hoses, wrap a
cloth around fuel hoses to absorb any leaking fuel. Catch
and dispose of escaped fuel. Plug all open fuel lines.
.
Always unscrew the fuel tank cap to release pressure
in the tank before working on the tank or lines
-
Start engine allow it to run for approximately one minute.
Note fuel pressure reading.
-
Observe fuel pressure reading after approximately 20 mln-
utes. The pressure should not drop more than 0.5 bar from
system pressure listed in fuel pressure specifications in ac-
companying table.
Fuel pressure specification
Ail E39 vehicles
3.5 i-0.2
bar (50.76 i2.9 psi)
-
When finished, disconnect pressure gauge and fitting, and
replace schraeder valve, if removed.
-
If fuel system does not maintain pressure:
Visually check for leaks in fuel lines and unions,
Check for leaking injector(s).
Check for faulty fuel pump check valve.
-
To test fuel pump check valve:
Repeat residual pressure test.
Immediately aftershutting off engine, pinch off fuel delivery
line using a fuel hose pinch clamp (BMW 13 3 010 or
equivalent).
If pressure is now maintained, fault is most likely at fuel
pump check valve.
If pressure again drops below specifications, fault is most
likely in fuel pressure regulator.