Fuel Supply; Relieving System Fuel Pressure - BMW 5251 Service Manual

130-6
Fuel Injection
Fuel supply to the injection system is provided by a high pres-
sure electric pump mounted inside the fuel tank. The pump is
powered via the fuel pump relay. If the relay is faulty, the fuel
pump will not run and the engine will not start.
NOTE-
* Fuel delivery volume and fuel pressure are critical to prop-
er operation. Fuel pressure direcfly influences fuel deliv-
ery
Fuel pump relay, fuel pressure and fuel volume tests are
given in 160 Fuel Tank and Fuel Pump.
Relieving system fuel pressure
The fuel system retains fuel pressure in the system when the
engine is off. To prevent fuel from spraying on a hot engine,
system fuel pressure should be relieved beforedisconnecting
fuel lines. One method is to tightly wrap ashop towel around a
fuel line fitting and loosen or disconnect the fitting. Cleanli-
ness is essential when working with fuel components. Thor-
oughly clean the unions before disconnecting fuel lines.
When system faults are detected, a diagnostic trouble code
(DTC) is stored in the engine control module (ECM). The MIL
also iiluminates if an emissions-related fault is detected
When faults arise, or if the malfunction indicator lamp (MIL) is
illuminated, begin troubleshooting by connecting BMW ser-
vice tester DISplus. MoDiC, GTI or equivalent scan tool. The
capabilities of OBD II software has the potential to save hours
of diagnostic time and to help avoid incorrect component re-
placement and possible damage to system components.
CAUTION-
-
The tests in this section may set fault codes (DTCs) in the
ECM and illuminate the MIL. Affer all testing is completed,
access and clear DJC fault memory using an OED I1 or
BMW compatible scan tool. See OBD On-Board Diag-
nostics.
0
Only use a digital multimeter for electrical tests.
.
Relay positions can vary Be sure to confirm relay posi-
tion by identifying the wiring in the socket using the wir-
ing diagrams found in ELE Electrical Wiring
Diagrams.