BMW 1989-1995 525i Service Manual page 173

FUEL INJECTION-DME
130-5
If the system pressure is too low, repeat the pressure test
while gradually clamping off the fuel return line. See Fig. 3.
The fuel pressure should quickly rise. If the pressure does not
respond quickly, and no leaks can be found , the fuel pump
may be faulty. Test the fuel pump delivery rate as described in
160 Fuel Tank and Fuel Pump.
5203
Fig. 3. Fuel pressure regulator return hose being clamped shut
\0
check fuel pump delivery pressure (M20 engine shown).
Residual pressure, checking
For quick restarts and to avoid vapor lock when the engine
is hot. the system is designed to retain fuel pressure after the
engine has been turned off. This residual pressure is primarily
maintained by a check valve at the fuel pump outlet and the
fuel pressu re regulator. The fuel pump check valve is not ser-
viceable as an individual part.
1. Relieve fuel pressure and connect a pressure gauge as
described earlier.
WARNfNG-
Fuel wiff be discharged. Wrap a shop towet around
the fuet line fitting when disconnecting the fuet line.
Do not smoke or work near heaters or other fire
hazards . Keep an approve d fire extinguisher handy.
2. Operate the fuel pump for approximately one minute by
bypassing the fuel pump relay as described in 160 Fuel
Tank and Fuel Pump.
3. Obse rve the fuel pressur e gauge after 20 minutes. The
pressure should not drop off more than 0.5 bar from the
system pressure listed in Table b.
4. When finished testing , disconnect the pressure gauge
and reconnect the fuel line. Tighten the hose clamp.
If the fuel system does not maintain pressure, there is a leak
in the system. Check visually for leaks in fuel lines or at
unions. Leaks can also be due to a leaking injector or a faulty
fuel pump check valve. Check the pump check valve by re-
peating the test. but before turning the fuel pump off pinch off
the return line at the fuel rail. If the pressure is now main-
tained, the fault is most likely the fuel pump check valve.
Fuel pressure regulator response
to engine load, checking
1. With the fuel pressure gauge connected, reinstall the
fuel pump relay. Start the engine and allow it to idle.
NOTE-
The fuel pressure should be slightly lower than the
specifications listed in Table b. Engine vacuum acting
on the fuel pressure regulator diaphragm reduces the
fuel pressure.
2. With the engine idling, remove the vacuum hose from
the regulator. The pressure shou ld increase.
3. Reconnect the hose and check that the pressure re-
turns to the lower pressure.
4. When finished testing, disconnect the pressure gauge
and reconnect the fuel line. Tighten the hose clamp.
If any faults are found. either the fuel pressure regulator or
the vacuum hose at the pressure regulator is faulty.
NOTE-
A cracked or leaking ptessute-requlstor vacuum hose
may cause an erratic idle.
ELECTRICAL CHECKS AND
COMPONENT TESTING
The tests given under this heading are made using a digital
multimeter. Always check for On-Board Diagnostic fault
codes as the first step in troubleshooting DME problems. See
100 Engine-General.
Main relay, testing
The main relay is energized via the DME engine control
module and supplies plus(+) power to many of the DME com-
ponents and subsystems, including the fuel pump relay. If this
relay is faulty , the engine will not start.
ELECTRICAL CHECKS AND COMPONENT TESTING