Jeep Cherokee 2000 Service Manual - page 1230
Actual electric fuel timing (amount of advance) is
accomplished by the fuel timing solenoid mounted to
the bottom of the injection pump (Fig. 3). Fuel timing
will be adjusted by the ECM, which controls the fuel
timing solenoid.
An overflow valve is attached into the fuel return
line at the rear of the fuel injection pump (Fig. 3).
This valve serves two purposes. One is to ensure that
a certain amount of residual pressure is maintained
within the pump when the engine is switched off.
This will prevent the fuel timing mechanism within
the injection pump from returning to its zero posi-
tion. The other purpose is to allow excess fuel to be
returned to the fuel tank through the fuel return
line. The pressure values within this valve are preset
and can not be adjusted.
The fuel injection pump supplies high–pressure
fuel of approximately 45,000 kPa (6526 psi) to each
injector in precise metered amounts at the correct
time.
For mechanical injection pump timing, refer to
Fuel Injection Pump Timing in the Service Proce-
dures section of this group.
FUEL INJECTORS
DESCRIPTION
The fuel injectors have controlled internal leakage
which colls the injectors. Fuel drain tubes (Fig. 4) are
used to route this fuel back to the overflow valve at
the rear of the injection pump. This excess fuel is
then returned to the fuel tank through the fuel
return line.
The injectors are connected to the fuel injection
pump by the high– pressure fuel lines. A separate
injector is used for each of the four cylinders. An
injector containing a sensor (Fig. 5) is used on the
number one cylinder injector. This injector is called
instrumented injector #1 or needle movement sensor.
It is used to tell the ECM when the #1 injector’s
internal spring-loaded valve seat has been forced
open by pressurized fuel being delivered to the cylin-
der, which is at the end of its compression stroke.
When the instrumented injector’s valve seat is force
open, it sends a small voltage spike pulse to the
ECM. This tells the ECM that the #1 cylinder injec-
tor is firing. It is not used with the other three injec-
tors.
Fuel enters the injector at the fuel inlet (top of
injector) and is routed to the needle valve bore. When
fuel pressure rises to approximately 15,000–15,800
kPa (2175–2291 psi), the needle valve spring tension
is overcome. The needle valve rises and fuel flows
through the spray holes in the nozzle tip into the
combustion chamber. The pressure required to lift
the needle valve is the injector opening pressure set-
ting. This is referred to as the “pop-off” pressure set-
ting.
Fuel pressure in the injector circuit decreases after
injection. The injector needle valve is immediately
closed by the needle valve spring and fuel flow into
the combustion chamber is stopped. Exhaust gases
are prevented from entering the injector nozzle by
the needle valve.
A copper washer (gasket) is used at the base of
each injector (Fig. 5) to prevent combustion gases
from escaping.
Fuel injector firing sequence is 1–3–4–2.
FUEL TUBES/LINES/HOSES AND CLAMPS—
LOW-PRESSURE TYPE
DESCRIPTION
Also refer to the proceeding section on Quick–Con-
nect Fittings.
Fig. 4 Fuel Injectors and Drain Tubes
Fig. 5 Fuel Injector Sensor
1 – NEEDLE MOVEMENT SENSOR
2 – FUEL INJECTOR (NUMBER 1 CYLINDER ONLY)
3 – COPPER WASHER
4 – SENSOR CONNECTOR
14 - 4
FUEL SYSTEM—2.5L DIESEL ENGINE
XJ
DESCRIPTION AND OPERATION (Continued)