fuel pressure JEEP CHEROKEE 1988 Service Repair Manual

emission control devices. This permits optimum engine performance
with minimum emissions.
OPERATION
The engine control system is divided into 6 sub-systems:
electronic control unit (also called the ECU or computer), sensors and\
switches, fuel control, emission control, idle speed control, and
ignition advance control.
ELECTRONIC CONTROL UNIT (ECU)
The ECU is located under the instrument panel, above the
accelerator pedal. It receives information from the 13 engine sensors
or switches to determine engine operating conditions at any particular
moment. The ECU responds to these signals by sending a control signal
to the fuel injector, fuel pump, ignition control module, idle speed
actuator (ISA) motor, EGR solenoid, and canister purge solenoid. It
also controls the Load Swap relay, and on Man. Trans. models, the up-
shift indicator lamp.
SENSORS & SWITCHES
Exhaust Gas Oxygen (EGO) Sensor
The amount of oxygen in exhaust gases varies according to the
air/fuel ratio of the intake charge. The exhaust gas oxygen sensor,
located in the exhaust pipe, detects this content and transmits a low
voltage signal to the ECU.
The outer surface of the sensor is exposed to exhaust gases,
the inner surface to outside air. The difference in the amount of
oxygen contacting the inner and outer surfaces of the sensor creates a
pressure, which results in a small voltage signal. This signal, which
is a measure of the unburned oxygen in the exhaust gas, is transmitted
to the ECU.
If the amount of oxygen in the exhaust system is low (rich
mixture), the sensor voltage signal will be high. If the mixture is
lean, the oxygen sensor will generate a low voltage signal.
The sensor has a heating element that keeps the sensor at
proper operating temperature during all operating modes.
Manifold Air/Fuel Temperature (MAT) Sensor
The manifold air/fuel temperature sensor is installed in the
intake manifold. This sensor provides a voltage signal to the ECU
representing the temperature of the air/fuel mixture in the intake
manifold. The ECU compensates for air density changes during high
temperature operation.
Coolant Temperature Sensor (CTS)
The coolant temperature sensor is located in the intake
manifold coolant jacket. This sensor provides a voltage signal to the
ECU. The ECU uses this signal to determine engine temperature. During
cold engine operation, the ECU responds by enriching the air/fuel
mixture delivered to the injector, compensating for fuel condensation
in the intake manifold, controlling engine warm-up speed, increasing
ignition advance, and inhibiting operation of the EGR system.
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor detects absolute pressure in the intake
manifold as well as ambient atmospheric pressure. This information is
supplied to the ECU, through voltage signals, as an indication of
engine load. The sensor is attached to the plenum chamber near the
hood latch. A vacuum line from the throttle body supplies the sensor

with manifold pressure information.
Knock Sensor
The knock (detonation) sensor, located in the cylinder head,
provides an input signal to the ECU whenever detonation occurs. The
ECU then retards ignition advance to eliminate the detonation at the
applicable cylinders.
Speed Sensor
The speed sensor (or crankshaft position sensor) is mounted
at the flywheel/drive plate housing. The sensor detects the flywheel/
drive plate teeth as they pass during engine operation and sends an
electrical signal to the ECU, which calculates engine speed.
The flywheel/drive plate has a large trigger tooth and notch
located 90
and 12 small teeth before each top dead center (TDC)
position. When a small tooth or notch pass the magnetic core in the
sensor, the build-up and collapse of the magnetic field induces a
small voltage signal in the sensor pick-up windings.
The ECU counts these signals representing the number of teeth
as they pass the sensor. When a larger trigger tooth and notch pass
the magnetic core, a higher voltage signal is sent to the ECU. This
indicates to the ECU that a piston will be at the TDC position 12
teeth later. The ECU either advances or retards ignition timing as
necessary according to sensor inputs.
Battery Voltage
Battery voltage input to the ECU ensures that proper voltage
is applied to the injector. The ECU varies voltage to compensate for
battery voltage fluctuations.
Starter Motor Relay
The engine starter motor relay provides an input to the ECU,
indicating the starter motor is engaged.
Wide Open Throttle (WOT) Switch
The WOT switch is mounted on the side of the throttle body.
The switch provides a voltage signal to the ECU under wide open
throttle conditions. The ECU responds to this signal by enriching the
air/fuel mixture delivered to the injector.
Closed Throttle (Idle) Switch
This switch is integral with the idle speed actuator (ISA)
motor. The switch provides a voltage signal to the ECU, which
increases or decreases the throttle stop angle in response to engine
operating conditions.
Transmission Gear Position Indicator
The gear position indicator is mounted on vehicles equipped
with automatic transaxles. It provides a signal to the ECU to
indicate that the transaxle is in a driving mode and not in Park or
Neutral.
Power Steering Pressure Switch
The switch increases the idle speed during periods of high
power steering pump load and low engine RPM.
A/C Switch
The A/C switch sends a signal to the ECU when the air
conditioner is operating and when the compressor clutch must be
engaged to lower the temperature. The ECU, in turn, increases engine
speed to compensate for the added load of the air conditioner.
FUEL CONTROL

An electric in-tank fuel pump supplies fuel through the fuel
filter located under the right rear floor pan to the throttle body,
maintaining a constant operating pressure. Fuel enters the fuel bowl
reservoir of the throttle body through the injector and overflow type
fuel pressure regulator. The fuel pump is controlled by the ECU. A
ballast resistor attached to the right side of the plenum chamber,
reduces fuel pump speed after engine is running. The resistor is
by-passed in the "Start" position.
The fuel injector and fuel pressure regulator are integral
components of the throttle body. The injector is electronically-
controlled by the ECU. See Fig. 15.
Fig. 2: Cross Section View of Injector
The fuel pressure regulator is a diaphragm-operated relief
valve which maintains fuel pressure of 17.3 psi (1.2 kg/cm

). See
Fig. 16. Fuel in excess of this pressure is returned to fuel tank by a

fuel return line. The regulator is not controlled by the ECU. The
regulator's spring chamber is vented to the same pressure as the tip
of the injector.
Fig. 3: Cross Section View of Fuel Pressure Regulator
Since fuel pressure at the injector is kept constant, the
volume of fuel injected is dependent only on the length of time that
the injector is energized. The injection time duration is based on
engine operating conditions, which are provided to the ECU by the

Fig. 4: Location of ECU-Controlled Relays
Load Swap Relay
The Load Swap Relay is used on models with A/C and power
steering. The relay works in conjunction with the power steering
pressure switch to disengage the A/C compressor clutch.
If the compressor clutch is engaged when the power steering
pressure switch contacts close, the input signal from the switch to
the ECU also activates the load swap relay. The relay contacts open,
cutting off electrical feed to the compressor clutch. The clutch
remains disengaged until the pressure switch contacts reopen and
engine idle returns to normal.
NOTE: The load swap relay does not reengage the compressor clutch
immediately. The relay has a timer that delays energizing
the clutch for .5 second to ensure smooth engagement.
Fuel Pump Control Relay
Battery voltage is applied to the relay through the ignition
switch. The relay is energized when a ground is provided by the ECU.
When energized, voltage is applied to the fuel pump See Fig. 15.
A/C Clutch Relay

1) If all components have been checked and/or repaired, but
a system failure or problem still exists, the ECU may be at fault.
However, the ECU is a very reliable unit and must always be the final
component replaced if a doubt exists concerning the cause of a system
failure.
2) The only way to confirm an ECU malfunction is to take the
unit to an AMC dealer to have it tested. This is the only sure way to
avoid replacing a good ECU.
SYSTEM DIAGNOSIS
PRELIMINARY CHECKS
Be sure fuel is actually reaching the injector. Make sure no
air is entering the intake or exhaust system above the catalytic
converter. Before assuming an engine control system malfunction,
inspect the following systems to ensure components are in good
condition and are operating properly.
* All support systems and wiring.
* Battery connections and specific gravity.
* Electrical and vacuum connections on components and sensors.
* Emission control devices.
* Ignition system.
* Vacuum hoses.
CAUTION: Never connect or disconnect a component without turning the
ignition switch off. Never apply more than 12 volts or AC
voltage to system terminals. Disconnect battery cables
before charging it. Remove ECU if temperatures are expected
to exceed 176
F (80C), such as in a paint shop bake oven.
DIAGNOSTIC TEST CHARTS
Following are 6 different diagnostic test flow charts,
providing the shortest means of testing the system. These include:
* Ignition Switch "OFF" Chart - Tests system power for ECU
memory keep-alive voltage.
* Ignition Switch "ON" Power Chart - Tests system power
function and fuel pump power function.
* Ignition Switch "ON" Input Chart - Tests closed throttle
(idle) switch, wide open throttle (WOT) switch, manifold
absolute pressure (MAP) sensor, park/neutral switch, coolant
temperature sensor (CTS), manifold air/fuel temperature (MAT\
)
sensor and the respective switch or sensor circuits.
* System Operational Chart - Tests engine start-up and fuel
injector circuits, plus function of closed loop air/fuel
mixture, coolant temperature sensor, manifold air/fuel
temperature sensor, knock sensor and closed loop ignition
retard/advance, EGR valve and canister purge solenoid, idle
speed actuator, and A/C control.
* Basic Engine Chart - Indicates possible failures within other
engine related components.
* Man. Trans. Up-shift Chart - Tests up-shift indicator lamp
function on manual transmission vehicles.

THROTTLE BODY ASSEMBLY
Removal
1) Remove throttle return spring, throttle cable and cruise
control cable, if equipped. Disconnect wiring harness connector from
injector, WOT switch, and ISA motor. Remove fuel supply and return
pipes from throttle body.
2) Identify and tag vacuum hoses for installation later.
Disconnect vacuum hoses from throttle body assembly. Remove throttle
body-to-manifold retaining nuts from studs.
3) Remove throttle body assembly from intake manifold. If
being replaced, transfer ISA motor and WOT switch and bracket
assembly to new throttle body.
Installation
To install, reverse removal procedure using a new gasket
between components. Adjust ISA motor and WOT switch.
FUEL INJECTOR
Removal
Remove air cleaner assembly, injector wire connector, and
injector retainer clip screws. Using a pair of small pliers, gently
grasp center collar of injector (between electrical terminals), and
carefully remove injector with a lifting and twisting motion. Note
back-up ring fits over upper "O" ring.
Installation
1) Lubricate new lower "O" ring with light oil and install
in housing bore. Lubricate new upper "O" ring with light oil and
install in housing bore. Install back-up ring over upper "O" ring.
2) Position new injector in fuel body, and center nozzle in
lower housing bore. Seat injector with a pushing and twisting motion.
Align wire connector terminals properly. Install retainer clip and
screws. Connect injector wire connector.
FUEL PRESSURE REGULATOR
Removal
Remove 3 retaining screws, securing pressure regulator to
fuel body. After noting location of components for reassembly
reference, remove regulator assembly.
Installation
Position pressure regulator assembly with a new gasket.
Install 3 retaining screws, securing regulator to throttle body.
Adjust regulator. Operate engine and inspect for leaks.
IDLE SPEED ACTUATOR, MOTOR & WIDE OPEN THROTTLE SWITCH
NOTE: Closed throttle (idle) switch is integral with ISA and motor
assembly.
Removal
1) Remove air cleaner assembly. Disconnect throttle return
spring, throttle cable and cruise control cable, if equipped.
Disconnect wiring harness connector from ISA motor and WOT switch.
2) Remove ISA motor and WOT switch bracket from throttle
body. Remove motor-to-bracket retaining nuts. See Fig. 23. Do not
remove nuts from motor studs.
CAUTION: Do not attempt to remove ISA motor attaching nuts without

Fig. 28: Jeep 2.5L TBI System Wiring Diagram
FUEL PRESSURE REGULATOR
1) Remove air cleaner assembly. Connect a tachometer to
terminals 1 and 3 of small diagnostic connector D1. Remove screw plug
and install special Fuel Pressure Test Fitting (8983 501 572).

2) Connect accurate fuel pressure gauge to fuel pressure
test fitting. Start engine and accelerate to 2000 RPM. Turn torx head
adjustment screw on bottom of fuel regulator to obtain 17.3 psi (1.2
kg/cm(C)) of fuel pressure.
NOTE: Turning screw inward increases pressure; turning screw
outward decreases it.
3) After specification is reached, install a lead seal ball
to cover regulator adjustment screw. Turn ignition switch off, and
disconnect tachometer. Disconnect fuel pressure gauge, remove test
fitting, and install original plug screw. Install air cleaner
assembly.

\003
4.0 L 6 -C YL - V IN [ M ] & 4 .2 L 6 -C YL - V IN [ C ]

1 988 J e ep C hero ke e
1988 ENGINES
Chrysler Motors 4.0L & 4.2L 6-Cylinder
Jeep; Cherokee, Comanche, Wagoneer, Wrangler
ENGINE CODING
ENGINE IDENTIFICATION
NOTE: For engine repair procedures not covered in this article,
see ENGINE OVERHAUL PROCEDURES - GENERAL INFORMATION article
in the GENERAL INFORMATION section.
The Vehicle Identification Number (VIN) is located on upper
left side of dash and is visible through windshield. The fourth
character identifies the engine size. The tenth character identifies
the model year.
ENGINE IDENTIFICATION CODES TABLE









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Application VIN Code
4.0L 6-Cylinder MPFI ........................ M
4.2L 6-Cylinder 2-Bbl. ...................... C









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SPECIAL ENGINE MARKS
Some engines are produced at factory with oversize or
undersize components. These engines are identified by a letter code
stamped on a boss between ignition coil and distributor. Letters are
decoded as follows:
* "B" indicates all cylinder bores are .010" (.25 mm)
oversize.
* "C" indicates all camshaft bearing bores are .010" (.25 mm)
oversize.
* "M" indicates all main bearing journals are .010" (.25 mm)
undersize.
* "P" indicates all connecting rod journals are .010" (.25 mm)
undersize.
REMOVAL & INSTALLATION
ENGINE REMOVAL
See ENGINE REMOVAL in this section.
INTAKE & EXHAUST MANIFOLD (4.0L)
WARNING: Fuel system is under pressure. Use care when removing fuel
lines to prevent personal injury.
Removal
1) Disconnect negative battery cable. Remove air inlet from
throttle plate assembly. Disconnect throttle, cruise control and
throttle valve (A/T models) cables (if equipped).
2) Disconnect and mark all vacuum and electrical connectors