1988 JEEP CHEROKEE Service Repair Manual

leaking) ............... A ............ Require replacement.
Sound quality
unsatisfactory ......... 2 .. Suggest replacement to address
customer need and/or request.
Split (exhaust leaking) . A ............ Require replacement.
Weak due to corrosion, but
no leaks present ....... 1 ............ Suggest replacement.









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FU EL E VA PO RATIO N S YSTE M
1988 J e ep C hero ke e
1988 Exhaust Emission Systems
FUEL EVAPORATION SYSTEMS
JEEP
DESCRIPTION
The Evaporative Emission Control System (EECS), prevents raw
fuel vapors from entering the atmosphere. It consists of a venting
system which allows only vaporous fuel to be drawn into the system.
During engine operation, vapors are drawn through system
vent lines and into intake manifold. When engine is off, fuel vapors
are stored in vapor storage canister charcoal. Vapors are drawn into
intake manifold when engine is running again. See Fig. 1.
Fig. 1: Open-Bottom Canister (2-Tube Type)
Courtesy of General Motors Corp.
OPERATION

Roll-Over Check Valve
When valve is inverted, a stainless steel ball pushes a
plunger against its seat, blocking fuel flow through valve.
Charcoal Canister
All models are equipped with a dual-purge type canister. An
inlet is provided for carburetor fuel bowl vapors and another one for
fuel tank vapors. The outlet is connected to intake manifold vacuum.
A secondary purge fourth nipple connects to carburetor ported vacuum.
During engine operation, manifold vacuum draws fresh air
through inlet filter in bottom of canister and purges stored vapors.
When ported vacuum reaches 12 in. Hg, secondary purge circuit is
opened and canister purges at a much higher rate.
Carburetor Bowl Vent
The carburetor bowl vent provides an outlet for fuel vapors
when engine is not operating. During engine operation, fuel bowl is
vented to inside of air cleaner. The bowl is automatically closed by
mechanical link to throttle when engine is started.
MAINTENANCE
No adjustments are required with this system. Replace air
inlet filter (if equipped) in bottom of charcoal canister every
30,000 miles. Regular inspection should be made and defective
components replaced as necessary.

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FU EL IN JE C TIO N S YSTE M - M ULTI- P O IN T
1988 J e ep C hero ke e
1988 Electronic Fuel Injection
JEEP MULTI-POINT
4.0L Cherokee, Comanche, Wagoneer
DESCRIPTION
The Multi-Point Electronic Fuel Injection (EFI) system is an
electronically controlled system which combines electronic sequential
fuel injection and electronic spark advance systems. Main sub-systems
consist of: air induction, fuel delivery, fuel control, emission
control, Electronic Control Unit (ECU), data sensors and switches.
Air induction system includes air cleaner, throttle body,
Throttle Position Sensor (TPS) and the Idle Speed Stepper (ISS) moto\
r.
Fuel delivery system provides fuel from fuel pump to the
fuel control system. Fuel system is composed of an in-tank electric
fuel pump, fuel filter and return line. Power is provided to operate
fuel pump through a fuel pump relay located on right inner fender
panel.
Fuel control system handles actual fuel delivery into the
engine. Fuel pressure regulator maintains a constant fuel pressure of
31-39 psi (2.1-2.7 kg/cm
). In addition to the regulator, fuel
system consists of the fuel rail and 4 fuel injectors. On MPI engine,
ECU controls EGR/EVAP solenoid operation.
The ECU is a digital microprocessor computer. ECU receives
input signals from various switches and sensors. ECU then computes
fuel injector pulse width ("on" time), spark advance, ignition module
dwell, idle speed, canister purge cycles, EGR flow and feedback
control from this information.
OPERATION
AIR INDUCTION
Air is drawn into combustion chamber through air cleaner and
intake manifold. Amount of air entering engine is controlled by
position of throttle body valve. Throttle body houses throttle
position sensor (TPS) and idle speed solenoid (ISS) motor. TPS is an\
electrical resistor which is connected to throttle valve. TPS
transmits a signal to ECU in relation to throttle valve angle. This
signal is used in calculations to determine injector pulse width to
provide adequate air/fuel mixture.
ECU controls idle speed by providing appropriate voltage
outputs to move ISS motor pin inward or outward to maintain a
predetermined idle speed. ECU continuously monitors TPS and ISS motor
and issues change commands to injectors to increase or decrease
amount of fuel injected.
FUEL DELIVERY
Power to fuel pump relay is supplied from ignition switch
when in "ON" or "START" position, at which time the ECU supplies a
ground for fuel pump relay. When relay contacts are closed, power is
applied to fuel pump.
Fuel is drawn through one end of a roller-type electric fuel
pump, compressed and forced out opposite end. Pump capacity is
greater than maximum engine consumption so that pressure in fuel
system is always maintained.

FUEL CONTROL
Fuel control system handles actual delivery of fuel to
engine. See Fig. 1. Fuel from fuel pump enters fuel rail, injectors
and pressure regulator. Based upon a manifold vacuum signal, pressure
regulator maintains a constant fuel pressure in fuel system of
approximately 31-39 psi (2.1-2.7 kg/cm
) by allowing excess fuel to
return to fuel tank.
Fuel injectors are electrically operated solenoid valves
which are energized by the ECU. The ECU determines injector pulse
width ("on" time) based upon input from the various sensors.
Fig. 1: Fuel Control Components
Courtesy of Chrysler Motors.
EMISSION CONTROL
ECU controls EGR valve and fuel evaporative operation. By
energizing the EGR/EVAP solenoid, vacuum is shut off, making this
system non-operative. When engine reaches normal operating
temperatures, ECU de-energizes solenoid. When de-energized, solenoid
allows vacuum to flow to EGR valve. ECU will energize solenoid
whenever EGR action is undesirable, during idle, cold engine
operation, wide open throttle and rapid acceleration or deceleration.
ELECTRONIC CONTROL UNIT (ECU)

ECU is a digital microprocessor computer. Data sensors
provide the ECU with engine operating information in varying
electrical signals. ECU calculates this information and corrects
air/fuel ratio, ignition timing, and emission control as needed to
maintain efficient engine operation. Other ECU output signals control
upshift indicator light (manual transmission only), ignition module
dwell and A/C clutch operation.
UPSHIFT INDICATOR
On vehicles equipped with a manual transmission, ECU
controls upshift indicator light. Indicator light is normally
illuminated when ignition is turned on without engine running.
Indicator light is turned off when engine is started.
Indicator light will be illuminated during engine operation
in response to engine load and speed. If transmission is not shifted,
ECU will turn light off after 3 to 5 seconds. A switch located on
transmission prevents indicator light from being illuminated when
transmission is shifted to highest gear.
DATA SENSORS & SWITCHES
Manifold Absolute Pressure (MAP) Sensor
MAP sensor is located in engine compartment on firewall,
behind engine. MAP sensor monitors manifold vacuum via a vacuum line
from intake manifold to sensor.
MAP sensor supplies an electrical signal which keeps ECU
informed of manifold vacuum and barometric pressure conditions. This
information is combined with data supplied by other sensors to
determine correct air/fuel ratio.
Oxygen Sensor
Oxygen (O2) sensor is mounted in exhaust manifold where it
is exposed to exhaust gas flow. Its function is to monitor oxygen
content of exhaust gases and to supply ECU with a voltage signal
directly proportional to this content.
If oxygen content of exhaust gases is high (lean air/fuel
mixture), voltage signal to ECU is low. As oxygen content decreases
(mixture becomes richer), signal voltage increases.
In this way, ECU is kept constantly informed of air/fuel
ratio. ECU can then alter fuel injector "on" time, in response to
these signals, to obtain best air/fuel ratio of 14.7:1 under any
given operating conditions.
O2 sensor is equipped with a heating element that keeps
sensor at proper operating temperatures. Maintaining correct sensor
temperatures at all times guarantees a more accurate signal to ECU.
By using an O2 heater, fuel control system may also enter "closed
loop" operating mode sooner and maintain this mode, even during
periods of extended idle.
Temperature Sensors
There are 2 temperature sensors used on this system.
Manifold Air Temperature (MAT) sensor, mounted in intake manifold,
measures temperature of incoming air/fuel mixture and Coolant
Temperature Sensor (CTS), located on left side of cylinder block just
below the exhaust manifold, measures temperature of engine coolant.
Information provided by these 2 sensors to ECU allows ECU to
demand slightly richer air/fuel mixtures and higher idle speeds
during cold engine operation.
Throttle Position Sensor (TPS)
TPS is regulated by movement of throttle shaft. It is

mounted on throttle body and senses angle of throttle blade opening.
A voltage signal of up to 5 volts at wide open throttle is
produced by TPS. Voltage varies with throttle angle changes. This
signal is transmitted to ECU where it is used to adjust air/fuel
ratio during acceleration, deceleration, idle, and wide open throttle
conditions.
A dual TPS is used on models with automatic transmissions.
This dual TPS not only provides ECU with input voltages but also
supplies automatic transmission control unit with input signals
relative to throttle position.
Knock Sensor
Knock sensor (detonation sensor) is located on lower left
side of cylinder block just above oil pan. Knock sensor picks up
detonation vibration from engine and converts it to an electrical
signal for use by ECU.
ECU uses this information to determine when a change in
ignition timing is required. Knock sensor allows for engine operation
on either "premium" unleaded or "regular" unleaded fuel.
When knock occurs, ECU retards ignition timing in one or
more cylinders until detonation is eliminated.
Speed Sensor
Speed sensor is secured by special shouldered bolts to
flywheel/drive plate housing. Speed sensor is nonadjustable and
preset at factory. Speed sensor senses TDC and engine speed by
detecting flywheel teeth as they pass pick-up coil during engine
operation. See Fig. 2.
Flywheel has a large trigger tooth and notch located 12
small teeth before each TDC position. When a small tooth and notch
pass the magnetic core in sensor, concentration and collapse of the
magnetic field created induces a small voltage spike into sensor
pick-up coil windings. These small voltage spikes are sent to ECU,
allowing ECU to count the teeth as they pass sensor.
When a large tooth and notch pass magnetic core in sensor,
increased concentration and collapse of the magnetic field induces a
higher voltage spike than smaller teeth. Higher spike indicates to
ECU that a piston will soon be at TDC position, 12 teeth later.
Ignition timing for cylinder is either advanced or retarded by ECU
based upon "sensor input".
Fig. 2: Speed Sensor Operation
Courtesy of Chrysler Motors.
Engine Switches
Several switches provide operating information to ECU. These

include Park/Neutral switch (automatic transmission only), air
conditioning clutch, and Sync Pulse switch. When A/C or Park/Neutral
switches supply ECU with an "on" signal, module signals ISS motor to
change idle speed to a specific RPM.
With A/C on and throttle blade above a specific angle, ECU
de-energizes A/C relay, preventing A/C clutch from engaging until
throttle blade angle is reduced.
Sync pulse switch, located within distributor, generates a
signal to ECU, helping to properly synchronize sequential fuel
injection opening with intake valve opening.
ADJUSTMENTS
NOTE: Idle speed and air/fuel mixture are controlled by ECU and
are non-adjustable. On-car adjustment procedures for other
components should not be necessary during normal vehicle
operation or maintenance. Adjustments of components should
only be required when a faulty component is replaced with a
new one.
THROTTLE POSITION SENSOR (TPS)
1) Turn ignition on. Check throttle position sensor input
voltage. Connect voltmeter negative lead to terminal "B" (M/T), or
terminal "D" (A/T) of sensor connector. Connect voltmeter positive
lead to terminal "A" (M/T and A/T) of sensor connector.
NOTE: On (A/T) models, connector terminals are identified by
letters molded into back of connector. On all models, do not
disconnect TPS harness connector. Insert voltmeter test
leads through back of wire harness connector. On some
models, it may be necessary to remove throttle body from
intake manifold to gain access to sensor wire harness.
2) Move and close throttle plate completely (M/T and A/T).
Ensure throttle linkage contacts stop. Note voltmeter reading. Input
voltage at terminals "B" and "A" (M/T), or terminals "A" and "D"
(A/T) should be 5 volts.
3) Return throttle plate to closed throttle position (M/T
and A/T). Check sensor output voltage. To do so, disconnect voltmeter
positive lead from terminal "A" and connect it to terminal "C" (M/T),
or terminal "B" (A/T).
4) Maintain throttle plate in closed position (M/T and A/T).\
Ensure throttle linkage contacts stop. Note voltmeter reading. Output
voltage should be .8 volt (M/T), or 4.2 volts (A/T).
5) If output voltage is incorrect, loosen bottom sensor
retaining screw and pivot sensor in adjustment slot for a coarse
adjustment. Loosen top sensor retaining screw for fine adjustments.
Tighten screws after adjustment.
TESTING & TROUBLE SHOOTING
PRELIMINARY CHECKS & PRECAUTIONS
Subsystem Checks
The following systems and components must be in good
condition and operating properly before assuming a fuel injection
system malfunction.
* Air filter.
* All support systems and wiring.