JEEP CHEROKEE 1988 Service Repair Manual

SPK ..................................................... Spark
TAC .................................. Thermostatic Air Cleaner
TPS .................................. Throttle Position Sensor
TSD ......................................... Throttle Solenoid
TP ........................................ Throttle Positioner
TWC ........................................ Three-Way Catalyst
VA-CTO .................. Vacuum Advance Coolant Temp. Override
VSA .................................... Vacuum Switch Assembly
VTP ................................ Vacuum Throttle Positioner









\









\









\









\









\









\









\

EM IS SIO N C O M PO NEN T ID EN TIF IC ATIO N

1988 J e ep C hero ke e
1988 Exhaust Emission Systems
JEEP SYSTEMS
NOTE: Information not available from manufacturer for Jeep 2.5L
TBI and 4.0L MPFI emission systems.
DESCRIPTION
Several systems are used to control emissions. System usage
depends on model, engine and transmission combinations. Each system
is designed to control vehicle emissions. In addition, specially
calibrated carburetors (carbureted models), fuel injection system,
distributors and modified combustion chambers are used with these
systems.
AIR INJECTION
Air injection system consists of air pump, diverter valve,
check valve, and various air distribution lines necessary to inject
fresh air adjacent to exhaust valves. Injection of fresh air adjacent
to exhaust valves creates an afterburn which further consumes
unburned gases in engine's exhaust.
CATALYTIC CONVERTER (CAT)
Converter is installed in vehicle's exhaust system to aid in
reduction of exhaust emissions. This unit changes unburned
hydrocarbons (HC) and carbon monoxide (CO) into water vapor and
carbon dioxide.
COMPUTERIZED EMISSION CONTROL (CEC) SYSTEM
CEC system closely controls air/fuel ratio through a
feedback system from an oxygen sensor in exhaust system. Major
components of this system include exhaust gas oxygen sensor, vacuum
switches, temperature switches, Micro Computer Unit (MCU), fuel
injection system or computer controlled carburetor (carbureted
models) to maintain a constant air/fuel mixture. For additional
information, see appropriate article in COMPUTERIZED ENGINE CONTROL
section.
EVAPORATIVE EMISSION CONTROL
All models use closed tank (sealed) system, which returns
raw fuel vapors and routes them to intake manifold for burning.
Carbon canister stores vapors until engine draws them off for burning.
OTHER EMISSION SYSTEMS
For additional information on description, operation,
testing and adjusting other exhaust emission systems, refer to the
following articles in this section.

EM IS SIO N C O NTR O L V IS U AL IN SPEC TIO N P R O CED URES

1988 J e ep C hero ke e
1983-98 GENERAL INFORMATION
Emission Control Visual Inspection Procedures
All Models
* PLEASE READ THIS FIRST *
This article is provided for general information only. Not
all information applies to all makes and models. For more complete
information, see appropriate article(s) in the ENGINE PERFORMANCE
Section.
EMISSION CONTROL LABELS
The vehicle manufacturer's emission control label, also known
as the underhood tune-up label or Vehicle's Underhood Emission Control
System (VECI) label, is located in the engine compartment. Information\
regarding year model of vehicle, engine size, number of cylinders,
emission equipment or type, engine tune-up specifications, whether
vehicle was manufactured for sale in California or is a Federal
vehicle, vacuum hose routing schematic, etc., can be found on this
label. See Fig. 1.
In addition to the VECI label, some emission control
inspection and maintenance programs may require an additional label to
be affixed to the vehicle in special circumstances. For example, in
California, a Bureau Of Automotive Repair (BAR) engine label may be
affixed to the left door post. A BAR engine label is only used when
the vehicle has an engine change, approved modification or is a
Specially Constructed (SPCN) or an acceptable Gray market vehicle.
Check your state's emission control inspection and maintenance laws to
determine if a similar label is used.
Fig. 1: Typical Emission Control Label
Courtesy of General Motors Corp.
EMISSION CONTROL VISUAL INSPECTION
* PLEASE READ THIS FIRST *
NOTE: The following emission control visual inspection procedures
should be used as a guide only. When performing a visual
inspection, always follow your state's recommended

inspection procedures.
A visual inspection is made to determine if any required
emission control devices are missing, modified or disconnected.
Missing, modified or disconnected systems must be made fully
operational before a vehicle can be certified.
POSITIVE CRANKCASE VENTILATION (PCV)
PCV controls the flow of crankcase fumes into the intake
manifold while preventing gases and flames from traveling in the
opposite direction. PCV is either an open or closed system. See Fig. 2
.
Ensure PCV system is installed as required. Verify valve,
required hoses, connections, flame arresters, etc., are present,
routed properly and in serviceable condition.
Fig. 2: Typical Open & Closed Type PCV System
THERMOSTATIC AIR CLEANER (TAC)
The TAC supplies warm air to air intake during cold engine
operation. This system is active during cold engine warm-up only.
Under all other operating conditions, air cleaner function is the same
as any non-thermostatic unit.
Ensure required exhaust shroud, hot air duct, vacuum hoses
and air cleaner components are present and installed properly. See
Fig. 3 . Ensure any required thermostatic vacuum switches are in place
and vacuum hoses are installed and in serviceable condition. Also
ensure air cleaner lid is installed right side up. Check for oversized
air filter elements and for additional holes in the air cleaner
housing.

Fig. 3: Typical Thermostatic Air Cleaner System
FUEL EVAPORATIVE SYSTEM (EVAP)
The EVAP system allows for proper fuel system ventilation
while preventing fuel vapors from reaching the atmosphere. This means
that vapors must be caught and stored while the engine is off, which
is when most fuel evaporation occurs. When the engine is started,
these fuel vapors can be removed from storage and burned. In most
systems, storage is provided by an activated charcoal (or carbon)
canister. See Fig. 4. On a few early systems, charcoal canisters are
not used. Instead, fuel vapors are vented into the PCV system and
stored inside the crankcase.
The main components of a fuel evaporation system are a sealed
fuel tank, a liquid-vapor separator and vent lines to a vapor-storing
canister filled with activated charcoal. The filler cap is normally
not vented to the atmosphere, but is fitted with a valve to allow both
pressure and vacuum relief.
Although a few variations do exist between manufacturers,
basic operation is the same for all systems. Check for presence of
vapor storage canister or crankcase storage connections when required.
Ensure required hoses, solenoids, etc., are present and connected
properly. Check for proper type fuel tank cap. Check for any non-OEM
or auxiliary fuel tanks for compliance and the required number of
evaporation canisters.

Fig. 4: Typical Fuel Evaporative System
CATALYTIC CONVERTERS
Oxidation Catalyst (OC)
This type of converter is the most common. It may use pellets
or monolith medium, depending upon application. See Fig. 5. Platinum
and palladium (or platinum alone) are used as catalyst in this type of\
converter.
Visually check for presence of catalytic converter(s). Check
for external damage such as severe dents, removed or damaged heat
shields, etc. Also check for pellets or pieces of converter in the
tailpipe.
Fig. 5: Typical Oxidation Catalytic Converter (Pellet Type) Shown;
Typical Three-Way Catalytic Converter Is Similar
Courtesy of General Motors Corp.
Three-Way Catalyst (TWC)
This type of converter is nearly identical to a conventional

converter with the exception of the catalyst. See Fig. 5. The TWC
converter uses rhodium, with or without platinum, as its catalyst.
Rhodium helps reduce NOx emissions, as well as HC and CO.
Visually check for presence of catalytic converter(s). Also
check for presence of any required air supply system for the oxidizing
section of the converter. Check for external damage such as severe
dents, removed or damaged heat shields, etc. Check for pellets or
pieces of converter in the tailpipe.
Three-Way Catalyst + Oxidation Catalyst (TWC + OC)
This system contains a TWC converter and an OC converter in a
common housing, separated by a small air space. See Fig. 6. The 2
catalysts are referred to as catalyst beds. Exhaust gases pass through
the TWC first. The TWC bed performs the same function as it would as a
separate device, reducing all 3 emissions. As exhaust gases leave the
bed, they pass through the air space and into the second (OC)
converter catalyst bed.
Visually check for presence of catalytic converter(s). Check
for external damage such as severe dents, removed or damaged heat
shields, etc. Check for pellets or pieces of converter in the
tailpipe.
Fig. 6: Typical Three-Way + Oxidation Catalytic Converter
Courtesy of General Motors Corp.
FILL PIPE RESTRICTOR (FR)
A fuel tank fill pipe restrictor is used to prohibit the
introduction of leaded fuel into the fuel tank. Unleaded gasoline pump
dispensers have a smaller diameter nozzle to fit fuel tank of vehicle
requiring the use of unleaded fuel (vehicles equipped with catalytic
converter).
Visually inspect fill pipe restrictor(s) for tampering, i.e.,\
restrictor is oversize or the flapper is non-functional. If vehicle is
equipped with an auxiliary fuel tank, ensure auxiliary fuel tank is
also equipped with a fill pipe restrictor.
EXHAUST GAS RECIRCULATION (EGR) SYSTEM

Single Diaphragm EGR Valve
This type uses a single diaphragm connected to the valve by a
shaft. Diaphragm is spring-loaded to keep valve closed in the absence
of vacuum. As throttle valves open and engine speed increases, vacuum
is applied to the EGR vacuum diaphragm, opening the EGR valve. This
vacuum signal comes from a ported vacuum source. Variations in the
vacuum signal control the amount of exhaust gas that is recirculated.
See Fig. 7 .
Verify EGR valve is present and not modified or purposely
damaged. Ensure thermal vacuum switches, pressure transducers, speed
switches, etc., (if applicable) are not by-passed or modified. Ensure
vacuum hose(s) to EGR valve is not plugged.
Fig. 7: Typical Single Diaphragm EGR Valve
Courtesy of General Motors Corp.
Dual Diaphragm EGR Valve
This type uses 2 diaphragms with different effective areas
and 2 vacuum sources. Although similar to the single diaphragm type,
the second diaphragm is added below the upper diaphragm and is rigidly
attached to the valve seat. See Fig. 8. These diaphragms form a vacuum
chamber which is connected to manifold vacuum.
During highway cruising when manifold vacuum is high in the
center chamber, manifold vacuum tends to pull the valve closed.
However, the vacuum signal applied to the top side of the upper
diaphragm overcomes the downward spring force and the manifold vacuum
pull, due to the diaphragm's larger piston. This regulates the amount
of EGR.
When manifold vacuum is low during acceleration, the higher
vacuum signal opens the valve, permitting more EGR. When manifold
vacuum is high during highway cruising, the valve is only partially
opened, reducing the amount of EGR.

Verify EGR valve is present and not modified or purposely
damaged. Ensure thermal vacuum switches, pressure transducers, speed
switches, etc., (if applicable) are not by-passed or modified. Ensure
vacuum hose(s) to EGR valve is not plugged.
Fig. 8: Typical Dual Diaphragm EGR Valve
Courtesy of General Motors Corp.
Positive Backpressure EGR (BP/EGR) Valve
This type uses both engine vacuum and exhaust backpressure to
control the amount of EGR. It provides more recirculation during heavy
engine loads than the single diaphragm EGR valve.
A small diaphragm-controlled valve inside EGR valve acts as a
pressure regulator. The control valve gets an exhaust backpressure
signal through the hollow valve shaft. This exhaust backpressure
exerts a force on bottom of control valve diaphragm. The diaphragm
plate contains 6 bleed holes to bleed air into the vacuum chamber when
backpressure valve is in open position. See Fig. 9.
Verify EGR valve is present and not modified or purposely
damaged. Ensure thermal vacuum switches, pressure transducers, speed

switches, etc., (if applicable) are not by-passed or modified. Ensure
vacuum hose(s) to EGR valve is not plugged.
Fig. 9: Typical Positive Backpressure EGR Valve
Courtesy of General Motors Corp.
Negative Backpressure EGR (BP/EGR) Valve
This type has the same function as the positive BP/EGR valve
except valve is designed to open with a negative exhaust backpressure.
The control valve spring in the transducer is placed on the bottom
side of the diaphragm. See Fig. 10.
When ported vacuum is applied to the main vacuum chamber,
partially opening the valve, the vacuum signal from the manifold side
(reduced by exhaust backpressure) is transmitted to the hollow stem of\
the valve. See Fig. 10. This enables the signal to act on the
diaphragm, providing a specific flow. Thus, the EGR flow is a constant
percentage of engine airflow.
Verify EGR valve is present and not modified or purposely
damaged. Ensure thermal vacuum switches, pressure transducers, speed
switches, etc., (if applicable) are not by-passed or modified. Ensure
vacuum hose(s) to EGR valve is not plugged.