1988 Jeep Cherokee

1988 Exhaust Emission Systems



On models equipped with carburetor, a system for pre-heating

air entering carburetor is used. This system is part of the air

cleaner and maintains air temperature at a point where carburetor can

be calibrated at leaner setting to reduce hydrocarbon emissions and

improve driveability during warm-up.

These systems are vacuum-operated and consist of heat shroud

on exhaust manifold, hot air duct, thermal sensor switch, vacuum

motor, air valve assembly and reverse delay valve.

Fig. 1: Thermostatic Air Cleaner (TAC) Assembly

Courtesy of Chrysler Motors.


During engine warm-up, temperature sensor switch applies

vacuum to vacuum motor. Air diverter valve is held to "ON" position.

Exhaust manifold heated air flows to air cleaner. As temperature of

incoming air increases to 90

vacuum line to atmosphere allowing spring pressure to push valve to

"OFF" position. Air now flows from outside, through air cleaner duct

to carburetor.

o F (32o C), temperature sensor opens


On California vehicles, spring-loaded trap door is built

into air cleaner to close off air cleaner when engine is shut off.

Door is vacuum operated.


Reverse delay valve is installed in vacuum line in some

vehicles to prevent trap door from closing during low engine vacuum

periods. Valve provides about 9 seconds delay before allowing trap

door to close.

Fig. 2: Cutaway View of Thermostatic Air Cleaner Assembly

Courtesy of Chrysler Motors.



1) Remove air cleaner assembly from vehicle and allow to

cool to room temperature. Look through air cleaner duct and observe

position of air diverter valve. It should be fully open to outside


2) Reinstall assembly on carburetor and connect hot air duct

and manifold vacuum hose. Start engine and observe position of air

valve in snorkel. It should be fully closed to outside air.

3) Move throttle lever rapidly to 1/2 to 3/4 opening and

release. Air diverter valve should open and then close again. Allow

engine to warm to operating temperature and observe position of air

valve in snorkel. It should be fully open to outside air.

4) If valve does not move to fully close off outside air at


vacuum leaks in hose connections or disconnected vacuum motor. If

valve mechanism operates freely and no vacuum leaks are detected,

connect hose from intake manifold vacuum source directly to vacuum


5) If air valve now moves to close off outside air, replace

thermal sensor switch. If valve still does not move to close off

outside air, replace air cleaner assembly and vacuum motor assembly.

o F (28o C) or less with vacuum applied, check for binding of duct,


1) With engine off, remove air cleaner and check position of

trap door. It should be closed.

2) Remove vacuum hose from intake manifold vacuum source and

apply an external vacuum source of approximately 2-4 in. Hg vacuum.

Trap door should open.

3) If door does not open, apply vacuum directly to vacuum

motor. If door does not open, check for binding and adjust as

necessary. If door swings freely, replace vacuum motor.

4) If door opens during step 3), check vacuum hose for

blockage, cracks or leaks. Correct as necessary and retest as

specified in step 2).

5) If hoses are not defective, remove reverse delay valve,

join vacuum hose and retest from step 2). If door opens, replace

reverse delay valve.


1) Connect external vacuum source to port on White side of

delay valve. Connect one end of 24" (610 mm) section of rubber hose

to vacuum gauge and other end to port on colored side of valve.

2) With a constant 10 in. Hg vacuum applied, note time

required for vacuum gauge pointer to move from 0-8 in. Hg.