Thermostat; Viscous Fan Drive; Radiator Coolant Flow Check - Jeep 1997 Grand Cherokee Service Manual

ZG
DIAGNOSIS AND TESTING (Continued)

THERMOSTAT

DIAGNOSIS
Diesel engines, due to their inherent efficiency are
slower to warm up than gasoline powered engines,
and will operate at lower temperatures when the
vehicle is unloaded. Because of this, lower tempera-
ture gauge readings for diesel versus gasoline
engines may, at times be normal.
Typically, complaints of low engine coolant temper-
ature are observed as low heater output when com-
bined with cool or cold outside temperatures.
To help promote faster engine warm-up, an electric
engine block heater must be used with cool or cold
outside temperatures. This will help keep the engine
coolant warm when the vehicle is parked. Use the
block heater if the outside temperature is below 4°C
(40°F). Do not use the block heater if the out-
side temperature is above 4°C (40°F).
TESTING
NOTE: The DRB scan tool cannot be used to mon-
itor engine coolant temperature on the diesel
engine.
(1) To determine if the thermostat is defective, it
must be removed from the vehicle. Refer to Thermo-
stats for removal and installation procedures.
(2) After the thermostat has been removed, exam-
ine the thermostat and inside of thermostat housing
for contaminants. If contaminants are found, the
thermostat may already be in a "stuck open" position.
Flush the cooling system before replacing thermostat.
Refer to Cooling System Cleaning/Reverse Flushing
in this group for additional information.
(3) Place the thermostat into a container filled
with water.
(4) Place the container on a hot plate or other suit-
able heating device.
(5) Place a commercially available radiator ther-
mometer into the water.
(6) Apply heat to the water while observing the
thermostat and thermometer.
(7) When the water temperature reaches 80°C
(176°F) the thermostat should start to open (valve
will start to move). If the valve starts to move before
this temperature is reached, it is opening too early.
Replace thermostat. The thermostat should be fully
open (valve will stop moving) at approximately 89°C
(192°F). If the valve is still moving after the water
temperature reaches this temperature, it is opening
too late. Replace thermostat.
(8) If the valve refuses to move at any time,
replace thermostat.

VISCOUS FAN DRIVE

TESTING
If the fan assembly free-wheels without drag (the
fan blades will revolve more than five turns when
spun by hand), replace the fan drive. This spin test
must be performed when the engine is cool.
The cooling system must be in good condition. This
is checked prior to performing the following test. It
also will ensure against excessively high coolant tem-
perature.
WARNING: BE SURE OF ADEQUATE FAN BLADE
CLEARANCE BEFORE DRILLING.
(1) Drill a 3.12-mm (1/8-in) diameter hole in the
top center of the fan shroud.
(2) Obtain a dial thermometer with an 8 inch stem
(or equivalent). It should have a range of -18°-to-
105°C (0°-to-220° F). Insert thermometer through the
hole in the shroud. Be sure that there is adequate
clearance from the fan blades.
(3) Block the air flow through the radiator. Secure
a sheet of plastic in front of the radiator (or air con-
ditioner condenser). Use tape at the top to secure the
plastic and be sure that the air flow is blocked.
(4) Be sure that the air conditioner (if equipped) is
turned off.
WARNING: USE EXTREME CAUTION WHEN THE
ENGINE IS OPERATING. DO NOT STAND IN A
DIRECT LINE WITH THE FAN. DO NOT PUT YOUR
HANDS NEAR THE PULLEYS, BELTS OR FAN. DO
NOT WEAR LOOSE CLOTHING.
(5) Start the engine and operate at 2400 rpm.
Within ten minutes the air temperature (indicated on
the dial thermometer) should be up to 93° C (200° F).
Fan drive engagement should have started to occur
at between 82° to 91° C (180° to 195° F). Engage-
ment is distinguishable by a definite increase in fan
flow noise (roaring).
(6) When the air temperature reaches 93° C (200°
F), remove the plastic sheet. Fan drive disengage-
ment should have started to occur at between 57° to
79° C (135° to 175° F). A definite decrease of fan
flow noise (roaring) should be noticed. If not, replace
the defective viscous fan drive unit.

RADIATOR COOLANT FLOW CHECK

There is coolant flow all of the time through the
coolant tank (bottle) before and after the thermostat
opens. If you remove the vent valve and insert a tem-
perature gauge through the opening, coolant willspill
out of the system and the engine will not be filled
with coolant up to the heads.
COOLING SYSTEM 7 - 13