Dodge Durango (DN). Manual — part 251
OPERATION
The coolant reserve/overflow system works in con-
junction with the radiator pressure cap. It utilizes
thermal expansion and contraction of coolant to keep
coolant free of trapped air. It provides a volume for
expansion and contraction of coolant. It also provides
a convenient and safe method for checking coolant
level and adjusting level at atmospheric pressure.
This is done without removing the radiator pressure
cap. The system also provides some reserve coolant
to the radiator to cover minor leaks and evaporation
or boiling losses.
As the engine cools, a vacuum is formed in the
cooling system of both the radiator and engine. Cool-
ant will then be drawn from the coolant tank and
returned to a proper level in the radiator.
THERMOSTAT—5.2L/5.9L ENGINES
DESCRIPTION
CAUTION: Do not operate an engine without a ther-
mostat, except for servicing or testing.
The thermostat on all gas powered engines is
located beneath the thermostat housing at the front
of the intake manifold (Fig. 4).
The thermostat is a wax pellet driven, reverse pop-
pet choke type.
Coolant leakage into the pellet container will cause
the thermostat to fail in the open position. Thermo-
stats very rarely stick. Do not attempt to free a ther-
mostat with a prying device.
Fig. 2 Engine Cooling System Flow—4.7L Engine—Typical
1 – LH CYL. HEAD
2 – BLEED
3 – THERMOSTAT LOCATION
4 – RH CYL. HEAD
5 – RH BANK CYL. BLOCK
6 – LH BANK CYL. BLOCK
7 – COOLANT TEMP. SENSOR
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COOLING SYSTEM
7 - 3
DESCRIPTION AND OPERATION (Continued)
The same thermostat is used for winter and sum-
mer seasons. An engine should not be operated with-
out a thermostat, except for servicing or testing.
Operating without a thermostat causes longer engine
warmup
time,
unreliable
warmup
performance,
increased exhaust emissions and crankcase condensa-
tion that can result in sludge formation.
OPERATION
The wax pellet is located in a sealed container at
the spring end of the thermostat. When heated, the
pellet expands, overcoming closing spring tension
and water pump pressure to force the valve to open.
THERMOSTAT—4.7L ENGINE
DESCRIPTION
CAUTION: Do not operate an engine without a ther-
mostat, except for servicing or testing.
A pellet-type thermostat controls the operating
temperature of the engine by controlling the amount
of coolant flow to the radiator. On all engines the
thermostat is closed below 195°F (90°C). Above this
temperature, coolant is allowed to flow to the radia-
tor. This provides quick engine warm up and overall
temperature control. On the 4.7L engine the thermo-
stat is designed to block the flow of the coolant
bypass journal by 50% instead of completely blocking
the flow. This design controls coolant temperature
more accurately (Fig. 5).
The same thermostat is used for winter and sum-
mer seasons. An engine should not be operated with-
out a thermostat, except for servicing or testing.
Operating without a thermostat causes other prob-
lems. These are: longer engine warmup time, unreli-
able
warmup
performance,
increased
exhaust
emissions and crankcase condensation. This conden-
sation can result in sludge formation.
OPERATION
The wax pellet is located in a sealed container at
the spring end of the thermostat. When heated, the
pellet expands, overcoming closing spring tension
and water pump pressure to force the valve to open.
RADIATOR PRESSURE CAP
DESCRIPTION
All radiators are equipped with a pressure cap.
This cap releases pressure at some point within a
range of 124-to-145 kPa (18-to-21 psi). The pressure
relief point (in pounds) is engraved on top of the cap
The cooling system will operate at pressures
slightly above atmospheric pressure. This results in a
Fig. 3 Upper Fan Shroud with Integral Coolant
Reserve/Overflow System
1 – RADIATOR
2 – WASHER FLUID RESERVOIR
3 – COOLANT OVERFLOW/RESERVOIR
4 – FAN SHROUD (UPPER)
5 – SCREW
6 – INTERLOCKING PINS
7 – FAN SHROUD (LOWER)
Fig. 4 Thermostat—Typical
1 – THERMOSTAT HOUSING
2 – GASKET
3 – INTAKE MANIFOLD
4 – THERMOSTAT
5 – MACHINED GROOVE
7 - 4
COOLING SYSTEM
DN
DESCRIPTION AND OPERATION (Continued)
higher coolant boiling point allowing increased radi-
ator cooling capacity. The cap contains a spring-
loaded pressure relief valve. This valve opens when
system pressure reaches the release range of 124-to-
145 kPa (18-to-21 psi).
A rubber gasket seals the radiator filler neck. This
is done to maintain vacuum during coolant cool-down
and to prevent leakage when system is under pres-
sure.
OPERATION
A vent valve in the center of the cap will remain
shut as long as the cooling system is pressurized. As
the coolant cools, it contracts and creates a vacuum
in cooling system. This causes the vacuum valve to
open and coolant in reserve/overflow tank to be
drawn through connecting hose into radiator. If the
vacuum valve is stuck shut, or overflow hose is
kinked, radiator hoses will collapse on cool-down.
RADIATOR
DESCRIPTION
The radiator is a aluminum cross-flow design with
horizontal tubes through the radiator core and verti-
cal plastic side tanks.
OPERATION
The radiator supplies sufficient heat transfer using
the cooling fins interlaced between the horizontal
tubes in the radaitor core to cool the engine and
automatic transmission oil (if equipped).
WATER PUMP
DESCRIPTION
The water pump is located on the engine front
cover, and has an integral pulley attached (Fig. 8).
The water pump impeller is pressed onto the rear
of a shaft that rotates in a bearing pressed into the
water pump body. The body has a small hole for ven-
tilation. The water pump seals are lubricated by
Fig. 5 Thermostat Cross Section View 4.7L
1 – FROM HEATER
2 – FROM RADIATOR
3 – TO WATER PUMP
4 – ENGINE BYPASS
5 – THERMOSTAT
Fig. 6 Radiator Pressure Cap—Typical
1 – FILLER NECK SEAL
2 – VACUUM VENT VALVE
3 – PRESSURE RATING
4 – PRESSURE VALVE
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COOLING SYSTEM
7 - 5
DESCRIPTION AND OPERATION (Continued)
antifreeze in the coolant mixture. Additional lubrica-
tion is not necessary.
OPERATION
A
centrifugal
water
pump
circulates
coolant
through the water jackets, passages, intake manifold,
radiator core, cooling system hoses and heater core,
this coolant absorbs the heat generated when the
engine is running. The pump is driven by the engine
crankshaft via a drive belt.
WATER PUMP BYPASS—4.7L
DESCRIPTION
The 4.7L engine uses an internal water/coolant
bypass system. The design uses galleries in the tim-
ing chain cover to circulate coolant during engine
warm-up
preventing
the
coolant
from
flowing
through the radiator. The thermostat uses a stub
shaft located at the rear of the thermostat (Fig. 9) to
control flow through the bypass gallery.
OPERATION
When the thermostat is in the closed position the
bypass gallery is not obstructed allowing 100% flow.
When the thermostat is in the open position the stub
shaft enters the bypass gallery obstructing bypass
coolant flow by 50%. This design allows the coolant
to reach operating temperature quickly when cold,
while adding extra cooling during normal tempera-
ture operation.
Fig. 7 Radiator—Typical
1 – RADIATOR
2 – A/C CONDENSER (IF EQUIPPED)
3 – TRANSMISSION AUXILIARY OIL COOLER
Fig. 8 Water Pump Location—Typical
1 – WATER PUMP BYPASS HOSE
2 – FAN BLADE ASSEMBLY
3 – VISCOUS FAN DRIVE
4 – WATER PUMP AND PULLEY
Fig. 9 Water/Coolant Bypass Flow and Thermostat—
4.7L Engine
1 – FROM HEATER
2 – FROM RADIATOR
3 – TO WATER PUMP
4 – ENGINE BYPASS
5 – THERMOSTAT
7 - 6
COOLING SYSTEM
DN
DESCRIPTION AND OPERATION (Continued)
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