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Lexus SC300 / Lexus SC400. Manual - part 697

DESCRIPTION

This engine utilizes a pressurized forced circulation cooling system which includes a hermostat equipped

with a by–pass valve mounted on the inlet side.

The cooling system is composed of the water jacket (inside the cylinder block and cylinderhead), radiator,

water pump, thermostat, cooling fan, hoses and other components.

OPERATION

CO–2

–

COOLIING SYSTEM

Description

DESCRIPTION

This engine utilizes a pressurized forced circulation cooling system which includes a hermostat equipped

with a by–pass valve mounted on the inlet side.

The cooling system is composed of the water jacket (inside the cylinder block and cylinderhead), radiator,

water pump, thermostat, cooling fan, hoses and other components.

OPERATION

CO–2

–

COOLIING SYSTEM

Description

Coolant which is heated in the water jacket is pumped to the radiator, through which a cooling fan blows

air to cool the coolant as it passes through. Coolant which has been cooled is then sent back to the engine by
the water pump, where it cools the engine.

The water jacket is a network of channels in the shell of the cylinder block and cylinderhead through which

coolant passes. It is designed to provide adequate cooling through the cylinders and combustion chambers
which become heated during engine operation.
RADIATOR

The radiator performs the function of cooling the coolant which has passed through the waterjacket and

become hot, and it is mounted in the front of the vehicle. The radiator consists of an upper tank and lower tank,
and a core which connects the two tanks. The upper tank contains the inlet for coolant from the water jacket
and the filler inlet. It also has a hose attached through which excess coolant or steam can flow. The lower tank
contains the outlet for coolant and drain plug. The core contains many tubes through which coolant flows from
the upper tank to the lower tank as well as cooling fins which radiate heat away from the coolant in the tubes.
The air sucked through the radiator by the cooling fan, as well as the wind generated by the vehicle’s travel,
passes through the radiator, cooling the coolant. Models with automatic transmission include an automatic
transmission fluid cooler built into the lower tank of the radiator. A cooling fan is mounted behind radiator to assist
the flow of air through the radiator. When the coolant temperature is low, the fan operates slowly to help the warm
up, and when the coolant temperature becomes high, the fan speed is increased to provide the air flow required
for cooling.
RADIATOR CAP (on Reservoir Tank)

The radiator cap is a pressure type cap which seals the radiator, resulting in pressurization of the radiator

as the coolant expands. The pressurization prevents the coolant from boiling even when the coolant tempera-
ture exceeds 100

C (212

F). A relief valve (pressurization valve) and a vacuum valve (negative pressure valve)

are built into the radiator cap. The relief valve opens and lets steam escape through the overflow pipe when
the pressure generated inside the cooling system exceeds the limit (coolant temperature: 110–120

(230–248

F) pressure; 29.4–98.1 kPa (0.3–1.0 kgf/cm2, 4.3–14.2 psi)). The vacuum valve opens to alleviate

the vacuum which develops in the coolant system after the engine is stopped and the coolant temperature
drops.
RESERVOIR TANK

The purpose of the reservoir tank is to catch coolant overflows created by volumetrix expansion when the

coolant temperature increases. The cap of the reservoir tank is a pressure type which prevents deterioration
of the LLC (Long Life Coolant) caused by contact with atmospheric air, increases vaporization performance and
reduces loss of the coolant volume.
WATER PUMP

The water pump is mounted on the front of the cylinder block and driven by the reverse side of the timing

belt.
THERMOSTAT

The thermostat has a wax type by–pass valve and is mounted in the water inlet housing. The thermostat

begins to open at the temperature of 80

C (180

F). When the coolant temperature is low, the valve closes to

prevent coolant flow to the radiator, thus permitting the engine to warm up rapidly. When the by–pass valve
opens the by–pass circuit, the engine coolant continues to circulate inside the engine, quickly and uniformly
warming up to the appropriate temperature. When the coolant temperature is high, the valve opens and coolant
flows to the radiator where it is cooled. When the wax inside the thermostat is heated, it expands and thus
creates pressure which overpowers the force of the spring which keeps the valve closed. When the wax cools,
its contraction causes the force of the spring to take effect once more, closing the valve.

ELECTRONICALLY CONTROLLED HYDRAULIC COOLING FAN (See page

CO–22

)

–

COOLIING SYSTEM

Description

CO–3

PREPARATION

SST (SPECIAL SERVICE TOOLS)

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Illustration

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Note

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09278–54012

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Drive Shaft Holding Tool

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Hydraulic pump pulley

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09286–46011

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Injection Pump
Spline Shaft Puller

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Hydraulic pump pulley

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09620–30010

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Steering Gear Box
Replacer Set

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(09627–30010)

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Steering Sector Shaft
Bushing Replacer

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Oil seal for hydraulic motor

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(09631–00020)

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(Handle)

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Oil seal for hydraulic motor

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09631–10030

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Oil Seal Remover

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Oil seal for hydraulic pump

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09843–18020

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Diagnosis Check
Wire

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RECOMMENDED TOOLS

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09025–00010

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Small Torque
Wrench

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For measuring preload

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09082–00015

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TOYOTA
Electrical tester

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09200–00010

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Engine Adjust
Kit

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09258–00030

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Hose Plug Set

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Plug for the vacuum hose, oil hose ect.

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09904–00010

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Expander
Set

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Oil seal for hydraulic pump

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09905–00013

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Snap Ring
Pliers

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Oil seal for hydraulic pump

CO–4

–

COOLIING SYSTEM

Preparation

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