Toyota 4A-FE Repair Manual page 172

COOLING SYSTEM - Description C0-3
The cooling system is composed of the water
jacket (inside the cylinder block and cylinder head),
radiator, water pump, thermostat, electric fan,
hoses and other components.
Coolant which is heated in the water jacket is
pumped to the radiator, through which an electric
fan flows 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 cylinder head
through which coolant passes. It is designed to
provide adequate cooling of the cylinders in the
combustion chambers which become heated dur-
ing engine operation.
RADIATOR
The radiator performs the function of cooling
the coolant which has passed through the water
jacket and become hot, and is mounted in the front
of the vehicle. The radiator consists of an upper
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 filter inlet. It also has
a hose attached through which excess coolant or
steam can flow. The lower tank contains the outlet
for coolant and the drain cock. 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
electric fan, as well as the wind generated by the
vehicle's travel, passes through the radiator, cool-
ing it. Models with automatic transmissions
include an automatic transmission fluid cooler built
into the lower tank of the radiator_ A fan with an
electric motor is mounted behind the radiator to
assist the flow of air through the radiator. The fan
operates when the coolant temperature becomes
high in order to prevent it from becoming too high.
RADIATOR CAP
The radiator cap is a pressure type cap which
seals the radiator, resulting in pressurization of the
radiator as the coolant expands. The pressuriza-
tion prevents the coolant from boiling even when
the coolant temperature exceeds 1 00°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: 11 O -
120°C, 230 - 248°F, pressure; 0.3 - 1.0 kg/
cm2, 4.3 - 14.2 psi, 29.4 - 98.1 kPa). The
vacuum valve opens to alleviate the vacuum which
develops in the coolant system after the engine is
stopped and the coolant temperature drops. The
valve's opening allows return of the coolant in the
reservoir tank to the cooling system.
RESERVOIR TANK
The reservoir tank is used to catch coolant
which overflows the cooling system as a result of
volumetric expansion when the coolant is heated.
The coolant in the reservoir tank returns to the
radiator when the coolant temperature drops, thus
keeping the radiator full at all times and avoiding
needless coolant loss. Check the reservoir tank
level to see if the coolant needs to be replenished.
WATER PUMP
The water pump is used for forced circulation of
coolant through the cooling system. It is mounted
on the front of the cylinder block and driven by a
timing belt.
THERMOSTAT
The thermostat is a wax type bypass valve and
is mounted in the water inlet housing. The ther-
mostat includes a type of automatic valve oper-
ated by fluctuations in the coolant temperature.
This valve closes when the coolant temperature
drops, preventing the circulation of coolant
through the engine and thus permitting the engine
to warm up rapidly. The valve opens when the
coolant temperature has risen, allowing the cir-
culation of coolant. Wax inside the thermostat
expands when heated and contacts when cooled.
Heating the wax thus generates pressure which
overpowers the force of the spring which keeps
the valve closed, thus opening the valve. When the
wax cools. its contraction causes the force of the
spring to take effect once more, closing the valve.
The thermostat in this engine operates at a tem-
perature of 82°C (180°F).