Toyota Supra 1990 Manual page 303

COOLING SYSTEM -
Description
CO-3
The cooling system is composed of the water
jacket (inside the cylinder block and cylinder head),
radiator, water pump, thermostat, cooling fan,
hoses and other components.
Coolant which is heated in the water jacket is
pumped to the radiator, through which an 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 cylinder head through
which coolant passes. It is designed to provide ade-
quate cooling of the cylinders and combustion
chambers which become the hottest during engine
operation.
RADIATOR
The radiator performs the function of cooling the
coolant which has passed through the water jack-
et and become hot, and 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 filter inlet. It also has
a hose attached through which excess coolant or
steam can flow. The lower tank contains the out-
let for coolant and the drain cock. The core con-
tains 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
cooling 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.
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 pressurization
prevents the coolant from boiling even when the
coolant temperature 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 pres-
sure generated inside the cooling system exceeds
the limit (coolant temperature: 110 - 120°C,
(230 - 248°F), pressure; 0.65 - 1.05 kg/cm2,
(9.2 - 14.9 psi, 63.7 - 103.0 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 the pressure in the cooling system
to return to the coolant in the reservoir tank.
RESERVOIR TANK
The reservoir tank is used to catch coolant which
overflows the cooling system as a result of volu-
metric 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
V -ribbed belt.
THERMOSTAT
The thermostat is a wax type one and is mount-
ed in the water outlet housing. The·thermostat in-
cludes a type of automatic valve operated 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 temper-
ature has risen, allowing the circulation of coolant.
Wax inside the thermostat expands when heated
and contracts when cooled. Heating the wax thus
generates pressure which overpowers the force of
the spring which keeps the valve closed, thus open-
ing 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 en-
gine operates at a temperature of 88°C (190°F).