Coolant Flow Chart
Permanent type antifreeze is used as a coolant to protect the cooling system from rust and corro-
sion. When the engine starts, the water pump turns and the coolant circulates.
The thermostat is a wax pellet type which opens or closes with coolant temperature changes. The
thermostat continuously changes its valve opening to keep the coolant temperature at the proper level.
When coolant temperature is less than 55°C (131°F), the thermostat closes so that the coolant flow
is restricted through the air bleeder hole, causing the engine to warm up more quickly. When coolant
temperature is more than 58 ∼ 62°C (136 ∼ 144°F), the thermostat opens and the coolant flows.
When the coolant temperature goes up beyond 100°C (212°F), the radiator fan relay conducts to
operate the radiator fan. The radiator fan draws air through the radiator core when there is not suffi-
cient air flow such as at low speeds. This increases up the cooling action of the radiator. When the
coolant temperature is below 97.5°C (208°F), the fan relay opens and the radiator fan stops.
In this way, this system controls the engine temperature within narrow limits where the engine op-
erates most efficiently even if the engine load varies.
The system is pressurized by the radiator cap to suppress boiling and the resultant air bubbles
which can cause engine overheating. As the engine warms up, the coolant in the radiator and the
water jacket expands. The excess coolant flows through the radiator cap and hose to the reserve tank
to be stored there temporarily. Conversely, as the engine cools down, the coolant in the radiator and
the water jacket contracts, and the stored coolant flows back to the radiator from the reserve tank.
The radiator cap has two valves. One is a pressure valve which holds the pressure in the system
when the engine is running. When the pressure exceeds 93 ∼ 123 kPa (0.95 ∼ 1.25 kgf/cm², 13 ∼ 18
psi), the pressure valve opens and releases the pressure to the reserve tank. As soon as pressure
escapes, the valve closes, and keeps the pressure at 93 ∼ 123 kPa (0.95 ∼ 1.25 kgf/cm², 13 ∼ 18 psi).
When the engine cools down, another small valve (vacuum valve) in the cap opens. As the coolant
cools, the coolant contracts to form a vacuum in the system. The vacuum valve opens and allows the
coolant from the reserve tank to enter the radiator.
COOLING SYSTEM 4-5