Yanmar 3JH4E, Service Manual

Page: 297 / 350

9. Coolant system
277
9.6 Thermostat
9.6.1 Functioning of thermostat
The thermostat opens and closes a valve
according to changes in the temperature of the
fresh water inside the engine, controlling the
volume of water flowing to the heat exchanger
from the cylinder head, and in turn maintaining the
temperature of the fresh water in the engine at a
constant level.
The thermostat is bottom bypass type. It is located
in a position connected with the cylinder head
outlet line at the top of the top of fresh water pump
unit.
When the fresh water temperature exceeds the
above temperature, the thermostat opens, and a
portion of the water is sent to the heat exchanger
and cooled by seawater, the other portion going
from the bypass line to the fresh water pump
intake.
The bypass line is closed off as the thermostat
valve opens and is completely closed when the
fresh water temperature reaches 81.5
°
C (valve lifts
4 mm (0.1575 in)), sending all of the water to the
heat exchanger.
9.6.2 Thermostat construction
The thermostat used in this engine is of the wax
pellet type, with a solid wax pellet located in a
small chamber.
When the temperature of the coolant rises, the
wax melts and increases in volume. This
expansion and construction is used to open and
close the valve.
9.6.3 Characteristics of thermostat
9.6.4 Thermostat inspection
Remove the thermostat cover on top of the fresh
water pump and take out the thermostat. Clean off
scale and rust and inspect, and replace if the
characteristics (performance) have changed, or if
the spring is broken, deformed ar corroded.
9.6.5 Testing the thermostat
Refer to 2.5 in chapter 2.
Opening temperature 75-78°C
Full open temperature 90°C
Valve lift at full open 8 mm
Bypass valve lift 3.7 mm
Bypass valve close temperature 81.5°C
Thermostat cover
Gasket
Gasket
Thermostat
Fresh water
pump body
010656-00E