Lesson 5/Learning Event 2
If the thermostat appears to be in good condition, test it in the following manner. Force the thermostat valve
open and place a 0.003-inch thickness gage between the valve and frame. Release the valve. If the valve does
not close on the gage tight enough so the thermostat can be suspended by the gage, discard the thermostat.
If the thermostat valve holds onto the gage, place it in water that has been heated to 175F. If the gage is
released, discard the thermostat. If the gage is not released, continue to heat the water. A good thermostat
will release the gage when the water temperature is between 177 and 182F. If the thermostat holds onto the
gage above 182F, discard it.
Cylinder Block and Head
Coolant may be lost through external (outside) or internal (inside) leaks in the engine. External leaks occur at
the core hole expansion holes, at gaskets where joints are sealed, and through cracked parts. If coolant leaks
internally, it will be into the crankcase or a combustion chamber. Internal leaks usually occur at the head
gasket or through a cracked cylinder head or block.
Inspect the engine for external leaks by examining the expansion plugs, the outside surfaces, and the point
where the cylinder block and head join. When needed, use a mirror and flashlight to aid in the inspection.
Some engines contain expansion plugs that you cannot see because they are in the rear of the cylinder block
and are covered by the flywheel housing. If one of these plugs is leaking, the coolant will run out of the drain
hole in the flywheel housing cover.
Coolant leaks in the cylinder block and head will be affected by the amount of pressure in the coolant jacket.
Pressure is created by the coolant pump as well as by expansion of the coolant when the radiator cap is in
place. Also, expansion and contraction of the cylinder block and head, resulting from temperature changes, will
affect leaks. For these reasons, check the engine for leaks when it is stopped and cold and then again when it
is running and warm.