series III and earlier Land Rovers use a skirted thermostat. There
is a cylinder or skirt that moves when the thermostat opens. This
moving cylinder is actually a coolant flow valve.
The thermostat housing has an oval opening in the side which leads
to the bypass hose. The bypass hose forms a passage between the
radiator entrance and exit allowing coolant to be pumped through
the engine while the thermostat is closed.
If there were no coolant flow while the thermostat is closed local
hot spots will occur in the head that will eventually cause head
The skirt on the thermostat closes off the oval bypass opening
when the thermostat is open. When the thermostat is closed, the
skirt is away from the opening so that water can flow through the
2.25L petrol Land Rover thermostat
If you use the common skirtless thermostat the by-pass opening
remains open continuously regardless of engine temperature. The
pump of course will pump the same amount but the flow will take
the line of least resistance which is through the by-pass rather
than the long way around through the radiator.
This will cause your engine to run hotter in warm weather. The
temperature gauge sender is located at the front of the engine by
the thermostat. With most of the water bypassing the radiator, temperatures
at the rear of the head can be significantly higher than your temperature
gauge indicates. The high temperature gradient can cause the head
Engines from the 110 onwards have a different thermostat housing
and uses the common skirtless thermostat. Instead of the large oval
opening the housing has a bypass hole about 3/8" in diameter.
When the thermostat is closed, the small opening to the bypass
hose provides sufficient bypass flow to keep hot sports from occurring
in the head.
When the thermostat is open the small hole provides more restriction
than the radiator forcing most of the coolant to flow through the
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