Use The Correct Thermostat
For today's engines to operate at maximum efficiency in terms of
performance, fuel economy and emission levels it is important for
the engine to get to the correct operating temperature as soon as
possible and ensure that temperature is maintained during all
In order to do this manufacturers have redesigned their engines to
utilise what is called a reverse poppet or bypass style thermostat.
This style of thermostat has two valves instead of the one valve that
is seen on a non-bypass thermostat. The primary valve operates
exactly the same as the non-bypass thermostat and opens allowing
coolant to flow to the radiator when the engine is at normal operating
temperature. The secondary valve allows coolant to be circulated
back through the engine during its warmup stage. The temperature
of the engine is able to rise more evenly, minimising hot and cold
spots in the engine. The primary valve begins to open and the
secondary valve closes when the engine temperature rises. All
coolant is then directed through the primary valve to the radiator
ensuring that the correct operating temperature is maintained.
A vehicle fitted with a bypass style thermostat must always have
the correct bypass thermostat fitted. Vehicles fitted with non-bypass
thermostats must always be fitted with the correct non-bypass
thermostat. The fitting of an incorrect thermostat will cause the
engine to run differently to how it was designed. Overheating and
subsequent engine damage can be caused by using an incorrect
Correct Bypass Thermostat In
The Correct Application
Figure (1) shows the installation of the correct bypass style
thermostat. When the engine is cold the primary valve is closed
preventing the flow of coolant to the radiator. The secondary valve
is open and directs the flow of coolant back through the engine
allowing it to warm up faster. As the engine warms up the primary
valve begins to open and the secondary valve begins to close. Figure
(2) shows the secondary valve completely closed when the engine
is up to proper operating temperature. The primary valve is then
also completely open allowing full flow of coolant from the engine
to the radiator.
Incorrect Bypass Thermostat
In A Bypass Application
Figure (3) shows the use of an incorrect bypass thermostat in a
bypass application. If the secondary valve is incorrect it may not
close completely and coolant continues to flow through the bypass
port even though the primary valve is fully open. This will cause hot
coolant to return to the engine before it has been cooled. The engine
temperature will be hotter than required and may cause premature
failure of other components in the cooling system.
Non-Bypass Thermostat In A
In this situation there is no secondary valve that will block off the
bypass port when the engine warms up (Figure 4). This will cause
hot coolant to continue to circulate through the engine without
going to the radiator. The engine will run hotter than required and
may cause a failure of other components in the cooling system.
Correct Non-Bypass Thermostat In
The Correct Application
In engines with no bypass port a non-bypass thermostat must be
used (Figure 5). No coolant flows until the thermostat opens. An
incorrect non-bypass thermostat used in these applications will
normally not physically fit or will be loose in the housing. Correct
opening temperatures must also be used.
Bypass Thermostat In A Non-Bypass
This is the worst possible combination as the secondary valve of
the thermostat will hit the bottom of the housing (Figure 6) and
prevent the thermostat from opening, as no coolant whatsoever
will circulate and the engine will overheat and boil. Major engine
damage may be caused as a result of this situation.
Selecting The Correct Thermostat
The best way to ensure that no problem occurs when fitting a new
thermostat is to make sure that the new thermostat is the same
style as the one being replaced. The following should always be
checked prior to installing a new thermostat.
1 ) Thermostat style (Non-bypass or Bypass).
2) Thermostat dimensions.
3) Correct operating temperature.
All thermostats in this catalogue are correct for the vehicle
applications listed in relation to operating temperature, thermostat
style and thermostat dimensions. Please use the vehicle
application guide in this catalogue to determine the correct
thermostat for your vehicle.
Why high-flow thermostats increase your
vehicles cooling system performance.
Todays engines tend to generate more heat than older vehicles. Most
vehicle engines are controlled by computers and sensors to specific
temperature ranges which makes fitting the correct temperature thermostat
even more important.
In the past if an engine had an overheating problem people tended to use a
cooler temperature thermostat to overcome this problem. This cannot be
done with the high-tech engines of today especially with vehicles that have
thermo fan switches fitted. These switches along with other sensors are
programmed to come on and off at specific temperatures set by the vehicle
manufacturers. Changing to cooler temperature thermostats could lead to
switches and sensors working differently to the settings as specified by the
vehicles manufacturer and in turn could lead to overheating and subsequent
engine damage. Vehicles with extra loads especially if towing can also
generate extra heat through the engine. The need to have cooling systems
working more efficiently becomes even more important for vehicles with
Tridon Australia now offers a High-Flow Thermostat to improve cooling
system performance. High-flow thermostats have a larger valve (Fig.l) and
allow approximately 30% more coolant to flow to the radiator than standard
type Thermostats. Using High Flow Thermostats eliminates any need for
using cooler temperature Thermostats as a temporary cure for overheating
problems as was done in the past. Using correct High Flow Thermostat at
correct temperatures specified by OEM manufacturers will not interfere with
the engine management system.
All High-Flow Thermostats are fitted with a jiggle pin that enables the system
pressure to be stabilised so that the valve opens easier. The jiggle pin also
allows air pockets to move past the Thermostat and the temperature to be
regulated more efficiently. Fitting High-Flow Thermostats will allow your
vehicle cooling system to operate more efficiently.
Thermostat Testing Procedures
- Correct Field Procedure
The thermostat may be tested in any commercial type of tester for
checking the thermostat operation. For accurate and valid results, the
tests should be pedormed under carefully controlled conditions as follows:
1. Fill test container with 50% water 50% coolant concentrate.
(minimum 2 litres)
2. Manually open the thermostat sufficiently to insert a fine thread under
the valve seat.
3. Suspend thermostat by the thread in the container. The thermostat
must be fully submerged and not touch the bottom or sides of the
4. Heat the liquid slowly to activate the power element properly. Agitation
of the test fluid is required for accurate results.
5. Place a thermometer into the coolant so that the bulb is adjacent to the
thermostat temperature sensing pill.
6. Observe the thermometer and note the temperature at once when the
thermostat drops from the thread. This reading is the approximate start to
open temperature of the thermostat.
7. The thermostat should be replaced if it opens at a temperature more
than 6øC (1 IøF) below or 3øC (5øF) above the specified opening
temperature. The temperature should be increased and the thermostat
observed to make sure it fully opens. The thermostat should be fully open
at a coolant temperature approximately 12øC (22øF) above the specified
8. When cool carefully examine the thermostat valve to make certain it is
properly seated. If it does not seat properly, it should be replaced. It is
important the replacement thermostat be of similar design with the proper
start to open temperature specifications.