A Little Valve with a big Responsibility: Thermostat

I recently came across an overheated 4WD on the tracks, which got me thinking about the importance of understanding thermostats.

A thermostat is a wax type valve that is used to regulate engine temperature. It warms the engine quickly and ensures that the engine stays at optimum operating temperature. Ensuring that the engine gets to the right temperature as quickly as possible improves fuel economy and keeps your engine running well.
Engine sensors control the amount of fuel delivered to the engine. To warm up, an engine requires more air and fuel than usual. Once it has reached its running temperature, these sensors bring the fuel trim back to normal to improve economy and emissions.

On this engine there is a bleed screw which must be removed when refilling the radiator.


On this engine there is a bleed screw which must be removed when refilling the radiator.
A thermostat can, however, faIl and become stuck in either the open or closed position. When the thermostat is stuck closed, it restricts the water flow through the engine and usually results in overheating, which happens very quickly.
When the thermostat is stuck open, it can cause the engine to run cool, which results in poor fuel economy, and also a cold running engine is a wearing engine. An open thermostat is not a huge problem because you will still be able to get home, but a thermostat that is stuck closed can be a very serious issue.

Once the thermostat has been cut, water can flow straight through. From here the thermostat housing must be refitted and the radiator refilled with water.


Once the thermostat has been cut, water can flow straight through. From here the thermostat housing must be refitted and the radiator refilled with water.


If an engine is overheating, is can be quite difficult to diagnose a thermostat problem. There are also a few important safety issues that should be mentioned. A hot engine is an engine that is under pressure and the radiator cap pressurises the engine’s radiator system. If the radiator cap is removed on a hot engine, it will cause the radiator water to steam up and explode out of the radiator. Do NOT open the radiator cap or undo a hose if the engine is hot. If you’re not sure, set up your chair and grab a drink, kick back and wait a bit longer for the engine to cool.
If the thermostat is stuck closed, you can usually feel this by checking the temperatures of the top and bottom radiator hoses. During warm up, the top and bottom hoses will start off at different temperatures and as the engine warms, the hoses will both become hot, which shows that water is circulating throughout the engine. If the thermostat is stuck closed, you will feel that one hose will be nice and hot and the other will still be cold (even as the engine temperature rises). This shows that there is a blockage in the system and the thermostat may be the cause.

If you cut along the joins of the thermostat the inside of the valve will simply come apart, creating flow.


If you cut along the joins of the thermostat the inside of the valve will simply come apart, creating flow.
Ultimately, if you’re stuck in the middle of nowhere and your 4WD is overheating, there is no harm in removing the thermostat temporarily while you find a place to have it fixed properly.
The thermostat is located in a housing that is generally quite accessible, but in some later model vehicles can be a little harder to get to. Remove the thermostat housing and the thermostat, then reassemble everything and refill it with water.
There are two types of thermostat seals, and you will need to reseal the thermostat housing when you put it back together. One type is the standard paper thermostat gasket and the other type is an O-ring seal, which is part of the thermostat itself. With this you will have to cut the thermostat so the valve component drops out. Then, refit the body of the thermostat back into the housing and reassemble.

 

An O-ring type thermostat with the O-ring fitted around the outside. The O-ring creates the seal between the thermostat housing and the cylinder head.


An O-ring type thermostat with the O-ring fitted around the outside. The O-ring creates the seal between the thermostat housing and the cylinder head.

The thermostat comes apart once cut.


The thermostat comes apart once cut.
When refilling the engine with water, you will need to fill it to the top of the radiator and then allow the engine to warm up and make sure there is no trapped air in the system. One easy way to do this is to make sure that both top and bottom radiator hoses are hot and to also have the heater set to hot (make sure it is blowing hot air). If the engine has a bleeder screw, the water will need to be bled from the screw before fitting back the radiator cap. It is important to bleed the system of air, as trapped air can also cause the engine to overheat.
When I go away, I make sure I am carrying enough water to refill the radiator in case of emergencies. When stuck, don’t worry too much about coolant, just fill the system with water until you are able to have the vehicle checked and repaired properly and the coolant flushed and replenished.
Removing the thermostat is a very common thing to do if your thermostat is sticking. In some extreme cases though, it is possible that water flow through the radiator may be too fast for efficient cooling. This could also result in the engine overheating. Most of us carry a pretty good box of spare parts with us when we go travelling. A thermostat and its correct seal or gasket takes up almost no room, so it would be a great addition to your set of spares.
If stuck, it’s better to do something than nothing, as long as it is done safely. But if you have tried and the vehicle continues to overheat, then don’t drive the vehicle further.  It will probably result in a very expensive repair. So be sensible, bite the bullet and organise a tow.
Travel safe and GO NUTS!

 


Qualified motor mechanic Adam Adler has spent half his life under the bonnet of a 4WD and has worked for some of the top accessory companies and workshops. He knows what it takes to get your vehicle out there and back home in one piece. He runs the online aftermarket store www.nutsabout4wd.com.au.


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