March 30th 2007.

The term "thermostat" is used for many different devices.

The one on the wall in your house or apartment is purely electrical in nature and in this day and age, is probably also electronic.

The thermostat in your car, by comparison, is purely mechanical and hidden from sight  inside your engine to regulate the flow of coolant.
This type of thermostat uses a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the
wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats
of this type are available to maintain different temperatures, typically in the range of 70 to 90 °C (160 to 200 °F).

Modern engines run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce emissions.

Most thermostats have a small bypass hole to vent any gas that might get into the system (e.g., air introduced during system flushing,
or when changing a radiator). Modern cooling systems contain a relief valve in the form of a spring-loaded radiator cap, with a tube
leading to a partially filled expansion reservoir. Owing to the high temperature, the cooling system will become pressurized to a
maximum set by the relief valve, usually about 15 psi. The additional pressure increases the boiling point of the coolant above that
which it would usually be at atmospheric pressure.

This is the same phenomenon as the fact that a good cup of tea cannot be brewed at the top of a mountain, because atmospheric pressure allows water to boil at a much lower temperature. Which is the opposite effect of applying pressure to a system.

This device dates back to neolithic times and I'm sure that racing engines use a completely different system.

There are plans afoot, for instance, to drive all engine accessories electrically. The power steering pump and the water pump, for example.
At that time, a variable speed water pump might be able to replace the thermostat, but until that happens, every day you drive, you're relying
on this ancient device to look after your engines' well being.

If the thermostat fails to close, the outcome is not so serious. The engine fails to warm up and the heater produces very little heat.
But continuing to drive with a very low engine temperature will eventually result in oil sludge and a drastic increase in fuel consumption
because the engine temperature sensor will force the control computer to increase fuel flow, thinking that the engine is cold and needs
a richer mixture. (What used to be called "more choke" in the days of carburettors).

If a thermostat fails to open, disaster is afoot, because now the engine will rapidly overheat and the cylinder head, or heads, will almost
instantly warp, causing the head gaskets to fail.

Do not EVER try to drive even one mile (Km) with an overheated engine. Each foot forward that you drive can cost a lot of money.

As summer approaches and particularly if you turn on your air conditioner, watch your temperature gauge closely.
If it begins to show signs of rising more than usual have it checked right away. The cause could be a rotten radiator or a defective
water pump, but the prime suspect is always the thermostat.

If you're using a high grade synthetic engine lubricant, then you have a better chance of surviving an attack of the vapours, but not
much and not for long.

Have a happy and cool summer season.