Self-checking the thermostat of the car cooling system is elementary. In most cases, you can determine its common breakdowns right on the machine. Draining the coolant and disassembling the assembly is required only to make sure the diagnosis is correct, and also to replace the faulty part after checking.

The principle of operation of the car thermostat

It is difficult to check something without understanding at least in general terms the principle of operation. Fortunately, a car's cooling system thermostat is a very simple thing. It is nothing more than a valve that opens or closes in response to changes in the temperature of the antifreeze. Thanks to this, the coolant is automatically directed "in the right direction."

The principle of operation of a classic car thermostat is based on the properties of a substance to expand and contract depending on temperature. It has a small sealed capsule with liquid that acts through the stem on the valve disc. The whole part is completely in antifreeze. When it warms up to a certain temperature, the liquid in the capsule expands, and the valve begins to open slightly. The hotter it gets, the more the thermostat opens.

When the coolant temperature drops, the valve should close. To do this, its design provides a spring that "supports" the plate. The valve closes as gradually as it opens, because the liquid in the capsule compensates for the spring force in proportion to the heat. Thanks to this, the temperature balance is better maintained in the car's cooling system, there are no jumps and sudden changes.

And it works even easier
And it works even easier

Thermostat functions

In order to quickly understand the functions of the thermostat, it is necessary to consider a simplified design of the system in which it operates. The engine cooling system consists of two main circuits - small and large. The small circuit covers the so-called water jacket of the motor and a small stove radiator in the cabin. A large circuit is when the main radiator and expansion tank are connected to a small one.

The task of the thermostat is to send overheated coolant to the radiator
The task of the thermostat is to send overheated coolant to the radiator

It is our thermostat that separates these subsystems. Its main function is to maintain the optimum temperature for engine operation in the small circuit. While it is low, the thermostat is fully closed. This means that the coolant circulates only in a small circuit, which allows the engine to warm up faster. In addition, the stove inside the car reaches the operating temperature faster.

When the coolant heats up more than it should, the valve starts to work. It opens slightly, and hot antifreeze flows to the main radiator. There it cools and returns to the small circuit. If you give the engine a serious load, it will overheat more intensely. In such cases, the thermostat opens all the way to allow as much coolant as possible to flow to the radiator for cooling.

When the thermostat "notices" that the antifreeze in the small circuit is cooled below the required temperature, it works in the reverse order. That is, gradually closed. In such a simple way, the temperature balance of the coolant is maintained, and the engine always works in optimal conditions for it. Including, there is no increased fuel consumption.

On many machines, the thermostat opens fully and more than high temperature
On many machines, the thermostat opens fully and more than high temperature

Let's summarize the above. The thermostat is needed in the car's cooling system in order to:

  • the cold engine warmed up as quickly as possible;
  • the stove in the cabin started blowing warm air earlier;
  • temperature conditions comfortable for the engine were maintained.

Accordingly, when a part fails, the engine reaches operating temperature for a long time (or does not go out at all), the stove does not heat the interior, fuel consumption increases, wear accelerates, and throttle response decreases. There are also more serious problems. In particular, when the valve is stuck closed, the engine quickly overheats, even to the point of failure.

Common thermostat failures

The next step on the way to a simple thermostat check is to get acquainted with its main malfunctions. Here, too, everything is quite easy, since such a simple node can have only five possible breakdowns. It is useful to know them not only for the sake of verification, but also for understanding the consequences of a particular malfunction. Moreover, some of them can lead to very expensive repairs.

Jamming in the open state

This thermostat malfunction is more common than others. It consists in the fact that the thermostat valve at one fine moment opens completely, and forever remains in this state. This happens because the force of the spring is not enough to return the plate to its original place. As a rule, the cause is corrosion, scale or hardening of the liquid in the capsule.

Indirect signs of the thermostat jamming in the open state:

  1. The motor does not reach operating temperature for a long time even in the warm season.
  2. In cold weather it doesn't warm up at all.
  3. The stove either barely heats up or constantly blows cold air.
  4. Engine power is reduced.
  5. Fuel consumption goes up.

Damage consequences:

  1. The fluidity of engine oil deteriorates.
  2. Reduced overall engine life.
  3. The car becomes uncomfortable in the winter season.

Generally speaking, jamming the thermostat in the fully open position is not a critical failure. A car with it can travel for a long time and without fail. In fact, many people drive like this, and they think about checking the thermostat only with the advent of cold weather.

Jamming in the closed state

This is perhaps the most dangerous thermostat malfunction. When the coolant remains "locked" in a small circuit, overheating of the engine is inevitable. This happens for several reasons. Again, scale and corrosion. Also, the liquid capsule sometimes ceases to be airtight, and because of this, it cannot do its job. With such a breakdown, it will not work for a long time, and therefore it is very important to always remember how to identify it in a timely manner.

Indirect signs of the thermostat jamming in the closed position:

  1. The arrow on the instrument panel goes off scale.
  2. If present, the corresponding signal lamps or errors light up.
  3. The motor may lose throttle response, stability, and even stall.
  4. When the stove is running in the cabin, a pungent smell of antifreeze is possible.
  5. White steam from under the hood.
Thick steam from under the hood is one of the signs of a stuck thermostat
Thick steam from under the hood is one of the signs of a stuck thermostat

The consequences of a breakdown depend on at what stage it will be noticed. If it's timely, then nothing bad will happen. Although it will be difficult to go further with your own, and in the summer, it is almost impossible. If the jamming of the thermostat in the closed state is missed, the engine will simply overheat and either stall or break. In general - a dangerous malfunction.

Premature opening or closing

For a working thermostat, the moment of opening and closing occurs at certain temperatures that are “comfortable” for the engine. If the valve operates prematurely, then the coolant performance is constantly outside the optimal range. This breakdown is not as terrible as jamming in the closed state, nevertheless.

When the thermostat begins to open or close prematurely:

  1. The engine does not warm up to operating temperature as it should.
  2. The oven never gets hot.
  3. Slightly increased fuel consumption.

Basically, it's okay. But unpleasant. The cause of such a malfunction is most often the natural wear of the thermostat. Although sometimes a marriage comes across, and he freaks out in the described way right out of the box.

Late opening or closing

In this case, the opposite is true - the engine regularly, although not critically, overheats, and the stove stinks of burnt antifreeze. The reasons for the breakdown are similar. Natural wear or marriage. It is also worth adding an incorrectly selected thermostat here. This sometimes happens, since it is quite realistic to find an option that is suitable in size for your car. It is installed easily (well, or with the help of a small refinement with a file), but the temperatures at which it opens and closes are calculated for a different engine.

Incomplete opening or closing

An extremely rare failure. Dangerous only if the thermostat does not open fully. It is clear that sooner or later this will lead to overheating of the engine. If the valve does not close completely, then it will be almost the same as in the case of premature operation. Therefore, we will not repeat ourselves. Let's start checking.

How to check the thermostat

To fully check the thermostat and identify absolutely any of its malfunctions, it is enough to know only three ways. Two of them allow you to carry out diagnostics without draining the coolant, and without disassembling anything. Let's start with them.

Checking the thermostat with bare hands

All that is needed to test the thermostat in this way is a working engine temperature gauge on the instrument panel. By observing its behavior, you can timely identify almost all the malfunctions described above. Naturally, one cannot do without understanding how the cooling system works in general, and the thermostat in particular.

The algorithm is this:

  1. During the engine warm-up, the temperature does not rise for a long time - the thermostat is either not closed completely, or it is generally stuck open.
  2. The temperature almost never reaches normal - the thermostat either opens a little early or is always open.
  3. The arrow reads off scale - it is obvious that the thermostat does not work out. It is either stuck closed or starts to close too early.

The disadvantage of this method is, of course, the low accuracy of diagnosis. Including, you need to remember that the off-scale temperature does not always indicate a broken thermostat. The engine may overheat for other reasons. For example, the main radiator is dirty, the fan does not work, the coolant level is low, or there is air in the system.

Checking the thermostat with thermometers

The second way. Its use allows you to check the thermostat with high accuracy without removing it from the machine. As the name implies, you will need thermometers. And those that can be attached to the car's cooling system. In addition, the measurement range of the instruments must be at least up to +150°C. The most convenient tool for these purposes is recognized as a multimeter with a thermocouple and the corresponding function. A kitchen thermometer, if available, will also work.

With a strong desire, you can do with one thermometer. But the check will have to be carried out in two stages, between which you need to give the engine time to cool down. The technique for two devices is also described here. However, it will be easy to navigate along it if there is only one.

The algorithm for checking the thermostat with thermometers is as follows:

  1. Initially, the engine must be cold.
  2. The first thermocouple is attached to the branch pipe of the small circuit of the cooling system closer to the thermostat. Be sure to make sure that the surrounding air does not introduce errors. Therefore, you should wrap the thermocouple with electrical tape.
  3. The second is wound on the other side. That is, between the thermostat and the main radiator. Closer to the first.
  4. We start the engine and look at the temperatures before and after the thermostat.
  5. We draw the appropriate conclusions about its performance, based on the information presented above.

Here is an example for clarity. Let's say you are warming up a cold engine, and the temperature climbed up on two instruments at once. This clearly indicates that the thermostat is stuck open. If it is serviceable, then the small circuit should warm up, and the large circuit should remain almost cold until the motor reaches operating temperature. When it is reached, the temperature in the large circuit should begin to rise, and in the small circuit, it should fluctuate back and forth.

Checking the thermostat in boiling water

Folk method for checking the thermostat - in boiling water
Folk method for checking the thermostat - in boiling water

Folk method, one might say. Simple, clear and precise. The only drawback is that the thermostat must be removed from the machine for testing. In addition, you will need a container of water, a stove and a thermometer to measure the temperature of liquids. As a matter of fact, if the thermostat is stuck in some position, then you will immediately understand this disassembly field. That is, it will not be necessary to check anything in boiling water. If visually everything looks workable, proceed to the test.

The algorithm is this:

  1. Fix the thermostat in a container of water so that it is completely immersed in it, but does not touch the walls and bottom.
  2. Install the thermometer in the same way.
  3. Heat the water until the thermostat begins to open slightly.
  4. Record the water temperature.
  5. Heat to a boil.
  6. Note the temperature at which the thermostat opens fully.
  7. Check the received numbers either with the inscriptions on the thermostat itself, or with the manual for your car.

It is clear that this method allows you to accurately identify all thermostat failures, including late or early operation.

Keep in mind that some thermostats cannot be forced to open fully in boiling water. This is not always a breakdown. Cooling systems vary, and many are designed to operate over a slightly larger range than the +100°C boiling point of water would reveal.

VIDEO: test thermostat

Brief summary

The thermostat is an extremely important part of a liquid-cooled internal combustion engine. It fails quite often, because it works in difficult conditions. Every motorist should be able to check it at least by indirect signs. Fortunately, this is not difficult to do, even without having anything at hand. Well, having a thermometer and the information presented above on hand, you can generally calculate any malfunction.