How to check the expansion tank or radiator cap

The expansion tank cap is one of the simplest mechanisms in a car. In this case, its failure can lead to serious engine damage. Many people have a very obvious question: “What can break there? It's just a cover." In fact, even on the first car models, the expansion tank was equipped with a reliable lock.

When disassembling the cover, curious car owners might be surprised by the presence of springs and valves in its design. The question arises, why such difficulties when closing the container with coolant.

In this article we will try to figure this out. Let's start with the purpose, move on to the symptoms, and then find out how to check the expansion tank cap. At the end, a couple of tips from experts.

Purpose of the tank lid

In the cooling system of a running power plant, pressure gradually increases. This is caused by heating the antifreeze. This parameter requires constant adjustment. This is what the expansion tank cap is for.

Sustained high or low pressure can shift the boiling point of a liquid. Under normal conditions, water will boil at +100 °C, antifreeze and antifreeze need about 120 °C. The operating temperature of modern car engines has already come close to this threshold.

Under certain operating conditions, the coolant heats up quickly. To prevent the motor from overheating, you need to raise the boiling point of the liquid. To do this, it is enough to increase the pressure in the system.

This gave engineers the idea that in order to prevent the boiling of the coolant in the engine, during short but severe overheating, it is necessary to maintain a stable pressure within the range of 1.1-1.5 kgf/cm2. This indicator is quite sufficient, because with an even higher value, the structure will have to be equipped with more durable and reliable elements.

The expansion tank cap maintains the required pressure and, if necessary, allows air into the tank or releases excess.

Signs and possible consequences of a part malfunction

Most often, the consequence of a malfunction of the cap is boiling of the coolant, sometimes accompanied by the ejection of the coolant from the expansion tank - a common occurrence for VAZ cars. If this happens at engine operating temperatures, then most likely the plug does not hold the required pressure.

Car coolant boiling

Another reason for the same phenomenon is that due to a malfunction of the vacuum valve, air entered the ODS and formed a plug, which disrupts normal circulation, and therefore the cooling of the coolant. In winter, due to an air lock, the stove may not work well. Unpleasant consequences: loss of coolant, which has to be topped up regularly. This happens when, due to increased pressure in the SOD, liquid is “expelled” through the joints between the fittings and pipes put on them, through clamps.

Disastrous consequences:

rupture of low-quality pipes or those that have not been changed for a long time (a picture familiar not only to VAZ owners);
the appearance of a leak in the main or heating radiator;
tears the thermostat housing (for Nexia it’s a common thing - into 2 parts);
burst expansion tank.

A leak appears in the main radiator

All these signs and consequences of a malfunctioning lid are the result of increased pressure in the SOD. The latter is not uncommon for VAZ 2108, 2109 and especially 2110 cars with new-style tanks. Of course, it is obvious that the plastic of these expansion tanks leaves much to be desired, but, nevertheless, this is also due to a malfunction of the plug. So the lid, by regulating the pressure in the SOD, also protects its elements from mechanical damage.

Expansion tank cap design

The lock to the container should fit tightly, without gaps. For better connection, the lid is equipped with a rubber seal. By unscrewing it from the tank and looking at the inner surface, you can see a kind of pressure regulation sensor.

As mentioned above, there are two valves in the lid. One is designed to prevent engine overheating, the second prevents depressurization due to increased pressure.

Each of them received a name corresponding to its purpose. On VAZ cars, the high pressure valve is called a safety valve, and the low pressure valve is called a vacuum valve. Both devices are at rest until the pressure value approaches the limit value.

It is worth knowing that car owners may not always notice malfunctions in the operation of the cover valves. Visual inspection in most cases does not provide accurate information about the serviceability of devices. But a breakdown of one of the valves is fraught with serious consequences and can lead to expensive repairs.

Refinement of a new part - what to do if it does not function?

When it turns out that the lid stubbornly refuses to relieve excess pressure and/or is not able to restore air losses in the SOD, it can be modified. For some reason, the most complaints come from owners of VAZ, model 2109, including those who buy new plugs. It is clear that the reason for the incorrect operation of the cover valves lies in the too much rigidity of their springs.

To modify the cover, for example, for a VAZ 2109, it must first be disassembled. This must be done carefully, using pliers and a thin flat-blade screwdriver. We must try to remember where everything stood so that the springs do not fly away to God knows where. After the lid for the VAZ 2109 tank has been disassembled, it’s time for the wire cutters. They need to shorten the springs: the large one, for the safety valve, usually by 1 turn, and the small one by 2.

Shortening the plug spring

For the second, the length of the removed segment is not critical - as long as it generally supports the vacuum valve when it is closed and does not resist the external air pressure of 0.03–0.1 kgf/cm 2 . With a large spring it is more difficult - as if not to overdo it. You need to look at its rigidity and the pressure at which the safety valve opened when checking the tank cap of the VAZ 2109. After shortening the springs, we assemble the cap in the reverse order. Before use, it is advisable to check again how it works.

Consequences of failure of the expansion tank cap

It depends on which valve has failed. If the mechanism for releasing excess pressure breaks down, at best, some pipe will rupture and coolant will leak out. If the driver does not notice this, the engine will overheat very quickly.

In the worst case, the expansion tank may rupture and a crack will form in the radiator or interior heater. In this case, you will have to spend a lot on repairs.

If the relief valve is stuck open, the system will not have the pressure needed for proper cooling operation. The boiling point will decrease accordingly.

If the vacuum valve fails, the consequences can also be unpleasant. This will lead to an air lock, and as a result, cold air will blow from the stove when the engine is well warmed up. In cold weather this will not improve driving comfort.

Signs of a faulty expansion tank cap

Failure of even one of the cover valves can lead to certain vehicle malfunctions and driving inconveniences. Indirect symptoms may indicate the need to check the functionality of the mechanisms. These include the following:

Thick white smoke poured out from under the hood;
cold air comes from the stove when the engine is warm;
the clamps on the pipes were torn off;
rupture of cooling system pipes;
a crack appeared on the radiator;
the expansion tank ruptured;
the rubber tubes were compressed.

Smoke appears more often if the pressure in the system has exceeded the permissible value. High engine temperature and cold air from the heater indicate low engine performance.

Normal pressure in the cooling system maintains optimal operation of the engine and interior heater. A breakdown of the expansion tank cap leads to an increase in engine temperature, which leads to failure of the valves and cylinders. Only a major overhaul will help return the car to working condition.

How does a radiator cap work?

In order to better understand the essence of checking the radiator cap, you first need to discuss its structure and circuit. First of all, it should be noted that the antifreeze in the cooling system is under increased pressure. This circumstance was done specifically in order to increase the boiling point of the coolant, since the operating temperature of the engine is slightly higher than the traditional +100 degrees Celsius. Typically, the boiling point of antifreeze is around +120°C. However, it depends, firstly, on the pressure inside the system, and secondly, on the state of the coolant (as the antifreeze ages, its boiling point also decreases).

Through the radiator cap, not only is antifreeze poured directly into the radiator housing (although antifreeze is usually added to the expansion tank of the corresponding system), but also through it, coolant, converted into steam, exits into the expansion tank. The design of a car radiator cap is quite simple. Its design involves the use of two gaskets and two valves - bypass (another name is steam) and atmospheric (another name is intake).

The bypass valve is located on a spring-loaded plunger. Its job is to smoothly control the pressure inside the cooling system. It is usually around 88 kPa (varies from vehicle to vehicle and also depends on the operating conditions of the particular engine). The task of the atmospheric valve is the opposite. Thus, it is designed to ensure a gradual equalization of atmospheric pressure and increased pressure inside the cooling system in a situation when the engine is turned off and cooling. Using an atmospheric valve provides two benefits:

A sharp jump in coolant temperature when the pump stops is eliminated. That is, heat stroke is excluded.
The pressure drop in the system is eliminated while the coolant temperature gradually decreases.

Thus, the listed factors are the answer to the question of what the radiator cap affects. In fact, its partial failure usually leads to a decrease in the boiling point of antifreeze, and this can lead to it boiling during engine operation, that is, overheating of the engine, which in itself is very dangerous!

How to check the functionality of the expansion tank cap

To avoid burns, the test is carried out on a completely cooled engine. In addition, the heated antifreeze in the radiator is under pressure. When the cap is unscrewed, it may splash out and cause serious burns.

There are several verification methods. Let's look at each of them:

Visual inspection - you need to carefully inspect the cover; it should not have any signs of damage (chips, dents, etc.). If there are any, then over time a corrosion center will appear in this place and the metal will gradually deteriorate. In this case, the cover should be thoroughly cleaned and repainted. It is better to replace the damaged product with a new one.
Checking the spring - there are springs in each cover, which belong to the safety valves. To check, you need to squeeze them with your fingers. If it offers virtually no resistance, it means it has failed. On a collapsible cover, you can replace a non-working spring. But more often the plugs are maintenance-free, so you will need to buy a new one.
Checking the vacuum valve - you need to pull it with your fingers so that it opens. Then throw it and watch it return to the opposite position. You also need to carefully inspect the mechanism seat for traces of dirt and deposits. If they exist there are two solutions. The first thing is to try to clean it. Second, install a new cover. The choice of solution method depends on the degree of contamination of the vacuum valve.
Checking the valve for operation - this will require special equipment. We'll talk about it a little lower.

You can use the popular method of checking the expansion tank cap. It is carried out as follows: the engine warms up well and the radiator pipe is probed with the car not turned off. If the pressure is felt, then the cap is working properly, if it is easily crushed, the valve is leaking.

There is another popular method, but experienced drivers will confirm that it does not work. Its essence is as follows: with the engine running, you need to squeeze the upper pipe with your hand. If the cap is working, then at this moment the liquid level in the expansion tank will rise. You can also remove one end of the pipe and see how the liquid flows out. But the point is that squeezing with your hands will not create the pressure required to lift the valve.

Checking the cover on the main VAZ models

Experienced owners of domestic cars have made it a rule to change the expansion tank cap every two to three years. On the one hand, such attention to detail greatly simplifies the operation of the internal combustion engine cooling system, and on the other hand, the cover costs less than 100 rubles, so absolutely every driver can afford to buy it.

And replacing the cover does not cause any difficulties. First, with the car cooled down, unscrew the cap from the tank. Then screw in the new one until it stops.

Video: diagnosing the performance of the lid

Checking the cover on the VAZ 2108/2109

The design of the “eights” and “nines” produced by AvtoVAZ allows you to check the functionality of the tank cap in one minute. The operation looks like this:

Open the hood of a car that has cooled down after driving.
Unscrew the cap of the expansion tank.
Squeeze the supply hose as tightly as possible with your hand.
At the same time, screw the lid back on.
After this, lower the hose.

If the compression area immediately regains its original elasticity, it means that the valves on the cover are working properly. If it still cannot recover for a long time, it is recommended to replace the tank cap with a new one.

You can check the operation of the coolant reservoir cap on a VAZ 2109 with your own hands by simply squeezing the pipe with antifreeze

How to check the functionality of the cover on a VAZ 2110/2111/2112

The procedure for checking the valves on the cover is the same as for previous VAZ models. The only nuance on the VAZ of the tenth family is that when unscrewing the cap from the tank, the valve mechanism itself may fall out of the cavity of the cap. This is not an indicator of failure, but, most likely, an insecurely fixed element will not work for long.

On a VAZ of the tenth family, the expansion tank cap tends to unscrew from the neck without valves

Checking the lid using available tools

Checking the functionality of the valves on the cover is quite simple if you have the right tools. First you need to disconnect any pipe of the cooling system. Then, using a compressor with a pressure gauge (this will help you find out the exact release pressure), you need to supply air to the system. A hissing sound from the cap will indicate that the valve has opened.

Please note: if the valve does not bleed air, it is not recommended to quickly release the pressure, since antifreeze may splash out when opening the lid or disconnecting the pipe. Under normal conditions this is prevented by a valve.

The verification is carried out in two stages:

First, the snout of the mechanism is lifted with your finger. It is ideal if he moves with minimal effort (virtually without resistance).
On a cold engine, the cover is installed in its seat. Then disconnect the pipe going to the expansion tank and blow into the radiator through it. The valve is triggered by a slight excess pressure, so anyone can create it. If the cover is in working order, you will hear a characteristic hissing sound as air escapes.

Important! For this procedure, it is better to use a compressor, if you have one, otherwise you risk drinking antifreeze.

How to check the cover yourself on Priora, Grant, Kalina

It seems that it is enough to unscrew the cap from the expansion tank and inspect its insides to understand the specifics of the problem. However, it is not. For example, service centers use special pressure pumps that can pump the required number of atmospheres . By creating artificial pressure in the cooling system, the device allows you to calculate the correct operation of the valves.

A device for diagnosing VAZ tank caps has a pressure gauge and a pump

But drivers don't have this expensive equipment, so they have to make do with what they have in a standard garage. Diagnostics of the quality of operation of the tank cap will be less accurate, but it will be possible to identify the main malfunctions of the valves:

Turn off the engine.
Unscrew the cap from the tank neck.
Inspect the product for cracks, dents, and chips. The seal may have ruptured or become severely worn.
If there is no visible damage, screw the cover back on and start the engine.
Wait until the engine warms up to its normal temperature.
Start slowly unscrewing the cap from the tank.
If, during the process of unscrewing, a whistle of escaping air is heard (this sound can be compared to the release of gases when opening lemonade), then the lid is in working order, since it restrains the pressure in the tank and hoses.
After this, you need to turn off the engine again.
Inspect the cooling system pipes. If they are pulled in, the vacuum valve of the lid does not work well, since the pressure in the system is extremely low.

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The expansion tank cap can only be unscrewed after the engine has cooled down. The working cap should not have expansions, cracks or dents. All pipes must have an equal diameter throughout their entire length, without thickening or shrinking. This is interesting: Preliminary diagnostics

This is a typical scheme for checking expansion tank cap valves on all models of AvtoVAZ vehicles.

Checking the cover gasket

When checking the valves, it makes sense to immediately make sure that the top gasket of the reservoir cap is tight. Even when air whistles through the cover, this only indicates that the valve is working properly. But if the lid does not close tightly, antifreeze will gradually evaporate through the gap. As a result, its level in the system will drop.

The reverse process is also possible, when the system sucks air from the atmosphere, instead of taking antifreeze from the expansion tank. This leads to the formation of air pockets (airing).

To carry out the test, you need to disconnect the pipe coming from the expansion tank and blow air into the system through it. The pressure required is low - approximately 1.1 bar. Then close the receiver and listen. If there is no hissing, everything is fine. For greater precision, you can make a soap solution and apply it around the perimeter of the cork. If the gasket is leaking, foam will appear. In this case, the part will need to be replaced.

Checking the expansion tank cap with a diagnostic device

Many who were interested in checking the cooling system plug have heard about a special diagnostic device. Such a tool is quite expensive; in 2022, I heard about an approximate amount of 15,000 rubles. Not every car owner can afford to buy such a tester. Therefore, it is only available to car services that constantly deal with such problems.

An ordinary car owner, if desired, can assemble a similar device on his own. To do this you will need the following components:

radiator - can be taken from any old car, the condition does not matter, the main thing is that there is a whole upper part where the cover is attached;
sandpaper;
cold welding;
camera nipple;
compressor with precise pressure gauge.

The expansion tank is cut off from the radiator and all honeycombs are muffled so that air does not pass through the bottom. The same needs to be done with the side walls. Then a hole is made, the nipple from the camera is inserted into it and carefully sealed with cold welding. That's all, the device is ready.

Now you can move on to testing. The cover being tested is installed on the seat. The compressor is connected to the nipple, and air is pumped into the tank through it. By comparing the pressure gauge readings with the moment the excess pressure is released, one can judge the performance of the cover.

The main advantage of such a homemade product is the low cost of the device. The disadvantage is the complexity of manufacturing and lack of versatility. That is, the device will only fit if the diameter of the lid and the thread match. Otherwise, you will have to make another device, but from a suitable radiator.

The tester will help determine the operating pressure range of the expansion tank cap. Depending on the engine type, the readings may differ:

Gasoline car. The safety valve opens at 83-110 kPa, for vacuum the opening pressure is -7 kPa.
Diesel transport. For a safety valve, the value should be 108 kPa, ±15 kPa. The vacuum will open at 83 kPa.

The above values are averages, but you can use them as a guide. More accurate data on the pressure of the safety and vacuum valves can be found in the data sheet for your car, or on the manufacturers’ websites. If, during testing, the values obtained differ greatly from those indicated, then we can judge that this unit is faulty. Therefore, the cover needs to be replaced with a new one or try to be restored.

Repair of expansion tank cap

Radiator caps on modern cars are often made non-removable, so it is unlikely to be repaired. So you can remove traces of corrosion yourself and repaint the product or replace the rubber gaskets.

But if one or both valves fail, it will not be possible to disassemble them, since the body is in most cases cast. In cases where you can get to them, there is an option to try to clean them of dirt and replace the springs. Although this is a temporary measure and soon you will still have to go for a new cover.

Which is the best cover to buy?

A question that almost every car owner asks sooner or later. It’s probably worth noting right away that in terms of characteristics it should completely correspond to the one that stood before. That is, have the same diameter, thread pitch, valve sizes, as well as similar operating pressure.

Most modern cars have plugs designed to operate at 0.9-1.1 bar. But before buying a lid, you should clarify these data, as sometimes there are exceptions.

Today on sale you can find so-called “tuned” expansion tank caps. They are designed to operate at higher pressures - about 1.3 bar. This is done to further increase the boiling point of the coolant, thereby increasing engine efficiency. Such products are installed on sports cars, the power plants of which operate under heavy but short-term loads.

For an ordinary car that is used for city driving, such covers are completely unsuitable. Their installation will lead to a number of negative factors, including the following:

Parts of the cooling system wear out - this leads to a decrease in their service life and premature failure. It’s good if after such an undertaking the pipe bursts or the clamp comes off, and you will have time to notice the leaked antifreeze. But everything can end much worse - the radiator or expansion tank may burst. And this will lead to very expensive repairs.
Reducing the service life of antifreeze. All technical fluids have a certain operating temperature range. Exceeding it leads to a decrease in the performance characteristics of antifreeze, which negatively affects its service life. If you decide to install a tuned cover, change the coolant more often.

Hence the conclusion - do not experiment, but use the parts recommended by the manufacturer. Regarding the question of which brand is better to buy an expansion tank cap, there is no definite answer; all products have their own characteristics. The main thing is that the part is original.

Cover device

It is important to understand that there is a locking element for expansion tanks on any car that has an internal combustion engine under the hood. And it doesn’t matter whether it’s diesel, gasoline or gas-powered.

Therefore, the problem with the failure of this unit can occur on any car, including such models as:

Lada Kalina;
Ford Focus;
Gazelle Next;
Daewoo Lanos;
Volkswagen Passat;
Lada Priora;
VAZ 2110;
Renault Logan;
Opel Astra;
VAZ 2114;
Chevrolet Lacetti, etc.

You already understand that the cap serves not only to prevent coolant from pouring out of the tank. This is also a pressure regulator in the system.

A valve is hidden inside the lid, usually made of plastic. It is he who takes on such responsibility. The valve is immersed inside the expansion tank and, as necessary, pumps or releases excess pressure.

Before buying a new element, if you suspect the old one is broken, you can check it. A similar type of locking mechanism should be selected.

Please note that all modern foreign cars, in the vast majority of cases, have switched to single-valve caps for expansion tanks. That is, only one valve design is physically present. It just can perform two functions at once. While on domestic cars you can still find a pair of valves. Each of them is responsible for performing a corresponding function. Whether this is good or bad is difficult to judge. But the fact remains a fact.

The lid is activated only when an excess or deficit of pressure is detected in the system. Each car has its own temperature and pressure standards for the element to operate.

I bought a new cover, but it doesn't work. What to do

Owners of VAZ 2110 and 2114 are sometimes faced with a situation where one or both valves on the purchased new expansion tank cap do not work. In this case, you can try to modify it. More often, valve malfunctions are associated with the manufacturer using too stiff springs.

To solve the problem, the first thing you need to do is disassemble the cover. This is done using a flat screwdriver and pliers. When disassembling, it is important to remember which part was placed where, as they can fly apart in different directions.

After removing the springs, they need to be trimmed a little - the large one (from the safety valve) is shortened by one turn, the small one (from the vacuum one) has two turns removed. When shortening the first part, it is important to take into account the moment the mechanism operates. The main thing here is not to overdo it and not to cut off too much. The second spring is a little easier - it should exert at least minimal pressure after closing.

Upon completion of the procedure, the components are assembled in reverse order. Now you need to check the functionality of the system and make sure that the valves operate at the right time.

DIY repair

Restoring the functionality of this valve element consists of three stages: disassembly, cleaning, and replacement of damaged elements.

Even a beginner can repair this part of the expansion tank with his own hands, being careful and careful. To do this, you only need a screwdriver with a thin flat end.

Work must be carried out in the following sequence:

Unscrew this part, carefully wipe its inner surface, getting rid of drops of antifreeze;
use a screwdriver to remove the rubber seal; remove the spring that is located under the rubber;

then remove the valve elements that have become accessible;
using cotton swabs and soap solution, rinse the inner cavity of the lid;
in case of visible damage or deformation, replace the spring or rubber seal;
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install all parts sequentially in reverse order.

A few tips for maintaining a healthy system

You can avoid most problems, from minor troubles to serious damage associated with the expansion tank cap, if you follow a few rules. Professionals advise the following:

Change the expansion tank cap at least once every 2-3 years. Aggressive coolant components attack mechanism parts and seals, gradually destroying them.
Use only the original cover to replace a worn one. Some analogues do not allow the engine to operate at the required temperatures.
Check and adjust the valves only on a cold engine, otherwise you risk getting burned from boiling water splashing out under pressure.
Use tuned expansion tank caps only temporarily. Such a part will not provide optimal engine performance parameters and is therefore poorly suited for installation on a car.

In most cases, to restore the functionality of the mechanism, it is enough to simply clean the valves and rubber seals from dirt and sediment. If this procedure does not help, then it is easier to buy a new one, since modern expansion tank caps are made non-removable.