Expansion tank cap: how it works, why is it needed and how to check


Since the combustion engine operates under increased thermal stress, most vehicles are equipped with a system in which coolant is circulated to maintain the unit's optimum temperature.

One of the important elements that ensures the stable function (motor cooling) of the system is the expansion tank cap. It not only closes the neck of the tank, preventing foreign objects from entering the line, but performs several important functions. Let's consider what they are.

Tasks of the expansion tank cap

When heat is exchanged in the engine, the antifreeze is very hot. Since the substance is based on water, when the temperature rises, it tends to boil. As a result, air is released, which is looking for a way out of the circuit.

Expansion tank cap: how it works, why is it needed and how to check

Under normal conditions, the boiling point of water is 100 degrees. However, if you increase the pressure in the closed loop, it will boil later. Therefore, the first function of the cover is to provide a pressure increase that increases the coolant boiling point.

In the case of antifreeze, it usually boils when it reaches a maximum of 110 degrees, and antifreeze - 120 Celsius. While the cooling system is closed, this figure increases slightly, preventing the formation of air bubbles that block circulation.

When the internal combustion engine is running, its temperature rises to approximately 120 degrees - in the region of the maximum boiling point of the coolant. If the reservoir is tightly closed, then excessive pressure will build up in the system.


A little earlier we have already considered motor CO device. Its main elements are made of metal, but the connection of the units is provided by rubber hoses of large diameter. They are fixed on the fittings with clamps. Since a pressure system is created in the circuit, the working fluid will look for a weak point in the line.

Expansion tank cap: how it works, why is it needed and how to check

An overpressure relief valve must be installed in the circuit to prevent the hose or radiator pipe from bursting. This is another function of the expansion tank cap. If the valve breaks, this problem will immediately manifest itself.

Device, principle of operation of the tank lid

So, firstly, the lid seals the reservoir tightly to increase the pressure in the system. Secondly, its device allows you to relieve maximum pressure. The design of any cover includes:

  • The body is mostly durable plastic. It has a hole for pressure relief;
  • A sealant so that air does not come out at the connection ahead of time;
  • Valve - Basically it consists of a spring and a plate that closes the outlet.

The spring loaded valve plate prevents excess air from leaving the system. The resistance of this element is strictly calculated by the manufacturer. As soon as the pressure in the circuit exceeds the permissible value, the spring is compressed by the plate and the outlet opens.

Expansion tank cap: how it works, why is it needed and how to check

Many cover models have a vacuum valve in addition to a pressure relief valve. It eliminates the need to open the reservoir when the engine is cold. When the coolant expands, excess air leaves the system, and when it cools down, volume begins to recover. However, with a tightly closed valve, a vacuum is created in the line. This deforms the plastic reservoir and may burst faster. The vacuum valve ensures that the system can be freely filled with air.

Why is the pressure in the cooling system so precise?

The pressure in the line that cools the power unit is important. Thanks to him, antifreeze does not boil in a modern car. If there is atmospheric pressure in it, the volume of the working fluid will decrease faster due to the evaporation of water. Such a problem will require frequent fluid replacement.

Expansion tank cap: how it works, why is it needed and how to check

Also, insufficient pressure will accelerate the boiling of antifreeze even before the motor reaches its maximum temperature regime. The operating temperature of the power unit is described in separate review.

What caps are there?

It is practical to use covers designed for the OS of a particular car model. If you install a non-standard modification (if it fits the thread), then it may not release in time or not relieve excessive pressure at all.

Regular covers are a cheap option, but they often have one malfunction. Since the materials in them are cheap, the metal elements corrode faster, losing their elasticity. Also, sometimes the elements are sintered, from which the valve either solidifies in the open position, or vice versa - in the closed position.

Expansion tank cap: how it works, why is it needed and how to check

Often the effectiveness of a cork can be determined by its color. There are yellow, blue and black caps. How each individual modification will work must be checked on a specific vehicle. Some maintain a pressure within 0.8 atm., Others provide an increase in this indicator to 1.4, and sometimes up to two atmospheres. The optimal indicator should be indicated in the car's manual.

If you put a part on the tank that can withstand more pressure than the tank itself, then it will need to be changed more often. And this is additional waste.

Signs of a bad expansion tank cap

The following "symptoms" may indicate the need to check the cover:

  • The car often boils (but earlier in the same operating mode, such a problem was not observed);
  • The radiator tube (heating or main) burst;
  • Nozzles burst;
  • The reservoir often bursts;
  • Even on an overheated motor, the stove does not heat the air. This often happens when airing appears in the circuit - pressure in the system is not created, from which the antifreeze boils;
  • When the car is started, an unpleasant smell of burning oil is heard from the air vents or white smoke comes from under the hood. This can happen when antifreeze is leaking onto a hot front pipe;
  • Coolant traces appear on the clamps of the pipes.
Expansion tank cap: how it works, why is it needed and how to check

Often, the situation may require not only replacing the tank cap, but also repairing other components of the cooling system. For example, if the radiator tube is torn, then it must be replaced with a new one. For more information about the design of radiators, and in what case they can be repaired, read here.

How to check the expansion tank cap

Visually, malfunctions of the expansion tank cap are revealed only in the case of rust formation, and then only protruding onto the outer part of the part. While the lid appears to be a simple element, testing it is not an easy procedure.

The problem lies in the fact that the serviceability of the valve can only be checked under pressure conditions. This is not a thermostat that you just put in boiling water to see if it opens. In the case of the cover, it will be necessary to create artificial pressure, which is not easy to do in the garage, and especially to fix the indicators (the easiest way is to use a car compressor).

For this reason, if you suspect a valve malfunction, you should contact a car service for help. In a workshop, it is easier to check the functionality of the valve.

Expansion tank cap: how it works, why is it needed and how to check

If there is no desire to pay for such a diagnosis, the procedure can be performed independently, but its results will be relative. So, the engine starts and warms up to operating temperature. Then we turn off the unit and, in conditions of complete silence, try to unscrew the cover (it is important to do this carefully so as not to get thermal injury).

If during the unscrewing process there were no sounds (for example, hiss or whistle), then the valve is working properly. However, it should be admitted that the valve is relieving excessive pressure, which means that a small pressure in the system will still arise.

The vacuum valve is checked as follows. We start the car, warm it up until the fan works, then turn it off. We are waiting for the unit to cool down. If the walls of the tank are deformed inward, then a vacuum has formed in the system, and the valve does not work.

Broken lids are usually not repaired. However, you can do this if you wish. Only the maximum that can be done in this case is to disassemble the part and clean it from dirt. Most car manufacturers recommend periodically replacing the tank cap.

Here's another option for how to check the traffic jam:

How to check the expansion tank cap for pressure relief


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