Ten expansion tank caps fitted on most VAZ models, excluding Vesta, XRAY, Largus and the classic lineup, were examined. Purchases were made across Russia through remote ordering.
Triune task
The expansion vessel plug serves to keep the liquid from splashing up and becoming contaminated. It also sustains higher system pressure as the fluid expands with heat. When the system cools, the plug must admit air to the expansion tank to avoid creating a vacuum that would squeeze the hoses.
A simple plug could handle the first function, but two valves are needed to maintain pressure and prevent vacuum. The exhaust valve governs overpressure and should open at 1.1 bar. The inlet valve opens when system pressure falls 0.03-0.13 bar below atmospheric pressure.
Plug components: 1 – valve holder; 2 – inlet valve; 3 – spring inlet valve; 4 – exhaust valve seal; 5 – exhaust valve; 6 – exhaust valve spring; 7 – body wrap; 8 – cover body.
Methodology
All plugs were checked for dimensional compliance by threading them onto the neck of a standard expansion vessel used on VAZ vehicles. Then pressure and vacuum were measured with both valves active.
Next, the samples underwent a “drying” process by placing the lids in a thermal chamber at 125°C for 24 hours. After drying, the caps were tested again against the full cycle.
Testing included verifying the outlet valve opening pressure on the stand. When the tank pressure exceeded 2 bar, the cork failed to open, indicating a potential explosion risk.
Members
AROKI 332080, Dimitrovgrad
| estimated price 130The cork shows no pressure retention, which can cause the engine to boil under light loads. Overall, a failure. |
Auto Tech AT 1065-001 CR
| estimated price 260Initially, the cork delivered satisfactory pressure, but after the drying phase it lost the correct geometry and would not seal around the tank neck. Failure. |
AVTOVAZ 2108-1311065-004
| estimated price 110The cork withstood pressure until drying, but after heating the pressure relief valve began sticking intermittently. The purchase is possible only if no alternatives exist. |
Automatic device 2108-1311065-01
| estimated price 280Thermally stable yet fails to maintain adequate system pressure. Engine boiling becomes a risk under load. Winter cork? Better to skip. |
Pramo 21080, China
| estimated price 190Maintains only 0.3 bar of overpressure, which is unacceptable for modern engines and loads. Do not apply! |
HPA 2108–1311065
| estimated price 200This cork causes no damage beyond wasted money. It could not even be screwed onto the tank neck. Total failure. |
ED Eurodetal, Rostov-on-Don
| estimated price 180Overpressure tolerance is only 0.1 bar. Engine boiling is inevitable. Do not buy! |
Ganz GRF 10001, China
| estimated price 250Appears to be hermetically sealed. It withstood overpressure above 2 bar, but after drying the cap stopped sealing around the tank neck. Total failure. |
Unnamed
| estimated price 190The cork allows free atmospheric communication with the tank. Such a product is useless. Do not buy! |
luzar
| estimated price 230The cork met factory requirements with minor flaws in the raw state and returned to normal after drying. Acceptable, with caution. |
The results apply only to a single sample of products and should not be used to judge all items bearing the same name.
Results
Readers were right to be cautious: selecting the simplest, cheapest part often leads to trouble. Among dozens tested, only two caps could be used on cars, and even those were not perfect. A basic device should not require microprocessors—yet a troubling question remains: why is quality so inconsistent?
A comment from the test engineer of NPO Talis LLC notes a disturbing trend in spare parts quality: more than half of the expansion vessel caps fail to release pressure properly, even when readings exceed 2 bar. Special insight from the specialist underscores a growing concern in the industry about part reliability. (Attribution: NPO Talis LLC, specialist’s remarks.)
Why do we cook?
Engine temperatures have risen with modern designs to boost efficiency. The cooling fan on early VAZ models activated around 99°C, while newer variants reach about 105°C. This shift aligns with stricter environmental standards, and many foreign cars operate even hotter.
The stated temperature reflects the sensor location; coolant can heat beyond that in some spots. To prevent boiling, coolant systems are designed to sustain higher pressures.
Modern antifreezes boil at atmospheric pressure near 108°C. By raising the cooling system pressure, the boiling point rises to roughly 120–125°C. In higher elevations or under heavy driving, boiling can occur earlier if pressure is insufficient.
- Further explanation on why many engines overheat can be found in related resources.
- Updates and discussions are available through messaging channels dedicated to car enthusiasts.