What is the function of the car's auxiliary water tank?

[MartinDella]

Automobile auxiliary water tank mainly functions as: 1. When the engine temperature is high, the liquid expands and part of the antifreeze flows into the auxiliary water tank through the radiator cap. 2. When the temperature drops and the cooling system pressure becomes negative, the system draws the antifreeze from the auxiliary water tank back into the cooling system through the radiator cap. 3. The auxiliary water tank has corresponding markings indicating the liquid level, which can intuitively show the current water level in the tank and whether it is sufficient. The auxiliary water tank is actually an auxiliary part of the radiator and an important component of the heat dissipation and cooling system.

[StephenDella]

I've been driving for over a decade and found this metal canister can be a lifesaver in critical moments. When the engine heats up, the coolant expands, with the excess flowing into the overflow tank for storage. As the car cools down, the hose sucks the coolant back to replenish the system. If a hose bursts on the road, the reserve in the overflow tank can last at least 20 minutes to get you to a repair shop. One time during a summer traffic jam on the highway, the AC cooling entirely relied on this little tank circulating coolant to survive! Now when I wash my car, I always wipe the radiator cap too – if the rubber seal ages, the cap won't hold pressure, causing coolant to spray everywhere when the engine's hot and making the fan run nonstop.

[DiElaina]

Last week at the repair shop, the mechanic pointed to the transparent plastic tank next to my engine and called it the overflow reservoir. When the car gets hot, you can hear it bubbling as it collects expanding coolant. The fun part is checking the fluid level when the engine's cold – if it's below the minimum mark, you need to top it up with pink antifreeze. I remember last winter when the overflow tank's hose developed a crack, and the AC started blowing this weird sour smell. Now I check all the hose connections every three months – if the rubber feels sticky or degraded, it's time for replacement.

[LeMatteo]

Every car enthusiast knows the overflow tank is the last line of defense in the cooling system. For high-performance racing engines, installing a larger overflow tank is essential—it can store an extra two liters of coolant during intense driving. I once witnessed a friend’s turbocharged car burst a coolant hose during a track day, but the overflow tank kept it running for a full lap. A transparent, graduated aluminum overflow tank is the most practical choice. When upgrading the intercooler, make sure to leave enough space for installation. Always use high-temperature silicone hoses, as stock rubber hoses can’t withstand the demands of track driving.

[CamilaLynn]

We taxi drivers know best how much money these little bottles save! Last year, the coolant reservoir was leaking unnoticed, and within three months, we burned through two whole bottles of antifreeze. Now, every shift change, we pop the hood and check the hose connections—replace them immediately if the rubber feels stiff. A mechanic taught me to check the fluid level only when the engine is cold; checking it hot can lead to misjudgment. Once, the radiator cap spring rusted, causing insufficient pressure in the cooling system and erratic temperature gauge readings—replacing the cap cost just fifteen bucks.

[Ferreira]

From a thermodynamic perspective, the overflow tank is essentially a pressure regulation vessel. When the internal pressure of the cooling system reaches 0.9-1.1 kg, the valve opens, allowing high-temperature coolant along with steam to rush into the overflow tank. As the temperature drops and creates a negative pressure, the one-way valve draws the liquid back into the main radiator. It is recommended to replace the radiator cap valve every two years, as aging rubber seals can lead to pressure imbalance. Using distilled water instead of tap water for refilling can prevent scale buildup from clogging the passages, especially since repairing a corroded and perforated aluminum radiator can cost thousands.