Back in the days of liquid cooled engines, almost all emissions systems used to release harmful vapors and chemicals right into the atmospheres. Catalytic converters, CCV and PCV lines weren’t really a thing yet, so all you had was dribbling coolant and blowing-by all over the roads. The cars had an amazing time, but the environment around them didn’t. As time passed and environmental restrictions became tighter, car manufacturers and engineers had to come up with ways to keep the byproducts of cooling chemicals and internal combustion inside the car.
And they did. Today, almost everything in the engine is recycled in one way or another, and stays there, including the liquid responsible for keeping the cylinders at appropriate temperature levels. As you probably know, when things get hot, they tend to expand, and when the coolant gets hot it creates gas pressure and steam that needs to go somewhere. As aforementioned, back in the days, that somewhere was just into the atmosphere and into the ground. So in order to preserve our environment, engineers designed special tanks to catch and reuse the coolant.
There are two types of tanks – expansion and overflow tanks. An overflow tank, also known as a recovery tank is the simpler type. And one of the best ways to figure out whether your car has an overflow tank is to look for a pressure rated cap on your radiator, because that’s what determines when the coolant starts flowing between the tank and the cooling system. Overflow systems rely on the pressure that’s a result of the expanding coolant.
Expansion tanks, also known as degassing bottles in the diesel world, are more complex. The tank is constantly under pressure, and the pressure cap is located on the tank itself rather than the radiator. Actually, the radiator doesn’t have a cap since you fill the coolant from the expansion tank. Since the cooling system is constantly under pressure, the coolant is constantly being recycled through the radiator and the tank. As pressure is released from the hot coolant, it goes to the top half of the tank and acts as a force to push the coolant back into the cooling system.
Both overflow and expansion tanks can be made from plastic and aluminium. Plastic is the more affordable solution, but aluminium is better at dealing with the constant and drastic temperature changes. You basically get what you pay for, but as long as the tank is in proper working condition, you’re not going to see much change in terms of performance.