To produce power, engines combust a fuel and air mixture. This pushes pistons and connecting rods down and turns crankshafts (and ultimately the wheel) to get vehicles going. A by-product of all this energy is heat. The bigger and more powerful the engine, the more heat generated from the engine block, exhaust, and transmission and in cars with forced induction, the turbo or supercharger. Without a proper cooling system, the engine, and all metal will melt within minutes. Temperatures exceed 2000 degrees in the combustion chamber and close to 700 exhaust manifolds and turbines.

For optimal working temperatures, coolant is circulated through hot engine parts, cooled through the radiator (with the help of fans and oncoming air), and checked by the thermostat (and temperature sensors) before completing another cycle. It’s pushed through the system by a water pump, while hot, expanding, or excess coolant is stored in expansion or overflow tanks. These parts maintain optimal fluid levels and prevent coolant leaking or gassing into the atmosphere, reducing the engine’s capability of lowering heat.

Overflow Tank Basics

Since cooling systems are pressurised and sealed, the coolant expands as it gets hot, meaning excess coolant has nowhere to go but into an overflow or expansion tank. The two parts have the same basic function of collecting expanding hot fluid, and then releasing it back into the system as temperatures subside. The aim is to maintain consistent pressure without loss of cooling liquid. They differ slightly in how they work and where in the engine bay they’re located.

How They Work

There are slight design differences between cars with overflow tanks and those that use a coolant expansion tank. Overflow or recovery tanks are simpler in that they don’t have their pressure cap (this sits on the radiator). As coolant gets hot and expands, a valve in the radiator cap opens and allows coolant to flow through hoses into the overflow tank, where it is stored until temperatures in the cooling system drop. The same valve reopens under the vacuum created as the coolant is cooled and allows it to circulate and absorb heat once more.

Expansion tanks have more complex designs, with integrated pressure caps. Here coolant is constantly being cycled through the system, moving from the radiator and tank. As it heats up and expands, hot coolant leaves the tank and fills the top half of the radiator through a separate by-pass or heater hose. This also means expansion tanks are located higher, to simplify cycling. The design is also more effective and features in most new and performance vehicles. If you’re not sure which type is in your car, pop up the hood and see if the tank has a pressure cap or if this is on the radiator.

When They Need Replacing

Engine overheating can be caused by one or more multiple faulty or damaged parts, leaking coolant, and issues with the sensors and electronics sending incorrect data to the ECU. Signs that your overflow or expansion tank may be at fault are:

• Overheating and poor performance – engines that warm up unusually fast, and pass recommended temperatures within minutes of turning the key are one short step away from complete engine failure. Often cracks, fissures and corroded (metal) tanks cause loss of coolant and leaks, killing cooling efficiency.
• Coolant smells or stains -leaking coolant is easy to notice. Either by the sweet smells or visible coloured leaks in the engine bay and under the car.
• Low coolant – while coolant levels change with different engine temperatures and loads, leaks and low coolant levels are signs of problems in the overflow tanks or the cooling systems as a whole.

To avoid any surprises (especially on longer trips), periodically check whether coolant levels are sufficient (there are MIN and MAX marks). Topping up is as simple as restoring the needed amount. Subpar parts may fail due to general wear and tear, from damage sustained in collisions, or during incorrect installation. Failed radiator caps are another reason why a radiator overflow tank fails prematurely, meaning they’re unable to maintain pressure and flow within the system. Additionally, besides coolant levels, check for any colour changes (meaning debris buildup) and the condition of hoses and thermostats.

Going With Aftermarket Options

If you suspect damaged tanks, OE or direct replacement parts offer much of the same performance and durability. Most tanks are transparent plastics that let you check coolant levels but constant temperature and pressure changes do take their toll. For better durability and parts that are fit for any engine rebuild, aluminium tanks last longer, endure lower wear and are corrosion-resistant. And they don’t cost much more than regular replacements. Just ensure you get what’s compatible with your engine and car, as designs, sizes and how much coolant each tank can take differs.

Summing Up

Overflow and expansion tanks may not have a wow factor, but they serve a vital function in the cooling system. They store excess expanding and cycling coolant, keeping engines at optimal working temperatures. The parts are cheap and easy to replace, and you can go with better-built metal options or larger sizes if you’re doing a car rebuild or modifying vehicles for more power. Shop for OE or aftermarket parts at respected retailers and get a professional fit, with topped-up coolant.