If it weren’t for the car’s cooling system, the engine will be overheating just minutes after turning the ignition key. Engines need heat to work, but too much will cause damage to the point that metal parts start melding together and complete engine failure soon follows. Keeping parts cool is assigned to an elaborate network of components that extract the excess heat from the engine block.
The aim is to maintain optimal working temperatures. For this to happen, coolant is circulated in a labyrinth of passageways in the engine block, collecting heat as it passes. It then exits through a system of hoses and is redirected toward the radiator, where it is cooled. Helping out are parts like water pumps to push the coolant on its way, thermostats to control the movement of coolant when required, and additional parts like coolant temperature sensors that gauge how hot things are and inform the car’s ECU. In modern vehicles, this part has a large say in how everything works.
One cooling component that is often overlooked and misunderstood is the radiator overflow container. This is where hot, expanding coolant is collected to keep the system at the needed pressure and prevent it from leaking.
What is a Radiator Overflow Tank?
Overflow and expansion tanks are containers that collect coolant that is heated and starts to expand. Since the whole system is sealed, the tanks provide the necessary space for storing the expanding coolant without the excess pressure damaging connected parts such as the radiator or the hoses that carry it. The tanks also ensure that the quantity of coolant is constant, and prevents it from leaking or evaporating.
There are two designs that work in slightly different ways. An overflow tank is simpler and doesn’t have its own cap through which the built-up air can be vented. Instead, this is found on the radiator. As the coolant expands a valve opens to let the expanding quantity flow through hoses and into the overflow tank. Once the pressure drops, the radiator cap opens again and draws the required coolant back into the system. You’ll recognise that your car has a radiator overflow container if it sits in line or lower than the radiator itself.
Systems revolving around expansion tanks are pressurised, so coolant is constantly being cycled between the radiator and tank. Expansion tanks are thus named because they let the coolant expand out of the radiator. They have a pressurised cap that performs the same role as that found on the radiator. The tanks can be easily located as they are higher up than the radiator, and have the advantage as this is where you can easily refill any lost coolant.
The differences between an overflow and an expansion tank are small, but expansion tanks offer a more efficient way of managing heat. As such they’re seen in racing cars, and are the most common type in newer vehicles.
Signs of a Faulty Radiator Overflow Tank
There are obvious signs that the cooling system is at fault. Overheating can cause dozens of related issues, but the main cause is that coolant isn’t doing its job. Often this is down to a damaged or faulty tank. The most common signs when this happens are:
- Visible cracks and damage – Physical damage, general wear and tear and discolouring due to heat, pressure and chemicals in the coolant will make the outer housing brittle and cause cracks and possible coolant leaks. This can also be because of the impact sustained during an accident or excessive vibrations on rougher roads. In addition, engines that have overheated more than once have been known to produce large enough amounts of pressure to rupture tanks.
- Low coolant levels – Any damage to the tank, or misconnected hoses can be the reason that some of the coolant evaporates over time. Coolant that has escaped the system will also leave stains under the car, and can additionally corrode nearby engine parts, causing further problems. If you notice unusual sweet smells while driving, or with the bonnet up, there’s a high possibility that this is coolant. Periodically check that it is topped up.
- Overheating – This is the last thing you’d want to see. While overheating can be down to any cooling system parts that have failed (often the water pump), coolant that has seeped through cracks and fissures in the overflow tank can be a possible cause.
What to Look for in Replacement Tanks
Overflow tanks are simple in design and function, and are inexpensive to replace. Ensure that the tank is compatible with your car by referring to the part number and VIN. Most tanks are made of translucent plastic, as this is tough enough to handle the changes in temperature and pressure, and also allows car owners to check how much coolant is inside.
If you’re modifying your vehicle for more power, other materials make more sense. Aluminium overflow and expansion tanks are not only built to a higher standard, but they also perform better and look nicer, especially in rebuilt cars. There are however harder to find and considerably more expensive.