Radiator Overflow Tank

Radiator Overflow Tank: A Comprehensive Guide

An internal combustion engine runs off a series of controlled explosions, so obviously it runs hot. And it’s the job of the cooling system to make sure that the engine doesn’t exceed operating temperature. The cooling system itself is very complex, and one of the parts that may seem minor but are very important is the radiator overflow tank. We’ll be discussing everything about the radiator overflow tank in this post, and the cooling system in general itself.

How The Cooling System Works

Before we get into the radiator overflow tank, let’s understand how the cooling system works first just in case you didn’t know. The system works by circulating coolant fluid through the engine to absorb the heat away from it. So, how does the system work?

The system starts with the thermostat, which is a valve that regulates coolant flow. After you turn on the engine, if the engine is still cold, the thermostat will keep the bypass valve open. This bypass valve recirculates the coolant that was in the engine and feeds it straight back into the engine. The purpose is to get the engine to operating temperature as quickly as possible.

After the coolant reaches a higher temperature – usually around 160°F – the bypass valve will close, and the main valve opens. This allows the hot coolant to flow into the radiator, where it will cool down. The radiator has a series of fins – usually made from aluminum – and a fan that’s blowing through it to disperse heat from the coolant.

The water pump will then pump the cooled coolant from the radiator back into the engine to absorb heat away once again. There’s also a temperature sensor that sits somewhere near the thermostat which detects the coolant’s temperature. This sensor feeds it into your car and displays it on the temperature gauge.

What’s The Radiator Overflow Tank Is For Then?

So, what’s the overflow tank for then? As the name suggests, this tank is used to store overflowing coolant. It’s usually made from plastic, also sometimes called the reservoir or recovery tank, but how does it work?

Well, you may notice your radiator has a cap on top of it. Not only this helps to keep the coolant in place, but it also helps to regulate pressure inside the system since the cap has a valve. As the system warms up, the pressure inside the system will increase. When pressure and temperature increase, the coolant will expand, essentially increasing the amount of coolant in the system.

When the system reaches a certain amount of pressure, it will push the valve on the radiator cap. This opens a passageway for excess coolant to flow through a hose and into the overflow or reservoir tank. As a result, pressure remains regulated, the temperature will remain optimal, the cooling system runs smoothly, and your engine won’t overheat.

After you drive the car, the system will shut off and pressure will decrease. As the pressure decreases, it will create a vacuum in the system. This vacuum will then draw the coolant back into the radiator, ready to circulate it once again the next time you drive the car. Here’s a great video explaining how the car’s cooling system works:

Radiator Overflow Tank: Why Is It Important?

In the early days of water-cooled engines (engines used to be air-cooled) in the 1960s, the system didn’t have an overflow tank. It simply had a tube on the radiator filler neck, which vented to the atmosphere or the ground. As the pressure rise and the coolant expand, it will vent steam out to protect the cooling system. Often expelling coolant in the process.

While this works just fine, it’s not exactly ideal for the system. It also means your car will need frequent coolant top-up, as coolant is expelled out rather than being recirculated. Additionally, coolant isn’t exactly kind to the environment. And this is where the overflow tank comes in.

As mentioned, the radiator cap is what regulates pressure inside the system. The overflow tank is there to catch the excess coolant, and stores it for use again once the pressure inside the system decreases. Regulating pressure helps to keep the system in good shape, as well as maintaining optimum temperature for your engine to operate.

Radiator Tank: Should You Upgrade?

Most cars will have a plastic radiator overflow tank right out from the factory. And this works just fine; plastic is durable and can withstand the heat from the coolant. But if you often search for aftermarket car parts online, you may notice that there are overflow tanks made from aluminum and stainless steel. They’re typically aftermarket and not OEM, meaning it’s made by a third-party company and not by carmakers.

The question is, should you upgrade? Are they worth your money? To answer that question, we need to take a look at the advantages of aluminum or stainless steel over plastic overflow tanks:

1. Repairability

An aluminum or stainless steel overflow tank only has two advantages, the first one being repairability. With plastic overflow tanks, it’s unlikely you’ll be able to fix it once the tank develops a crack or a hole. You’ll have to replace them altogether because the chemical properties of polythene – the most common type of plastic – make bonding a problem.

With aluminum or stainless steel tank, you can weld to repair them if they have a crack or a hole. If you need to replace a plastic overflow tank, it’ll cost you somewhere around $160 – $200. Meanwhile, stainless steel welding is around $15 per pound and around $100 per hour for labor.

Keep in mind aluminum is quite expensive, and welding a broken aluminum overflow tank can be just as if not more expensive than replacing a plastic tank. Still, if you care about the environment – and you should – it does mean you won’t be throwing away plastic waste every time you need to replace the tank. A plastic overflow tank will typically last somewhere around 5 to 10 years unless it’s damaged by something else.

2. It Looks Cool

The second advantage is, well, it just looks cool. The plastic overflow tank just doesn’t look very nice; it’s just a flimsy-looking plastic tank with an even flimsier plastic cap. Additionally, plastic overflow tanks will get dark and cloudy-looking over time due to the heat. Once this happens, cleaning them to look like when it was brand new will be extremely difficult.

An aluminum or stainless steel overflow tank meanwhile just looks cooler. It doesn’t look flimsy and since they’re not transparent, they won’t look cloudy over the years. If it gets dirty, you can simply wipe them down and you’re good to go. One disadvantage is of course they don’t have maximum and minimum indicators like plastic tanks do, so you’ll have to guess whether or not your car has enough coolant.

Should You Upgrade?

An aluminum or stainless steel doesn’t actually offer many advantages other than the ones we’ve mentioned above. They won’t improve your car’s cooling system. To do that, you’ll need to make other upgrades to the system, such as to the thermostat, hoses, and possibly converting to an expansion tank system. We’ll explain all that in a bit, but let’s stick to the overflow tank for now.

Radiator Overflow Tank

So, unless you’re a car enthusiast that cares about how your engine bay looks when you open the hood, there’s no point in using an aluminum or stainless steel tank. If you want to open your engine hood confidently at a car meet, then yes, go right ahead and upgrade. But otherwise, the plastic one in your car will do the job just fine.

However, if for example, your current plastic overflow tank is broken and you need to replace it, then there’s no harm in replacing them with aluminum or stainless steel ones. The stainless steel ones especially aren’t all that expensive and they can be as cheap as $50. Just make sure that the tank you’re about to purchase will fit your car.

How To Replace Your Radiator Overflow Tank

If you’re planning to change your radiator overflow tank, we’ll give you a quick guide. Keep in mind that every car has its unique design, so your steps may differ a bit, and this a general instruction on how to do it:

  1. Make sure the engine is cold.
  2. You can either drain the cooling system first by opening the coolant drain plug or disconnecting the lower radiator hose (if your car doesn’t have a drain plug) or pinch the hose that connects to the overflow tank by using the hose clamp. Draining the system is recommended since you can do a radiator flush while you’re at it.
  3. Remove the nuts and bolts that keep the tank in place. The socket size you need will vary depending on your car’s make and model. Additionally, use WD-40 or penetrating fluids if you have difficulties removing them.
  4. Afterward, remove the lower overflow tank hose. If you didn’t drain the system, pinch it with the clamp and remove the hose.
  5. Connect the lower hose to the new overflow tank and tighten the clamp.
  6. Install the new tank and secure them with the nuts and bolts.
  7. Reconnect the upper hose to the new tank and tighten the clamp. Replace the clamp if it seems loose.
  8. Fill the coolant tank with coolant to the maximum indicator level.
  9. If you flushed the radiator before installing the new tank, refill the system from the radiator’s filler hole. Check with your owner’s manual to find the requirement and the type of coolant you should use.
  10. Turn on the engine and get it to operating temperature. See if the engine overheats, and see if there are any leaks. If not, job well done!

Here’s a video from 1A Auto to give you a visual guide:

Radiator Overflow Tank Vs Expansion Tank

Most cars these days use an overflow tank, and manufacturers will often refer to them as reservoir tanks. But you may have also heard of an expansion tank. So, what’s the difference between the two? An expansion tank is similar to an overflow tank, but the expansion tank is pressurized. Meanwhile, the overflow tank is not under pressure. If anything, they’re there to relieve pressure from the radiator and cooling system.

This is because, with an expansion tank, the pressurized cap is located on the tank rather than on the radiator itself. If the radiator in your car doesn’t have a cap, or the cap is not pressure-rated and doesn’t have a valve, then you have an expansion tank system.

In this system, the amount of fluid expands in the system because it doesn’t escape into a catch tank or a reservoir tank unless it has to, hence why it’s called an expansion tank. Whereas in an overflow tank system, the fluid escapes via the radiator cap. Additionally, the expansion tank has a lower hose that feeds coolant back into the radiator. Rather than letting the coolant just sit there until the pressure decreases.

What Are The Advantages?

The first advantage of this system is that you essentially get more coolant, around half a gallon more. As mentioned, the coolant expands when it’s hot and increases in amount. More coolant in circulation means it has more capacity for absorbing heat away from the engine.

Additionally, the system is more efficient in relieving the coolant pressure. This is because the expansion tank allows more room for the coolant to expand. This additional room is also great for pulling air out of the system since the hot air will sit on the top part of the tank. This separates the coolant from any air, while simultaneously pushing the coolant back into the radiator.

All this means that you can get better cooling performance in your car. Additionally, separating air from coolant can help to prevent rust in the cooling system, helping to maintain the system for a longer period. We recommend watching this video to learn more about expansion tanks:

Should You Convert To An Expansion Tank System?

If you haven’t made any performance upgrades to your car and you don’t take it for track driving, then you don’t need to. Most cars have a good cooling system that’s designed to work well with the engine. So if your engine has no significant performance modifications, you don’t need to go through the trouble of converting the system.

Even if let’s say your car has a flaw in the cooling system and it often overheats, you’re probably better off upgrading certain individual parts to fix these flaws. For example, better radiator hoses or a better thermostat may help to solve your problem.

However, if you have performance modifications in your car, you will need to upgrade the cooling system. Especially if you take it for a track event, as you will abuse the car to its limits. But an expansion tank may still not be necessary. You should consider an expansion tank conversion if you see coolant spewing out of the car but there are no leaks in the system. Otherwise, an overflow tank is perfectly fine.

Converting your car from an overflow tank to an expansion tank is likely to be a complicated process. This is quite a major modification job, and you’ll probably have to make significant alterations to the engine bay. You’ll have to go to a performance-tuning shop to be able to do this. Additionally, the process will vary greatly from one car to another. We recommend checking owner’s forums to see how other owners did the job, and whether or not it’s necessary in the first place.

How To Improve Your Car’s Cooling System

If you have significant performance upgrades to your car and you take it for track driving, you’ll benefit from upgrading the cooling system. Performance upgrades can often increase the operating temperature of the car, especially fitting an aftermarket or larger turbo to your car since turbochargers are very toasty.

If you don’t improve the cooling system, your car can overheat. This leads to damage to your engine, leaving you with an expensive engine rebuild bill and essentially making all your upgrades pointless. You should also upgrade your car’s cooling system if you’re experiencing overheating issues due to manufacturer design flaws.

Here are some of the parts that you can upgrade to improve cooling in your car:

1. Silicone Radiator Hoses

This upgrade won’t actually improve performance, but it will increase durability which is still necessary for a car that’s going to be abused. Radiator hoses in most cars are made out of rubber. And while they’re great, they’re more prone to cracking over the years.

Meanwhile, silicone hoses have greater flexibility. This means they’re less prone to cracking which reduces the possibility of coolant leaks from the radiator hoses. This also means they can last longer than rubber hoses, possibly even outlasting your car’s lifespan.

Radiator Overflow Tank

Additionally, silicone hoses have a higher operating temperature range. This means they can withstand more heat than rubber hoses, which contributes to their longevity. If you’ve installed a great big turbocharger on your car, you will benefit greatly from using silicone hoses. As mentioned, turbochargers run hot and can increase the engine’s operating temperature. Using silicone hoses means your cooling system can withstand the heat better.

Of course, they’re a bit more expensive. Rubber hoses are typically around $150 to replace, while a set of silicone hoses – such as from Mishimoto – start from around $230. Oh, and did we mentioned they look really cool? They come in various colors and not just boring old black.

2. Higher Pressure Rating Radiator Caps

As mentioned, the radiator cap has a valve that regulates pressure within the cooling system. Once it reaches a certain pressure, the valve will open, allowing coolant to flow into the overflow tank. However, using a high-performance radiator cap won’t give you any benefits unless in a very specific circumstance.

You may need a performance radiator cap when your car’s cooling system is running on water rather than a coolant mixture. This is because some race tracks may not allow glycol-based coolant to be used during track events due to environmental concerns, just in case of a spill happens at the track. Additionally, water will usually transfer heat better than the coolant, so you might consider using them, but they have a lower boiling point.

This is where performance radiator caps come in. Since they can withstand higher pressure, this helps to increase the boiling point of the water in your cooling system. This protects the cooling system and prevents the engine from overheating. So this is a very specific scenario, and you probably won’t need it. But now you know why you might need a high-pressure radiator cap. Summit Racing made a great article on how you may benefit from high-pressure radiator caps.

3. Thermostat

As a reminder, the thermostat is also a valve that regulates coolant flow during the engine’s operation. When the engine is cold, it will close the main valve and open a bypass valve instead. The coolant from the engine then directly recirculates back into the engine, helping the engine to get to optimal operating temperature. Once the engine and coolant warm up to around 160°F, the main valve will open which allows the hot coolant to travel to the radiator to be cooled down.

A performance thermostat will open the main valve at a lower temperature instead, usually about 25°F lower. Additionally, it ensures the valve to be fully open once the coolant gets to around 180°F. This helps the engine to remain cool for longer and helps to cool the engine during hard driving as the flow of coolant is not restricted.

Changing the thermostat itself won’t give you more power. But because this can allow the engine to run cooler, you can make other modifications to your car to help increase horsepower, such as higher compression and a richer fuel and air mixture.

Keep in mind that if you drive your car daily, this will make your car’s interior heater less warm. This is because the heater core uses hot coolant to warm your car’s cabin. Since the coolant is now at a lower temperature, the air coming from the heater core won’t be as warm. Additionally, excessive engine wear and poor fuel consumption may happen as well when the engine operates at a lower temperature.

4. Radiator Cooling Fans

If your car still runs hot, then the next thing you should consider is an auxiliary cooling fan. Chances are your car already has a radiator cooling fan that sits either in front or behind the radiator. But the standard cooling fan is usually just enough for normal daily driving, and not meant to cool an engine with performance upgrades.

A higher-performance cooling fan may be necessary in this case. The diameter, fan thickness, cubic feet per minute air rating, and fan blade direction of travel all can contribute to the performance of the cooling fan. Some cooling fans may also have manual switches, so you can turn them on and off on demand. Check the reviews and owner’s forums to see which cooling fan will work best for your car.

5. Performance Water Pump

A performance water pump will have stronger and larger impellers to move coolant more quickly. This helps with coolant circulation and helps to keep your engine cool. Additionally, they have stronger bearings to deal with high pump loads. And they’re often made from aluminum to maintain strength while saving weight.

You should also consider using an electric water pump if your car doesn’t already use one. A mechanical water pump is belt-driven, which robs horsepower from the engine. Using an electric water pump helps to remove the burden from your engine, allowing for more power to the drive wheels.

Radiator Overflow Tank: In Conclusion

The radiator overflow tank is a device used for storing excess coolant during the car’s operation. When the engine is on, the pressure inside the cooling system will increase and coolant will expand, essentially increasing the amount of coolant. The overflow tank stores this excess coolant for later use, preventing frequent top-ups and environmental damage.

You don’t need to upgrade your plastic overflow tank to an aluminum or stainless steel one, as it doesn’t improve performance. But if you want to for aesthetic purposes, then by all means you should go ahead. Hopefully, this post has been a helpful guide to overflow tanks and the car’s cooling system in general.

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