The purpose of the radiator coolant overflow tank is to store excess coolant that is released from the radiator due to pressure build up from heat. It works similar to an expansion tank on a water heater.
As you may know, an engine that is working very hard will heat up very quickly. The oil alone is not enough to keep the components of the engine cooled off.
The engine relies on coolant to ensure that it does not overheat during these high performing situations. While this heat is being generated, the coolant liquid will absorb all that heat so that the engine stays cool.
How a Radiator Overflow Tank Works
As coolant gets hot due to it absorbing heat from the engine, the liquid expands and creates additional pressure in the radiator. As the pressure causes the coolant to get higher than the pressure cap, the excess coolant needs to be captured somewhere in order to prevent leakage.
So, the radiator coolant overflow tank functions to serve this exact purpose. The excess fluid flows into the overflow tube and goes into the overflow tank. Once the driver parks the vehicle and turns off the engine, the heat dissipates which causes the coolant to not be as hot anymore.
The coolant will then contract instead of expand; resulting in the volume of the coolant decreasing. This is kind of like a vacuum effect where the pressure decrease allows the excess coolant in the overflow tank to flow back out of it, so it can return to the radiator.
Top 4 Bad Coolant Reservoir Overflow Tank Symptoms
Here are four of the most common signs of a bad radiator overflow tank.
1) Coolant Leak
If the radiator coolant overflow tank is damaged or cracked, then you can expect coolant fluid to leak out of it. Sometimes cracks will form on the overflow tank if it is too old and worn out. You should notice small puddles or drops of coolant on your garage floor or in your driveway.
Note: Coolant is very harmful to dogs and cats. If you notice a coolant leak on the ground, get it cleaned up right away so you family pet doesn’t clean it up for you. There have been many cases of dogs and cats getting very sick or even dying because they ingested engine coolant.
3) Coolant Odor
In addition to the coolant leak, there will be an apparent coolant smell coming from the front of your vehicle. If it gets bad enough then it may even circulate throughout the cabin of your vehicle.
While not the worst smell in the world (it’s slightly sweet), it’s not something you want to smell for long periods. So if you smell this warning sign, check the condition of the overflow tank and replace asap.
4) Overheated Engine
Since the coolant’s job is to cool the engine when it is overworked, a leak may mean that your engine is not getting enough coolant circulating through the engine to adequately cool it down.
As a result, you can expect the temperature of your engine to rise and become overheated. This will be indicated on the engine temperature gauge located on the dashboard.
5) Low Level of Coolant
If you check your coolant levels in the radiator coolant overflow tank and they are low, then you either have slowly evaporating or leaking coolant. Usually, people who don’t realize they have a leak may discover that it is a small leak which is barely noticeable on the outside.
To confirm, you need to actually check the coolant level to make sure you have a leak. The overflow tank cap is often the cause of the leak. Make sure it is correctly closed and replace the cap if that’s where you suspect coolant is escaping from.
Radiator Overflow Tank Replacement Cost
Before spending the money on a new overflow tank, make sure the problem is not actually the overflow cap or tubing that comes out of the tank at the bottom as these are common problem areas. They are a lot cheaper to fix than replacing the tank itself.
Otherwise, the cost to replace a radiator coolant overflow tank is anywhere from $90 to $260. The parts cost will be between $50 and $150 while the labor costs will be anywhere from $40 to $110.
On many vehicles, replacing the overflow tank is an easy DIY job. On others which are harder to get to, you should let the professional handle the replacement job. There will also be additional fees and taxes added to this as well.