So, A coolant recovery tank is a reservoir designed to collect hot coolant from the radiator as the coolant expands.
But, As the engine cools down the coolant in the radiator contracts, creating a vacuum in the radiator.
As a result, This vacuum draws coolant back into the radiator so it’s full at all times, preventing air locks.
Another function of the coolant recovery tank is to remove air bubbles from the cooling system.
Coolant without air bubbles is much more efficient than coolant with air bubbles, because it absorbs heat much faster.
With no recovery tank, air is sucked into the radiator instead of coolant which results in the formation of rust.
When rust forms inside the cooling system, you will have to flush out the system and replace the coolant.
One fairly common condition is an engine that’s overheating—but the coolant tank contains a normal level of coolant.
Although the tank may be full, the radiator itself is empty or very low because the radiator cap failed. The cap isn’t allowing the system to draw coolant back into the engine during cool down.
Before The Recovery Tank
Traditional radiators use a radiator cap that holds pressure until it reaches its preset pressure threshold.
Once cooling system pressure exceeds the cap rating, the cap releases coolant until pressure falls.
In older vehicles without a recovery tank, the excess coolant was dumped onto the ground.
So, The next time the engine starts with cold coolant, the coolant level in the radiator is low.
The remaining coolant would expand to fill the entire radiator once the engine warmed again.
Basic Recovery Tank (Overflow Tank)
To keep the radiator full at all times, car makers developed a new radiator cap and installed a tank.
The radiator cap operates just like the older versions to open and bleed off excess coolant.
But instead of expelling the coolant onto the ground, the coolant drained into a recovery bottle/tank.
So far, the cap operates just like the previous caps. However, as the engine cools and the coolant contracts, it creates a vacuum inside the radiator.
A vent mechanism in the cap allows the vacuum to draw coolant back into the radiator from the recovery tank. The recovery tank in these vehicles are simply storage containers. They are not under pressure at any time.
Pressurized Recovery Tank (Expansion Tank)
As car makers moved to transverse mounted engines and swept hood designs they faced an under hood space problem.
One way to gain more space was to move the radiator forward. Unfortunately, that limited access to the radiator cap.
So car makers decided to move the radiator cap from the radiator to the recovery tank.
In this design, the radiator, upper radiator hose, drain hose and recovery tank are all under pressure. The recovery tank is a much sturdier design and can handle more than 15-psi.
Coolant Recovery Tank Leaks
Both the non-pressurized and pressurized tanks can develop leaks along the seams.
The leaks are most noticeable when the coolant is hot.
So you may face a condition where you refill the coolant reservoir, drive the vehicle.
Then, when the vehicle cools down it’s low on coolant with no indication of a leak at the tank.
So, The use of a coolant recovery tank prevents a slightly overfilled radiator system from venting onto the ground. Also, The use of an expansion tank, with a conventional radiator, adds additional coolant to the system.