A coolant recovery tank, is a tank designed to collect hot coolant from the radiator, as the coolant expands.
However, 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.
Above all, coolant without air bubbles, is much more efficient, than coolant with air bubbles. Consequently, it absorbs heat much faster. So, with no recovery tank, air is sucked into the radiator instead of coolant. As a result, causing air locks and 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 sign, is an engine that’s overheating.
Another common cause can be, the radiator cap.
Although the tank may be full, the radiator itself is empty or very low. Most often because, the radiator cap failed. So, the radiator cap isn’t allowing the system, to draw coolant back into the engine, during cool down.
Before, The Coolant Recovery Tank
Traditional radiators, use a radiator cap that holds pressure, until it reaches its, preset pressure threshold. Once the cooling system pressure, exceeds the cap rating, the cap releases coolant, until the pressure falls. In older vehicles, without a coolant 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 up again.
Basic Recovery Tank (Overflow Tank)
So, 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 tank. So, 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. So, 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 stronger 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 tank and drive the vehicle. Then, when the vehicle cools down it’s low on coolant, with no sign 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.
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