What is the purpose of the radiator overflow bottle?

when the water in the radiator gets hot it pushes it into the surge tank,& as the engine cools off is sucks it back into the radiator! yes on many vehicles the overflow jug is the only way to add water!

When the coolant is hot, it expands. In stead of being vented overboard it goes into the overflow bottle. Since the radiator is air tight, as the coolant cools, it contracts and sucks the coolant from the overflow bottle back into the radiator.

Yes you can fill a radiator via the overflow bottle. On some cars you do not have a choice in that they do not have radiator caps. If the radiator is cool, slowly release the pressure via slowly loosing the radiator cap, or in some cases there is a pressure release lever on top of the radiator cap. Then fill the radiator directly. If you added too much, it will just be purged into the overflow bottle. The is a very good system, do not disable it.

Yes, hot expands, in a close system the hot water expands in to the over flow bottle, when the radiator cools down and the water contracts the water siphons back in to the radiator. Also keep the overflow with water and you will not have to worry about the radiator, but if you break the system, by open the radiator cap, to check the water level you may refill the radiator and O.F. Bottle. I have driven some big truck, and that was how you filled the radiator with a reserve tank. Any time there is a problem I check and fill the radiator. it is to easy to do, against loosing a engine or being broke down along side a road.

Radiator Overflow Tank

Yes. The coolant recovery system consists of this overflow bottle. This allows the coolant that normally escapes from the radiator, due to expansion from the heat of operating temperatures, to be retained. When the engine cools the excess is drawn back into the radiator due to a vacuum effect from the cooling thus maintaining the system at full capacity,( providing there are no leaks in the system). This is a continuous process and provided the level in the reservoir is maintained you shouldn't have to add coolant to the radiator. Never introduce liquids to a hot system, it can cause a need for major repairs.

Engine coolant expands as it is heated. Without a reserve reservoir, it would spill over and be lost. Then when the engine cools back down, there would be too little coolant to do the job, and it would result in the engine overheating and being permanently damaged.

This all works through valves. The thermostat remains closed until the engine heats up. At about 150° the thermostat opens and allows the hot liquid to enter the radiator, where it is cooled by the fans or ram air. It then flows back into the engine.

One must always add coolant or coolant mixture to the overflow bottle, never to the radiator.

#1 is fact that the cooling system in modern cars is pressureized! This is because water under pressure boils at a higher temperarture,- with 50/50 antifreeze-water mix it is a little over 235 degrees (as opposed at 212) Your system has a pressure cap on it designed to release pressure at a certian pre-determined pressure (generally around 12-18 psi)... Water expands when it gets hot, -- so some runs back to reserve bottle where it is captured and saved! The entry from radiator is near the bottom of the bottle -- when the engine cools off, the water "shrinks in volume", - so water is pulled back from the bottle to replentish level inside radiator! This is designed to assure that the radiator remains "topped off" all the time!

But you are supposed to check water in bottle occsionally and be sure it is full to a certian level marked somewhere on container! BUT if bottle has been "run dry" then you have no idea if radiator is ful or not, -- you may have overheating problem (that will tell you water is too low though)! Do not remove radiator cap when engine is hot (and system is presurized), - as it will spew out very hot water and possibly scald you if not done properly! It is safe to remove cap on cold engine, and look inside radiator and fill as needed, - but if only slighly low, it will do ok by just topping off,- and driving. It will "take care of itself" next time it cools off! The main thing here is that you do not want it low enough to make engine run too hot. So keeping bottle full will automatically take care of water level in engine, - if pressure cap is ok, and no leaks in system!

when the engine gets hot and the anti freeze expands and has to much antifreeze the radiator will over flow into the over flow bottle through the radiator cap which has a pressure that it opens at to allow the antifreeze to flow into the over flow,and this system also works in reverse.if radiator has too little antifreeze the radiator cap will open to allow the antifreeze to go back into the radiator from the overflow through the cap..most radiator caps have a pressure of 15 pounds.also if you have a small leak in the system could put a smaller pressure cap on so the radiator doesn't build up so much pressure that it leaks out antifreeze..

when the engine get to a certain temperature, the anti-freeze builds up and the radiator cap releases and it flows over into the bottle, when it cools down, it sucks it back in, more or less back and forth, and when it gets so hot, the fan will kick on, until it cools down, then it stops. if the bottle gets low, just fill the bottle, it will suck it into the radiator.