Once the driver parks the vehicle and turns off the engine, the heat dissipates which causes the coolant to not be as hot anymore. Coolant in the radiator is 100% full. Most car's have a radiator drain valve that will be visible at the bottom tank or on either side tank. The excess fluid flows into the overflow tube and goes into the overflow tank. A new hose is cheap. the small hose that goes into the overflow tank (reservoir you're talking about) runs to the top of the radiator right below the radiator cap. I replaced the radiator and thermostat on my 98 Subaru Forester but still when I drive it the temperature gauge reads over the hot line the heater doesn't work but the coolant doesn't seem to be circulating to where its supposed to be. If you have a leak somewhere it will draw in air instead as the coolant cools. With a couple of days driving the level returned to normal. Coolant Reservoir doesn't drain back into Radiator. Check your owner's manual for proper cap replacement. There may be times that the reservoir radiator cap may turn out to be faulty. flush but it has never worked when they get that bad take the rad. The coolant expands and contracts as it is heated and cooled. So you'll want to mark where the coolant level is when hot, and then come back when the car cools down and see if it's dropped. - Answered by a verified Subaru Mechanic If there are any leaks in the system, air will be drawn into the system via those leaks instead of drawing coolant from the reservoir. When you turn off your engine, and the cooling system starts to cool down, then there is a vacuum drawn from the radiator and the coolant in the reservoir is sucked/drawn back up the line into the radiator. The excess coolant goes into the reservoir. Open the drain valve by turning the it counter clockwise. Your problem may simply be a loose or damaged cap not keeping coolant where it needs to be. I noticed though that the overflow tank would fill but not siphon coolant back into the rad. Fill your reservoir with antifreeze up to the FULL line at the middle of the plastic tank, you don’t have to add water, because the water will evaporate in a few days and what is left is antifreeze which will not evaporate. Therefore i am constantly topping up the radiator when it has cooled down. to a rad. he put a chemical in the radiator fluid that is supposed to form a plug to stop a leak. Mix the coolant with distilled water so that you have a combination of 50% coolant and 50% water. Look at the level in your radiator in the morning for a few days without adding any coolant to the reservoir and if it does not go any lower this might be your problem. After ~5 months of waiting, still no plug…no change. The radiator cap comes in different pressures. So, the radiator coolant overflow tank functions to serve this exact purpose. Before coming home i topped up the rad again, and had overheat issues again after about 2.5 hours of highway driving. This allows for excess pressure to vent to the reservoir as well as any coolant. However, upon cooling, it still put a bunch into the overflow, leaving my level way below the cap. Coolant goes into reservoir but doesn't get drawn back into the radiator when it cools. This leakage is visible and may result from bad reservoir cap. I also "burped" a few of the hoses as I was refilling. This expelled coolant moves against the spring in your radiator cap. Seems that its not holding pressure or there is AIR in the system. In normal circumstances, the coolant gets ousted out from the reservoir overflow tube, the moment the engine starts warming up. When you start the engine the water begins to warm. While the engine is off, your coolant reservoir should be about 30% full. The video above shows you where the coolant reservoir in your Contour is located and how to add coolant. If you look at just about any overflow reservoir (not an expansion tank), the hose from the radiator run off goes to the bottom of the tank (or dips down to the bottom of the tank from the lid). As the system cools down, the contracting coolant creates a vacuum that draws coolant from the reservoir back into the radiator. Hey Victor, A radiator overflow tank collects the expanding coolant that is heated by the engine and recycles it back into the coolant system once it loses enough heat. This excess coolant stays here until the system cools down enough to create negative pressure and draw the excess coolant from the reservoir back into circulation. Overflow tank had filled and coolant had shot all over the front passenger wheel well in particular.. Once you are done driving, the coolant will slowly cool down as the engine does and the radiator cap will allow the coolant stored in the reservoir to flow back into the radiator. 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