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I noticed that when the cooling fan comes on, coolant is sucked out of the overflow tank, and back into the radiator. Is this normal? If not, how can I correct it?
 

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assuming you don't have a degas bottle you would consider that the coolant inside the radiator is under pressure and your coolant overflow is atmospheric pressure with engine at operating temperature.simply speaking if your engine is running hotter than it's supposed to the pressure inside your cooling system increases.the reason being is coolant is made so that the boiling temp is higher,not only that but whenever you put any liquid under pressure it instantly raises the boiling temp.pressure is regulated by the radiator cap.any time the coolant pressure inside the engine exceeds the limits of the cap rating it is forsed passed the cap and into the overflow tank-thus the name overflow tank
now it could be as simple as replaceing the rad. cap (don't use a cap that exceeds oem specs.)
could be old coolant or not the right water to coolant ratio which will cause the same thing that a tiny leak will cause and that is a lowering of the boiling point(which causes steam which causes high rad. pressures
what i suggest you do first off is pressure test your system and also your rad cap(or just replace it-they're cheap).if you havn't changed your thrmostat in a while the that would be concidered part of a cooling sys. service then flush your cooling system and add a 50-50 water coolant mix.
after thats done ,bring the engine to operating temp then take a pyrometer and ck to be sure
the temps are within specs. and ck around the engine to be sure that there aren't any hot spots where there may be a lack of cooling. so try these things first if your having any problems or if you havn't done any service on your system in a while.
well i kind of explained why your coolant level will rise in your over flow but the real answer to your question(i'm sorry i took so long to get to it) is when your radiator fan comes on it cools the liquid inside the rad. so pressure decreases and a vacuum is created inside the rad. and the coolant is basically sucked back in.
 
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