What would cause a vacuum in the cooling system? After my car has cooled the top radiator hose is collapsed. When I open the radiator cap I can hear air rush in. Isn’t the cooling system supposed to draw coolant from the reservoir as the car cools off and the volume of the coolant shrinks due to the cooling? There is coolant in the reservoir. I’m certain I’m going to have to replace that top hose. That hose now looks deformed from being collapsed for an extended period of time.

 

:oh:
Dude...theres SUPPOSED to be a vacuum in the cooling system.
Youve got a set amount of liquid that has been heated up. As it cools back down the volume that it occupies decreases and since the system is a closed system (so long as the reservoir is full) a vacuum is created. Havent you ever sat by and watched while the car cools down and draws fluid from the coolant reservoir?

Anyways there is nothing wrong with your car other than the need for a new upper radiator hose. Radiator hoses should be firm enough not to deform under pressure or vacuum.

 

I always thought it was supposed to be the same pressure as the atmosphere when it was cool. If you have the cap open the pressure inside and out would be the same. If you close the cap and start the car the heating would cause the coolant to expand and the pressure would force the extra coolant out into the reservoir. Once the engine cooled back down it would draw the fluid that was expelled into the reservoir back into the cooling system. So shouldn’t the pressures be the same? It almost seems to me like there is something blocking it from sucking fluid back into the cooling system after the engine cools. I’m thinking maybe there is some foreign debris in the reservoir that could be covering the end of the tube and acting as a one way valve blocking the cooling system from drawing fluid from the reservoir. I’m going to replace that hose in the next few days and take a look in the reservoir too.

 

Eventually as the car cools and coolant is sucked back into the radiator the total vacuum will decrease and the pressure inside the radiator will indeed be equal to that of atmospheric pressure.
But back to your problem; so youre telling me that loooong after the car has been sitting it still retains vacuum? If thats the case there definately could be some type of foriegn object floating around the resevoir that is expelled during engine operation but sucked back in during cool-down. Pretty simple check. Anyways the upper hose still requires attention since it shouldnt be deforming like that.