I know it doesnt seal 100 percent but there is a hose that goes down there. The hose was inside an ice block. So the cap doesnt really matter in this situation. I asked the guy who had the car before me if he had put water. I got this car not too long ago and the guy before me said oh it was under the FULL line so he added WATER in the overflow tank. And I guess it never mixed well. There was no freezing inside the engine though.