Rep Power: 0

So, I've had this Civic for about a month, and one day on a two hour ride home, it began to overheat. I pulled over and noticed the reservoir and that little overflow tank thing was dry, so I filled them both with water. The problem then seemed resolve. Until I decided to drive back to school. I checked the water, filled it. Checked the oil, it was fine. No leaks, nada.

An hour into the drive, bam. Overheat. Pull over, refill, and so on. It does this about every 45 minutes of driving now, and I have to refill the water every time before I turn the car on.

I noticed that there's a cap missing. Not the metal thing for the reservoir, but rather the overflow tank. Now that I think about it, that cap's been missing since the first time it overheated. I thought about purchasing a new one, but I'm not sure if that'd resolve the problem. So, I've come to you lot to see if that'd be the problem.

With my logic, the reservoir would pull coolant or water from the overflow using pressure. Absent a cap, there would be a lack of pressure, so the water/coolant wouldn't be able to get to where it's supposed to go. Is my thinking correct, or?

ANY help would be appreciated.

 
Rep Power: 519

Cooling system leak and/or head gasket leakage
Loss of coolant leads to overheating, overheating leads to severe engine damage including head gasket and head problems, and more.