1997 Honda Civic Temp raising when idling.

You may need to replace the hose if the clamp doesn't work.. There isn't much vacuum/pressure on that hose so if it's leaking usually its ripped.

If your vehicle is overheating only while it's idling, first thing I'd check is to see if the cooling fan is coming on. If that's in check, feel both rad hoses, they should be around the same temperature if the thermostat is fully opened.

Well, it happened again. What seems to be happening is that when it's running for a while, especially at highway speeds, the overflow tank just keeps filling up. Then, when I slow down, because there's not much air intake, the temp starts to rise within a couple of minutes. This last time, I let it cool down a bit (only for 30 mins or so) and opened the rad cap. There was no pressure. I took another look at the cap, it seems like the seal ring is compromised. I've just bought a new cap, and hopefully this will finally resolve the problem. I will take a look at the cooling fan, though. Would that cause the coolant to overflow, though?

Usually excessive/unwanted pressure in the cooling system will cause the overflow to overfill or bubble. Usually this is a headgasket issue unforunately.

Get a leakdown test done on your cylinders, this may be your issue.

Do you have to keep on refilling your coolant?

Only because the overflow tank itself overflows. As far as I can tell, there is no leakage into the oil or anything like that.

I'll get a mechanic to take a look tomorrow. Thanks!

Unfortunately like I said, if the coolant in the overflow tank bubbles or overflows it is usually unwanted combustion pressure entering the system. The oil and the coolant don't always mix, that depends on where the leak on the gasket is.

Let us know what happens Mythorn. Goodluck.

Yeah, it turned out to be the head gasket.
*sigh*

Ah well.

That is unfortunate.

If you opt to repair the vehicle you should send the cylinder head to a machine shop to get re-surfaced and checked for cracks or anything out of the ordinary.

Goodluck.