Rep Power: 0

I have a 97 Civic LX. about 250K miles on it. Had a new tranny put in 2 years ago, new Cat converter actually had put in today because i am due for emissions test next week. I noticed that the engine has started to overheat at an idol. I checked the coolant fluid and it is at the MAX mark. I have caught that reservoir boiling before. Any ideas what could potentially be causing this? There is right amount of fluid in the radiator as well.

 
Rep Power: 169

It could be your fan relay switch. Is the car only overheating during idle? If so, this is most likely the cause. Now there are 2 things that could be causing this. One is the ECT sensor located on the top left of your thermostat housing. This runs to a fuse inside the box that tells the radiator fan to kick on. Either one of these parts failing or becoming faulty would cause your engine to overheat at idle. If your check engine light is not on then it to most likely not the fuse.

Also check to make sure that both your intake (upper) and outflow (lower) radiator hoses are hot when your engine is hot, to ensure that the thermostat is working properly.

Also, maybe drain a little coolant out into a pan and see what it looks like. DONT DO THIS WHEN ENGINE IS HOT. If its brownish or rusty looking at all, flush your radiator and replace coolant. If not just dump it back in radiator.