cooling problems

I have noticed this problem for about a week now. I have a 97 neon and I just replaced my exhaust piping, (2.5") muffler, and headers and I have noticed when i'm driving at a constant speed like on the highway it doesn't run hot. But the moment I get off and i’m sitting at a red light the temp gauge sky rockets to the top. I'm wondering on what to do. This is what I did last weekend changed the thermostat and flushed out my radiator. And it’s still running hot. I’m thinking that it’s because I left my stock cat on and I didn't replaced it with a high flow one or like a resonator or something of that sort. Anybody else have any ideas on why it’s running so hot...Thanks

it could be a plugged cat but id think itd get hotter when youre flooring it.. check to make sure you have enough coolant in your engine..you might have a air bubble..

I cant imagine it would be the catylytic converter. Even when they are partly clogged, the car will idle okay. And at an idle the engine isnt generating much heat or much exhaust gas flow.
My first question is whether both fans come on when the gauge goes upscale? (Need to have the a.c. turned 'off' to check this). Once the car is warmed up, you should be able to let it idle, and after a few minutes the temp gauge will creep up past the midpoint of the gauge, maybe to the upper 1/3rd - and the fans should come on, run for less than 5 minutes, and shut off. As you are in Hawaii, you would need to park the car so the prevailing breeze wasnt blowing into the radiator - or the fans might never come on, and the temp gauge would/should never get above the 2/3rds upscale point. There is an electronic goody on the drivers side of the engine comartment, about even with the radiator - and hidden under the battery tray. This is the item that the PCM feeds a signal to, to make the fans run at the different speeds.
Is there coolant showing in the overflow bottle before the car is first started in the morning? I think it should show about 1/3rd of the way up the plastic bottle, and about halfway up when the car is warmed up. And of course it should not use any coolant at all from one week to the next.
How fast does the gauge get to the upper end of the scale? Does it happen in 5 minutes, or a minute, or in 30 seconds?

Check the electric fan/s and make sure they are running as she heat up (fan relay is bolted to drivers side inner fender well). If you have AC, turn on AC and see if the fans run. if no fans then the relay or fan motor is probably bad.

I don't have a/c so i only have the one fan. And yes i can hear it come on and then go off like normal. I also have plenty of engine coolent in the resvior and in the engine. Im going to recheck the fuses tommarrow but when i change out the thermostat and did a flush and fill they all check out o.k. Thanks guys

sounds like an air pocket did you burp the cooling system

I had the same problem not too long ago, and was diagnosed as a bad radiator relay. One way to check is to listen for the radiator fans to turn on right after idle. If they don't turn on, or are barely running, that might be the problem.