I have an 05 Neon that I bought used in November. I noticed that it started overheating in January. The antifreeze level was full and there were no leaks, and after having a mechanic friend look at it and test the temperature, we determined it was a bad thermostat. I went to Autozone, bought a thermostat, and had our mechanic friend replace it. It ran completely fine after that, and wasn't overheating anymore, so we thought we fixed the problem. Then in March, around the time when the weather warmed up to about 70°, my car started to overheat again. I live in Denver, CO, and I only drive short distances (10-15 min at a time). The temp gauge would start to rise in stop-and-go traffic, when I would stop at a red light and idle. Then it would decrease as I started to drive. A few different people looked at it and none could determine a problem, each saying everything looked and sounded fine. So we had our friend come back over and look at it, and again, the level of antifreeze was full at the mark, and there was no leaks. He took the radiator cap off and let the car run so that antifreeze sputtered out of the cap opening for a few min, and then ended up adding a bit more antifreeze into it, and that seemed to help, as it drove normal for a couple weeks after that. But at the same time, the weather got cold again, so i don't know if that had anything to do with it. Now it is warming up again and my car is overheating in the same way, while idling at stop lights, or if I drive for longer distances (more than 15 min). People keep telling me if it's got enough antifreeze, and doesn't have any leaks, then it has to be my thermostat, but I just replaced it less than a month and a half ago. Could it already be busted this soon? Does anyone have any ideas for what other reasons could be causing it to overheat? Or why it only seems to overheat on warmer days?

Any feedback would be much appreciated,
Thanks.

 

It might need a coolant flush, because a hose or pipe might have gotten clogged or contaminants might be reducing the flow. The 2 other things I would think to check is your water pump, and your radiator fans.
The water pump is ran off the timing belt.
If your radiator fans are not working properly, it would explain why you overheat in stop and go traffic, because there isn't as much airflow going through the radiator.

 

2000 Neon here. Tons of problems with over heating. Replaced thermostat. Hose ruptured so replaced that. Then I was told I needed to replace the radiator so I did. All solutions worked for a while. Thought radiator replacement would do the trick. Over heated again. This time I noticed that the radiator cap was loose when I went to check it. Car towed to mechanic. He replaced the radiator cap. Solved the problem. Probably my problem all along.