If you feel it is raising to a point of concern and it has not done it before, there is a problem.
The fact that it lowers with the heater on is not uncommon. The heater core acts as an additional (small) radiator where coolant has extra time to cool.
On the other hand, the AC places an extra load on the engine that can cause it to run hotter.
Now, if the car only heats up when you are at a stop, make sure the cooling fans are running (when the gauge shows hot). If they are not, they need to. When the car is sitting, there is no air flow through the radiator to cool the coolant. As a result, the fans draw the air through it to keep it cool. When you drive, air flows. Therefore, if it is heating up while you are moving, I would suggest replacing the thermostat. It is a cheap part to start with and if it is sticking, it could be causing the problem as well.
If you want, we have a "how to" for checking the thermostat on our homepage under the DIY section. The hardest part is removing it. However, I would just replace it if I remove it. They are like $5.00.
Let me know if this answers your question and helps. If you have other questions, let me know.
Thursday, September 15th, 2011 AT 1:36 AM