Occasionally when I have the AC on and I am stopped at a light, the air will become warm and my engine temperature will go up significantly (to about 3/4 of the guage). Once I start moving again everything returns to normal. I live in a pretty hot and humid climate in the summer. My 2000 Accord has ~145k miles on it has has been well maintained.
The first thing that I would check is to be sure that the electric cooling fans are operating as they should. When the car is stationary, the only way that cooling air is drawn through the radiator and the A/C condenser is if the fans are operating as they should. And, if those fans are not operating properly, both the engine temperature and the temperature of the A/C air will increase.
Also, we need to know more about maintenance than “it has been well maintained”.
More often than not when we press people for details on maintenance, it turns out that “well” is actually more like “not-so-well”, but I will withhold judgment until you give us at least the recent maintenance history of the car, particularly when the coolant was last flushed and replaced.
Can you give us some details?
Check the radiator level, when the engine is cool. Sometimes, leaks happen.
Do you hear the radiator fans turn on? If the fans are on, and it starts to get hot, raise the engine speed. This will get more water flowing through the cooling system, to increase cooling.
At least when I park the car after a long drive when the AC was on the fan(s) appear to be running properly since they remain on for a while.
I am not exactly sure when the radiator was last flushed since I do not have the maintenance records with me (all of which I have kept), but I have stuck with all of the factory recommended maintenance schedules.
I agree with VDCdriver. Start by checking the radiator fans. Start the car and let it idle. As the engine warms to operating temperature one fan, and perhaps both cooling fans, should cycle on and off to cool the radiator. If it doesn’t there’s a problem.
When you switch on the AC, the radiator fans should also come on.
As you’re watching this, keep your hands and everything else away from the fans, because they can come on at any time.
First check the front of the condensor for debris such as bugs and leaves, and remove any that are found.
When the AC is operating, heat is emitted from the condensor thru the radiator. This extra heat thru the radiator reduces the radiator’s ability to remove engine coolant heat. When driving at speed, the ram air effect thru the condensor and radiator removes this heat more efficiently. When stopping, the electric radiator fan is supposed to remove this heat. However, that little fan doesn’t flow nowhere near as much air thru the condensor/radiator when idling as when moving at speed where the ram air effect is taking place. Less air passing thru the condensor/radiator at idle equals less heat removed, which results in an increase of engine coolant temperature.