Why does engine overheat while sitting at idle ?

When running the Air conditioner, the engine on my 2005 Envoy tends to overheat while running at idle, like at a stop light.

But once I begin to drive on down the road the engine heat reduces and comes back to a normal setting.

Why?

The variable mechanical/electric fan clutch, and added heat from the AC condenser can contribute to the overheating.

It is also important to have a fan shroud that is intact, as well as a clean condenser and radiator. Debris in the front and between the condenser and radiator can impede cooling.

Check the fuse and relay for the fan clutch first.

Agree with Blue, make sure that clutch fan is running.....

Could a radiator flush be done first, as perhaps the coolant is old?

Could old coolant be an issue here?

If the system cools normally with air flowing through the radiator (moving along the road) the problem is not coolant.

That doesn't mean that coolant shouldn't be replaced on schedule, but that's more to prevent corrosion as much as it is for actual cooling. If the coolant were inadequate or not flowing properly, it would overheat FASTER while driving along the road (more load, more heat generated) and cool off at idle.

If you want to test this theory, place a large poultry or industrial fan in front of the vehicle and allow it to idle with the hood open and fan running full. It will probably never overheat. However, it will be more effective to simply diagnose the fan clutch to see if it is operating properly - or at all.