Why does my car temperature rise when I idle but when i drive it, it's goes back to normal?

So I started noticing this problem 2 days ago. I thought I fixed it by refilling the coolant. It was working fine till today again. When I stop on a cross street or I idle, the temperature slowly rises up to the "H" but once I accelerate or move the car, it slowly goes back to normal. My A.c works perfectly either way. What could be the problem? How much would it cost? It's a 2005 Honda civic ex.

Your ac isn't related at al XD

Your vehicle isn't cooling correctly. It if ever hits H, then you've got a problem. Remember, with all mechanical things, heat is the enemy. I wouldn't worry about the A/C. When you end up with a warped or cracked cylinder head, then you won't have an A/C anyways.

Check and replace the thermostat. It's a cheap fix and a bad thermostat can cause all sorts of issues.

Flush the radiator completely and refill with a proper coolant-water mix. 50/50 is usually fine, unless you're in a colder climate, in which case you'll need to up the concentration of coolant to water

Given the time of year we're in, it's probably not freeze plugs. They're another cheap part that will go wrong when you don't realize it and suddenly you'll find yourself sitting on the side of the road screaming and cussing.

I'd inspect the water pump, too. The impeller might be shot.

Also, make certain that your fans are working, too.

Check your hoses and make certain there aren't any leaks.

That's a classic sign of the cooling fans not turning on when they should.

Rad fan is not working so probably burnt out fuse. Or relay. Look for rad fuse first. What is happening right now is that as you drive cool air is going through the radiator cooling the coolant. At idle no air flows, which is why the electric fan motor needs to run.

It sounds more like a coolant flow problem rather than an air flow problem, if I'm understanding the rate at which it rises and falls. The usual cause in a Honda is a bubble in the coolant. Look at the housings where the radiator hoses connect to the engine. One should have a bleed nipple that looks like an oversize brake bleed nipple. With the engine off and cool, open it 1/2 turn (I use a 12 mm socket) and close it when a steady stream of coolant comes out. Top up the radiator and see how it goes.

The radiator fans are not turning on. Likely their fan relay is shot. Replace it. $15 Plug n play part.

If all the fans run with the AC on (front and back electrical radiator fans), the fan relays and circuits are good. I would check the radiator thermostat fan switch usually mounted on the engine thermostat housing, using a short jumper wire with spade terminals crimped on both ends, insert that jumper wire in the 2 wire thermostat switch connector and the fan should run constantly with the key on (the first stage rear radiator cooling fan, not the front additional AC operation fan). It's usually the thermostatic coolant fan switch or the cooling fan relay inoperative. Shorting that thermostat housing fan control temperature switch connector with the key on and engine not running should give you 12VDC at the rear engine compartment side electrical fan connector and energize the rear radiator cooling fan with the ignition switch on, regardless if the engine is running or not because the jumper wire is bypassing the cooling fan temperature switch.

See this picture link for the fan thermostatic control SW location -

My guess is your electric fan is not working. While parked there's no airflow through the radiator so the radiator does not cool as effectively so while idling at lights and stop signs the heat builds in the cooling system. As soon as you drive off the air starts to flow through the radiator again and things start to cool down again. The cause may be a faulty heat sensor switch, faulty fuse or wiring, or even a faulty fan motor.
Warning-- Allowing your engine to get too hot will risk the head of the engine warping which is a serious mechanical problem that will cost a lot to repair. Modern cars have very good cooling systems as long as all is working correctly, but must be repaired quickly if things are not working correctly. As the heads on the engines are now alloy instead of the old cast iron heads of 30years ago it is important they are not allowed to overheat.

Overheating when stopped usually means the electric radiator fan isn't operating. Some cars have 2 electric fans. Open the hood and see if the fan works while the car sits still idling.

If you find that no fan operates, you next have to find the reason. Sometimes the fan motor is defective. Sometimes the fan relay is defective. Some cars have a thermostatic switch somewhere in the cooling system. In many cars the ECM turns the fan(s) on, based on what the the coolant temperature sensor reports. Most cars have a temperature sensor for the computer, and a second temperature sensor if there's a temperature gauge on the instrument panel.

-Engine overhaul mechanic since 1972

Check the coolant and oil. Do not run AC and get it fixed you can cause a lot of damage letting it overheat.