My a/c in my 2004 Sunfire blows hot when idling but cold when I step on the gas and go.?

If it gets cold right away when you step on the gas, it's none of the things these guys are talking about. AC can't change temp that quick. It would probably be a vacuum leak to your AC controls, making the hot/cold air mix door in the vent malfunction. Increased engine speed restores the vacuum enough for the controls to work again.

The most common cause is low refrigerant caused by a small leak. A inoperable radiator fan could cause this also by creating high pressures in the system and cuttingthe compressor at idle. You should take it to a good repair shop and have it properly diagnosed because there are a lot of things that could be contributing factors also. This will cost anywhere from $80 to $400 depending on what is wrong and where you live.

This looks like you only need a couple ounces of refrigerant- R134A. Be careful the wrong mechanic (Most mechanics are honest) will tell you that you have a leak. If this is the first time chances are that what you have is a "natural" leak that is caused by the weather changes over the past few years. On the other hand if this is not the first time chances are you do have a leak and the system will have to be serviced. Be careful of some answers that you may receive about "doing it yourself" as I mentioned before you may only be a couple of ounces low - over charging the system will cause you more problems. good luck

Your fan may not be working when your A/C is on. The A/C unit in front of your car (looks like a small radiator) need to have air blowing over it to keep you cool. If your fan isn't turning on for any reason, your A/C will be warm at idle and low speeds. Tell your mechanic the symptoms rather than a part to change. If he changes a part because you told him too, and it doesn't fix the problem, your both going to be upset.

You probably need to replace the old A/C gas. What happens is that a/c compressor moves the gas through the pipes and cools it at low idling RPMs (about 750-1100rpms) when the gas is new, now when it gets old, it needs to move a lot faster in order to cool down. This means that you have to accelerate to about 2000-2500rpms so most likely you just need to replace the gas. And as Jack w said (on top), it could be a natural leak and DON'T try to do it your self because in order for the new gas to last longer, you have to pump out the old gas with a special vacuum. Also if you exceed 30 psi in gas you might even damage you're compressor.

take a look under the nose of the car for damaged air dams that may have been broken by a speed bump or parking lot curb