Clean a throttle body
If your car runs rough when idling, the cause just might be a dirty throttle body. The throttle body controls the amount of air the engine takes in, and when it gets dirty, the engine can't idle smoothly. So before you take the car into the shop, consider taking 10 minutes and cleaning it yourself with a spray can of throttle body cleaner and paper towels. You just might save a bunch of money and there's a good chance you'll cure the problem.
Here's how the throttle body gets dirty. When an engine shuts down, hot exhaust gases and unburned fuel float to the top of the engine. As the vapors bake from the engine heat, they form black sooty carbon deposits inside the throttle body.
Follow the plastic duct from the air filter housing up to the throttle body. Remove the duct and locate the throttle cables and rotating mechanism. Rotate the throttle mechanism and spray the cleaning solvent around the inside of the throttle body. Give it a moment to work and then wipe out the crud.