In most modern vehicles some of the exhaust is routed back into the engine through the throttle body, part of the air intake system. This leads to less air pollution, although some of the post combustion products in the exhaust can coat the inside of the throttle body. This can lead to a loss in vehicle performance, especially stalling and stumbling when the engine is warm. There are several indicators that your throttle body needs cleaning.
Check your gas mileage. This can be as easy as filling your tank to full and setting the trip odometer to zero. When the tank is empty, observe how any miles you were able to go on that one tank. If you notice that you are getting fewer miles to the gallon than your car should be getting, you may need to clean your throttle body.
Observe your car when it is idling. When in idle, if the engine idles erratically (slowing and/or speeding up suddenly) or sputters then you may need to clean the throttle body.
Pay attention to your car when accelerating. Uneven acceleration or slowed acceleration can be indications that your throttle body is dirty and needs to be cleaned.
Remove the throttle body and visually inspect it for residue. First remove the two screws on the circular clamps that hold the intake hose coming into the throttle body. Next, remove the two bolts on either side of the intake opening to remove the throttle body from the rest of the engine. Remove the four or five screws or bolts on the top of the throttle body to open it up. The residue, if present, will be black and tar-like. If you see this residue you need to clean your throttle body, using a solvent made for this purpose.