If you've noticed that your car is acting strangely when the engine is idling, there could be a number of problems causing the issue. Some issues may only arise under certain circumstances, but others can present themselves any time the engine idles. When the engine runs rough or even stalls while it is idling, but operates normally at faster speeds, you could be looking at a vacuum leak, clogged fuel injectors, maladjusted ignition timing, a busted cylinder, faulty spark plugs, a damaged distributor cap or other issues.
Vacuum leaks could cause rough idling regardless of engine conditions and circumstances, so the first thing you should check is the vacuum hoses. Inspect them for damage and replace if needed. If no visible damage is noticed, you can perform a leak test. To do this, you will need a vacuum gauge. Disconnect the hose from the engine and hook it up to the gauge. Start the engine and let it idle. The correct reading specifications for your vehicle can be found in the owner's manual. If the readout is too low, then there is either a leak or a blockage in the line.
An engine that idles too fast may force the driver to put extra pressure on the brake pedal to keep the car from rolling forward. Accelerated idling may be due to an overheated engine, a faulty fuel pressure regulator, a bad idle speed control unit or a damaged alternator.
If your car stalls when you come to an abrupt stop, the engine control system may need to be repaired or replaced. When stalling is more intermittent, the fuel pressure regulator could be out of whack and need to be replaced. The idle speed settings may not be correct, either, and should be reset. Idling problems could also be caused by a faulty EGR valve. This part is used to improve emissions by running exhaust through the combustion process a second time to ensure the noxious gases burn off before exiting the car.
There are many other issues that could be causing the car to idle roughly, including a faulty part in a camshaft or other mechanical issues in the engine. Regardless of the underlying cause, you should take heed when your car begins to idle inappropriately, as this is a clear sign that something is going wrong.