If your car's engine RPM (revolutions per minute) goes up and down at idle, it could indicate a problem with the engine or fuel system. Here are some possible reasons why this could be happening:

  1. Vacuum leaks: A vacuum leak can cause a fluctuation in engine RPM at idle. When air enters the engine through a vacuum leak, the air/fuel mixture becomes unbalanced, causing the engine to idle rough.

  2. Dirty or malfunctioning idle air control valve: The idle air control valve regulates the engine's idle speed. If the valve is dirty or malfunctioning, it can cause the engine RPM to fluctuate at idle.

  3. Faulty mass airflow sensor: The mass airflow sensor measures the amount of air entering the engine and adjusts the fuel injection accordingly. If the sensor is faulty, it can cause an incorrect air/fuel mixture, which can cause the engine to idle roughly.

  4. Dirty or clogged fuel injectors: Dirty or clogged fuel injectors can cause an incorrect fuel mixture, which can cause the engine to idle roughly and fluctuate.

  5. Faulty ignition system: A faulty ignition system, such as a bad spark plug or ignition coil, can cause the engine RPM to fluctuate at idle.

  6. Failing alternator: If the alternator is failing, it may not be supplying enough power to the engine at idle, causing the RPM to fluctuate.

If you're experiencing engine RPM fluctuation at idle, it's essential to have your car diagnosed by a qualified mechanic to determine the underlying cause and address any necessary repairs promptly.