What causes an engine idle to go up and down?

An engine idle that fluctuates can be caused by several factors, including a vacuum leak, which allows extra air into the intake system, disrupting the air-fuel mixture. Issues with the idle air control valve (IAC), which regulates airflow during idle, can also lead to irregular idling. Additionally, problems with fuel delivery, such as a clogged fuel filter or failing fuel pump, or faulty sensors, like the mass airflow sensor or throttle position sensor, can contribute to this issue. Regular maintenance and diagnostics are essential for identifying and resolving the underlying cause.