* Faulty Idle Air Control (IAC) Valve or Throttle Body: The IAC valve regulates the air intake at idle. A malfunctioning valve can cause erratic idling, surging RPMs, and vibrations. A dirty throttle body can also cause similar symptoms.
* Vacuum Leaks: A leak in the vacuum system can disrupt the engine's air-fuel mixture, leading to unstable idling and potentially higher RPMs.
* Problems with the Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A faulty MAF sensor can provide inaccurate readings, resulting in incorrect fuel delivery and erratic idling.
* Transmission Issues (Less Likely, but Possible): While less probable given the symptoms are primarily engine-related, a problem within the transmission's control system *could* indirectly affect the engine's behavior. This is less likely if the issue is only present when the car is in Park.
* Engine Mounts: Worn-out engine mounts can amplify engine vibrations, making them more noticeable, even if the root cause is something else.
* Faulty Sensors (Various): Several other sensors (crankshaft position sensor, throttle position sensor, etc.) can cause erratic engine behavior if they're malfunctioning. A diagnostic scan can pinpoint this.
What You Should Do:
* Do NOT continue driving the car extensively with this problem. It could worsen the underlying issue and potentially cause more damage.
* Get a diagnostic scan: This is the most important step. A mechanic can use a scan tool to read the car's onboard computer and identify any diagnostic trouble codes (DTCs) that might point to the specific problem.
* Take it to a qualified mechanic: Attempting DIY repairs without the proper knowledge and tools could be dangerous and may not solve the problem. A Volvo specialist is ideal, as they have experience with the specific systems in your S60.
Ignoring this issue could lead to more significant and costly repairs down the line. Have it checked out promptly.
