* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. If it's sticking or malfunctioning, it might allow too much air into the engine when the transmission shifts into Park, causing the RPMs to increase. The increased load on the transmission when in Park (compared to Drive) might be enough to trigger this malfunctioning behavior after the engine has warmed up and the IAC is under a different operating parameter.
* Throttle Position Sensor (TPS): This sensor tells the computer the position of the throttle plate. A faulty TPS can send incorrect signals, leading to erratic idle speed, especially after the engine reaches operating temperature. The change in load from Drive to Park could be enough to expose a subtle TPS problem.
* Vacuum Leak: A vacuum leak after the throttle body can cause the engine to run faster than it should. When the transmission is shifted into Park, the engine's load changes, and a small leak might become more noticeable, resulting in a higher RPM.
* Transmission Issues (Less Likely): While less probable, a problem with the transmission's linkage or a malfunctioning park position switch *could* indirectly influence the engine's idle speed by altering engine load signals or causing a delayed signal to the PCM. This is less likely than the IAC or TPS issues.
* Computer issue (PCM): In rare cases, a failing Powertrain Control Module (PCM) or a software glitch could be the culprit. This is typically diagnosed after other possibilities have been eliminated.
Why after warming up? Many of these problems, especially those related to sensors and valves, are more likely to manifest after the engine is warm because the heat can exacerbate existing issues like sticking valves or inconsistent sensor readings.
Troubleshooting: A mechanic would likely start by inspecting the IAC valve, cleaning or replacing it if needed. Then they'd check the TPS for proper voltage readings and look for vacuum leaks using a vacuum gauge or smoke machine. If those tests don't reveal the problem, further diagnostics, including checking the PCM, might be necessary.
