Why does the idle speed increase when shifting to P gear?

[StNorah]

An increase in idle speed when shifting to P gear can occur under different circumstances. If it is a temporary increase, it is a normal phenomenon. If the increase persists, it may be due to improper adjustment of the engine idle speed regulator or an air leak in the intake system. Below is a detailed explanation: Temporary increase is normal: After parking and shifting to P gear, the engine's RPM increases as it is no longer under load. This happens because the engine control unit sends a signal to the idle speed regulator to raise the idle speed, preventing the engine from stalling when the load is suddenly removed. Improper adjustment of the idle speed regulator: If the idle speed regulator is improperly adjusted, it may cause the idle speed to drop too low, leading to the vehicle stalling. This can be resolved by readjusting the regulator. Air leak in the intake system: An air leak in the intake system can result in an insufficient air-fuel mixture at idle, causing the engine to stall.

[VanElijah]

I also experienced the sudden surge in idle speed to 1500 RPM when shifting to P gear a couple of days ago, which was quite startling. The main cause could be excessive carbon buildup in the throttle body. After I cleaned it, the issue was resolved. In some older cars, vacuum line leaks can allow extra air to enter, causing the ECU to compensate by increasing fuel supply, which raises the RPM. The air conditioning compressor starting up can also cause a brief RPM increase, but if it stays high continuously, that's abnormal. If the engine control module receives incorrect signals, such as from a faulty coolant temperature sensor, it may also incorrectly increase fuel supply. Sticking idle control valves or unstable battery voltage are common causes as well. It's advisable to first check for trouble codes with a diagnostic scanner, then focus on inspecting the throttle opening and the sealing of the intake system. Long-term high idle speeds can severely damage the engine and significantly increase fuel consumption, so it's best to address the issue as soon as possible.

[OCollin]

I often encounter abnormal idle speed issues when repairing cars. In P gear, it should stabilize around 800 RPM. An increase usually indicates an intake control malfunction. Dirty throttle bodies can cause the throttle to stick in a more open position, while oil contamination on the mass airflow sensor leads to incorrect air intake readings. Older cars are prone to vacuum hose leaks, causing the ECU to compensate by injecting more fuel when detecting a lean mixture. Additionally, the ECU will actively increase idle speed when the alternator or AC is under high load, but prolonged operation suggests electrical issues. I recommend first checking OBD data streams for abnormal air-fuel ratios, then testing fuel pressure. Never ignore this, as abnormal RPM can worsen carbon buildup and fuel consumption.

[LaAlexis]

I totally relate to this issue. My car's RPM spikes in P gear with AC on during summer. The mechanic said there are mainly two categories: normal compensation and fault abnormalities. It's reasonable for RPM to rise when AC kicks in or the battery is low, but sustained high RPM requires hardware checks. Carbon buildup prevents the throttle valve from sealing properly, letting excess air leak in. Faulty sensors like oxygen sensors reporting errors can cause the ECU to inject more fuel. Older cars often have vacuum hose aging/leakage issues, messing up the air-fuel mixture. You can do simple checks: see if cold starts are normal, if symptoms worsen with AC on, or if restarting improves it. If all else fails, connect an OBD scanner to read real-time data.