What Causes a Sudden Increase in Neutral Gear RPM?
2 Answers
Neutral gear RPM suddenly increases due to faults in the carbon canister solenoid valve or vacuum sensor. Below are the specific reasons: Carbon Canister Solenoid Valve Fault: If the solenoid valve fails to close or does not seal properly, gasoline vapor from the carbon canister will continuously flow into the intake manifold and enter the combustion chamber, causing a sudden increase in RPM when in neutral gear. Vacuum Sensor Fault: If the vacuum sensor malfunctions, it cannot detect whether the vacuum level of the power assist device is sufficient. In this case, the engine will increase RPM to enhance the engine's suction force, thereby increasing the vacuum level in the intake manifold.
A few days ago while driving, when I shifted to neutral at a red light, the RPM suddenly surged to 3000, which really scared me. After researching, it might be due to severe carbon buildup in the throttle body getting stuck, accumulated dust from frequent city traffic congestion, or a malfunctioning idle air control valve sending wrong signals. Another possibility is the throttle pedal sensor sending incorrect signals to the ECU, causing it to randomly increase RPM. Aging vacuum hoses with leaks can also draw in extra air, causing RPM to rise. I recall a friend's car had a similar issue which was fixed after cleaning the throttle body. Unstable RPM not only creates loud noise but also burns fuel excessively and damages the engine over time. If left unresolved, it could lead to sudden unintended acceleration on the road. I recommend going to a repair shop immediately for a diagnostic scan instead of guessing and wasting time. For prevention, clean the intake system and replace the air filter every 20,000 km for peace of mind and safety.
