Why Does the Engine Speed Increase Automatically in Neutral Gear?

[CarlosMarie]

There are two main reasons for a sudden increase in engine speed when the vehicle is in neutral: a faulty carbon canister solenoid valve causing the engine speed to rise suddenly in neutral, and a malfunctioning vacuum sensor in the braking system leading to the same issue. Here is a detailed analysis: 1. Cause of carbon canister solenoid valve failure: When the vehicle is shifted into neutral, the engine should return to idle speed, and the carbon canister solenoid valve should close. If the solenoid valve fails to close or does not seal properly, gasoline vapors from the carbon canister will continuously flow into the intake manifold and then enter the combustion chamber, causing a sudden increase in engine speed in neutral. 2. Cause of vacuum sensor failure in the braking system: If the vacuum sensor malfunctions, it cannot detect whether the vacuum level in the power assist system is sufficient. In this case, the engine will increase its speed to enhance the intake vacuum, thereby raising the vacuum level in the intake manifold.