What Causes Fluctuating Idle Speed in Cars?

[BenjaminLee]

The reason for fluctuating idle speed in cars is due to unstable air intake volume. Factors causing unstable air intake: Faulty mass air flow sensor; Intake manifold leakage; Charcoal canister purge valve stuck open; Positive crankcase ventilation valve stuck open; Clogged air filter. Analysis of idle speed principle: During cold idle, the engine will do its utmost to maintain stable air intake volume and air-fuel ratio while ensuring it doesn't stall. At this time, the engine control unit will close the charcoal canister purge valve, crankcase ventilation valve, and oxygen sensor signals. More advanced engines also need to deactivate precise control functions like variable intake manifold and variable valve lift. Although deactivating these precision controls makes the engine less fuel-efficient and less environmentally friendly, the extremely simple control logic results in very stable RPM. In this situation, as long as there are no vehicle faults, the throttle opening angle will basically stabilize at a fixed value.