What Causes High Idle Speed in Small Cars?

[LeJoel]

High idle speed in small cars can be attributed to the following reasons: 1. Idle switch not closing: When the idle contact is disconnected, the ECU determines that the engine is under partial load. At this time, the ECU calculates the fuel injection amount and timing based on signals from the air flow sensor, crankshaft position, and engine speed. However, since the engine is actually operating under idle conditions with less air intake, this results in an overly rich air-fuel mixture and increased engine speed. 2. Overly rich air-fuel mixture: When the ECU receives a signal from the oxygen sensor indicating an overly rich mixture, it reduces the fuel injection amount and increases the opening of the idle control valve, which then leads to an overly lean mixture and decreased engine speed. Conversely, when the ECU receives a signal indicating an overly lean mixture, it increases the fuel injection amount and decreases the opening of the idle control valve, resulting again in an overly rich mixture and increased engine speed.