Why is the car idle speed unstable?

[Archie]

Below are the reasons for unstable car idle speed: 1. Intake manifold leakage: When unwanted air, gasoline vapor, or exhaust gas enters the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to unstable car idle speed. 2. Excessive carbon buildup in the throttle body: Too much carbon deposit in the throttle body and surrounding intake passages changes the cross-sectional area of the air passage, making it impossible for the control unit to precisely control the idle air intake, which also results in an overly rich or lean mixture, causing unstable car idle speed. 3. Faulty intake system components: There are many components in the engine intake system, such as the throttle motor, idle speed stepper motor, duty cycle solenoid valve, air flow meter, intake pressure sensor, etc. Damage to these components can cause unstable car idle speed. 4. Fuel system faults: Clogged fuel injectors, clogged fuel filters, clogged fuel pump screens, faulty fuel pressure regulators, clogged return fuel lines, etc., can all lead to incorrect fuel injection, causing the mixture to be too rich or too lean, resulting in unstable car idle speed. 5. Ignition system faults: Faulty ignition modules, incorrect spark plug gaps, eroded or damaged spark plug electrodes, excessive resistance in high-voltage wires, etc., can reduce the energy of the ignition spark or cause misfires, leading to unstable car idle speed that fluctuates between high and low.