What Are the Causes of Unstable Idle Speed in Sedans?

[MasonRose]

Here are the reasons for unstable idle speed in cars: 1. Intake manifold leakage: When unwanted air, gasoline vapor, or exhaust gases enter the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to unstable idle speed. 2. Excessive carbon buildup in the throttle body: Too much carbon buildup in the throttle body and surrounding intake passages changes the cross-sectional area of the air passage, preventing the control unit from accurately controlling the idle air intake. This also results in an overly rich or lean mixture, causing unstable idle speed. 3. Faulty intake system components: There are many components in the engine's intake system, such as the throttle motor, idle speed stepper motor, duty cycle solenoid valve, air flow meter, and intake pressure sensor. Any failure or damage to these components can cause unstable idle speed. 4. Fuel system issues: Problems like clogged fuel injectors, clogged fuel filters, clogged fuel pump screens, faulty fuel pressure regulators, or blocked return fuel lines can lead to incorrect fuel injection, resulting in an overly rich or lean mixture and unstable idle speed. 5. Ignition system faults: Issues such as faulty ignition modules, incorrect spark plug gaps, eroded or damaged spark plug electrodes, or excessive resistance in high-voltage wires can reduce ignition spark energy or cause misfires, leading to erratic idle speed that fluctuates between high and low.