What Causes Unstable Car Idling?
2 Answers
Below are the reasons for unstable car idling: 1. Intake manifold leakage: When unauthorized air, fuel vapor, or exhaust gases enter the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to unstable car idling. 2. Excessive carbon buildup in the throttle body: Too much carbon buildup in the throttle body and surrounding intake passages alters the cross-sectional area of the air passage, preventing the control unit from precisely regulating the idle air intake. This also results in an overly rich or lean mixture, causing unstable car idling. 3. Faulty intake system components: The engine's intake system has many components, such as the throttle motor, idle speed control stepper motor, duty cycle solenoid valve, air flow meter, and intake pressure sensor. Any malfunction or damage to these can cause unstable car idling. 4. Fuel system issues: Problems like clogged fuel injectors, blocked fuel filters, obstructed fuel pump screens, faulty fuel pressure regulators, or clogged return fuel lines can lead to incorrect fuel injection, causing the mixture to be too rich or too lean and resulting in unstable car idling. 5. Ignition system faults: Issues such as faulty ignition modules, incorrect spark plug gaps, eroded or damaged spark plug electrodes, or excessively high resistance in high-voltage wires can reduce spark energy or cause misfires, leading to erratic and unstable car idling.
My car used to have an unstable idle issue, where the engine would inexplicably shake when stopped at a red light, with the RPM fluctuating between high and low. This is usually caused by problems in the ignition system, such as worn spark plugs, poor contact in the ignition coil, or aging wires, leading to weak sparks and unstable combustion. Additionally, fuel system malfunctions are common culprits—like clogged injectors with excessive carbon buildup or insufficient fuel pump pressure, resulting in poor fuel delivery. Air system issues are also frequent, such as a dirty throttle body that hasn't been cleaned, a clogged air filter, or a faulty mass airflow sensor, preventing the engine from drawing in enough air. Sensor problems, like a stuck idle control valve or a vacuum leak, can also cause the idle to go out of control. I later rushed to the repair shop for an inspection, spent some money replacing parts, and the issue was resolved. Regular maintenance, such as changing the engine oil and cleaning carbon deposits, can help prevent such problems in advance.
