What Causes Unstable Idling?
2 Answers
Below are the reasons for unstable car idling: 1. Intake manifold leakage: When unauthorized air, gasoline vapor, or combustion exhaust enters the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to unstable idling. 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, preventing the control unit from precisely regulating the idle air intake. This also results in an overly rich or lean mixture, causing unstable idling. 3. Faulty intake system components: The engine's intake system has many components, such as the throttle motor, idle speed stepper motor, duty cycle solenoid valve, air flow meter, and intake pressure sensor. Malfunctions or damage to these can cause unstable idling. 4. Fuel system issues: Problems like clogged fuel injectors, blocked fuel filters, obstructed fuel pump screens, faulty fuel pressure regulators, or blocked return fuel lines can lead to incorrect fuel injection, causing the mixture to be too rich or lean and resulting in unstable idling. 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 spark energy or cause misfires, leading to erratic and unstable idling.
Just helped my neighbor with this issue yesterday - his old car was shaking like it was dancing. Unstable idle most commonly happens when carbon deposits build up behind the throttle body. Impurities in gasoline that don't burn completely stick to the intake passages, blocking airflow like rust clogging a water pipe. Another common culprit is spark plugs - if the electrodes are worn or the gap is incorrect, they can't ignite properly. A misfiring cylinder makes the car shudder like a person gasping for air. Also check vacuum hoses for cracks and leaks - rubber tubes harden and crack after 5-6 years of use, and idle is especially sensitive to this. The most overlooked part is engine mounts - when those three rubber cushions age and crack, all the engine vibration gets transmitted to the cabin. Replacing them makes an immediate difference.
