Why is the car idle speed unstable?
3 Answers
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.
My car also had an unstable idle issue before. When driving, I could feel the engine surging up and down, which was really annoying. There could be many reasons for this. A dirty throttle body is a common cause - when the car isn't driven for long periods or only driven at low speeds in the city, carbon deposits can block the air intake. Worn-out spark plugs are another problem; after several years of use, their ignition capability weakens, directly leading to unstable idle. Air intake system leaks, like cracked aging hoses or worn seals allowing air leakage, can also prevent sufficient oxygen from entering. Additionally, fuel system issues such as clogged fuel injectors or a dirty air filter affecting engine breathing can all cause erratic idle fluctuations and abnormal engine sounds. I recommend regular maintenance and periodic checks of these components - don't wait until problems occur, especially in congested areas where engine shaking can be particularly uncomfortable. It's best to have a professional mechanic handle the repairs as DIY attempts can be quite risky.
As an experienced driver, I've encountered unstable idling situations several times. Common causes include: when the throttle body is severely carbon-fouled, the car struggles to breathe; or when spark plugs are worn out, leading to poor ignition performance which easily causes shaking. Another example is air intake leaks, where small cracks allow extra air to sneak in, confusing the system. The fuel system shouldn't be overlooked either - clogged fuel injectors from carbon buildup affect fuel distribution. Idle control valve issues are more direct - once faulty, it can't adjust the RPM properly. Sensor failures like malfunctioning oxygen sensors also disrupt signals. These factors combined can make the car's idle jump around like it's drunk, especially noticeable at red lights. Regular maintenance and periodic cleaning of key components can prevent this. If you notice instability, stop and check immediately - safety first.
