What are the causes of unstable idle speed?

[MacCalvin]

Reasons for unstable idle speed in cars: 1. Intake manifold leakage: When unauthorized air, gasoline vapor, or exhaust gases enter the intake manifold, causing 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 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 regulate 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 actuators in the engine's intake system, such as throttle motor, idle speed control stepper motor, duty cycle solenoid valve, air flow meter, intake pressure sensor, etc. Malfunctions or damage to these components can cause unstable idle speed. 4. Fuel system faults: Issues like clogged fuel injectors, blocked fuel filter, obstructed fuel pump screen, faulty fuel pressure regulator, or blocked return line can all lead to incorrect fuel injection, causing the mixture to be too rich or too lean, resulting in unstable idle speed. 5. Ignition system faults: Problems such as faulty ignition module, incorrect spark plug gap, 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 fluctuations.

[Levi]

My decade-old car has been idling roughly lately, shaking violently as if it's about to fall apart. Based on my repair experience, it's most likely an issue with the ignition system – the spark plugs might be severely carbon-fouled from prolonged use or the ignition coils could be failing, causing engine misfires and rough operation. On the air intake side, cracked vacuum hoses or a dirty throttle body can disrupt proper airflow. Clogged fuel injectors or a dirty fuel filter may also lead to unstable fuel delivery, making the idle RPM fluctuate erratically. Faulty sensors like the MAF (mass airflow sensor) or a malfunctioning idle control valve are common culprits too, as they poorly regulate engine speed. Other potential causes include poor electrical grounding, ECU software glitches, or a faulty PCV valve. The best practice is to monitor unusual engine noises while driving – if you hear buzzing sounds or notice the tachometer needle jumping erratically, get it professionally diagnosed immediately to prevent compounding issues through delayed repairs.

[Natalie]

Last time my car had idle fluctuations, it shook violently when stopping at a red light. Later, the mechanic found it was due to a vacuum leak caused by a cracked intake manifold. Common causes also include aging spark plugs needing replacement, clogged air filters, or dirty fuel injectors causing uneven fuel supply. Sensors like oxygen sensors giving inaccurate signals can confuse engine control, and a stuck idle valve can cause similar issues. The problem is not only annoying but also increases fuel consumption and wastes money, potentially damaging engine components in the long run. It's recommended to regularly check these key points, such as cleaning the throttle body every six months to prevent carbon buildup. If you want to try fixing it yourself, check the engine light; if it's on, use a diagnostic tool to read the fault codes for quicker troubleshooting. For safety, avoid long-distance driving if symptoms occur.

[PrestonLynn]

The car idles unevenly and shakes constantly when driving? Simply put, the main reason is excessive carbon deposits, such as dirty spark plugs or throttle body, which affect ignition and air intake. Air leakage issues, like a broken vacuum tube causing uneven air entry, are also common. Fuel supply problems, such as a clogged fuel pump or filter, often lead to insufficient fuel delivery. A stuck idle valve or faulty sensor can cause control errors. If the tachometer needle jumps around, there's likely an issue. Preventive measures include avoiding low-quality fuel and regularly replacing filters, which are simple yet effective.