What are the causes of unstable engine idle speed?

[PiperDella]

Engine idle speed instability causes: 1. Intake manifold or various valve leaks - When unwanted air, gasoline vapor, or combustion exhaust enters the intake manifold, causing the air-fuel mixture to become too rich or too lean, resulting in abnormal engine combustion; 2. Excessive carbon buildup on the throttle body and intake passages; 3. Inaccurate idle air intake volume; 4. Fuel injector dripping or clogging, preventing proper fuel injection according to ECU commands, causing mixture imbalance and poor cylinder operation leading to unstable idle; 5. Fuel pressure malfunction - Low pressure results in poor fuel atomization or stream-like injection (sometimes only droplets), reducing injection volume and creating a lean mixture. High pressure increases actual injection volume, creating a rich mixture.

[TylerLee]

I've been dealing with idle instability issues in repair shops for over a decade. The most common cause is excessive carbon buildup in the throttle body, which blocks the intake passage and affects the air-fuel mixture flow. Another frequent issue is vacuum line leaks, where aging hose connections crack, allowing air to leak in and disrupt pressure balance. Changes in spark plug electrode gaps or aging ignition coils can cause individual cylinders to malfunction. A dirty or damaged mass airflow sensor can mislead the ECU in calculating fuel injection quantities. Insufficient fuel pump pressure or clogged fuel injectors can also lead to uneven fuel supply. I recommend first using a diagnostic scanner to read trouble codes and locate the problem. Regular throttle body cleaning and air filter replacement can effectively prevent issues. Spark plugs should generally be replaced every 60,000 kilometers – don't wait until severe shaking occurs before addressing it.

[SanEmmett]

Last time my car was shaking intermittently at idle, even at red lights it wasn't stable. After having a mechanic check it, it turned out the fuel injectors were clogged with carbon deposits, causing uneven fuel spray and cylinder imbalance. The idle air control valve might have been stuck and unable to adjust the RPM, while excessive carbon buildup in the throttle body also affected its smooth operation. Additionally, a leaking intake manifold gasket and cracked vacuum hoses were allowing excess air to mix in. My neighbor's car had fluctuating RPM due to incorrect ECU programming. Now I know: regularly adding high-quality fuel system cleaners and checking/replacing the fuel filter every two years helps. Long-term low-speed city driving easily leads to carbon buildup, so occasional high-speed driving helps clear out deposits.

[Emmanuel]

When modifying my car, I pay special attention to idle issues. Faulty signals received by the ECU, such as from a malfunctioning coolant temperature sensor, can report incorrect data. Aging ignition coils or excessively large spark plug gaps in the ignition system can easily cause misfires. A dirty mass airflow sensor with drifting readings may misjudge the intake air volume. Another common issue is a failed oxygen sensor unable to feedback the air-fuel ratio. Use a multimeter to check these sensors by measuring resistance changes. For modified cars, excessive intake air volume or improper exhaust backpressure can also cause unstable idling. Sometimes, cleaning the sensor connectors of dirt can solve the problem.