What are the causes of unstable engine idle speed?
2 Answers
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.
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.
