Reasons for Occasional Shaking at Idle Speed?

[Dean]

Possible causes of car shaking at idle include: faulty spark plugs or ignition coils; incorrect valve timing; uneven cylinder compression; poor valve sealing leading to air leakage; inadequate fuel pressure; clogged fuel injectors; leaks between the cooling system and cylinders; malfunctioning mass airflow sensor. Below is a classification of unstable idle conditions: 1. How to Observe Unstable Idle ① Observe the degree of engine block shaking, or check the movement of the oil dipstick handle—a stable handle appears clear, while a shaking one may appear doubled. ② Monitor the engine tachometer or read data blocks; the RPM fluctuates around the expected idle value or shakes violently on one side. The expected idle values in the program include standard idle, load idle (with lights on, automatic transmission engaged, etc.), A/C idle, and warm-up idle. ③ Start the engine while stationary and feel the severe shaking of the vehicle from the seat. 2. Classification by Pattern ① Rhythmic instability when the engine is cold (coolant temperature below 50°C). ② Rhythmic instability when the engine is hot (coolant temperature above 50°C). ③ Irregular severe shaking once or twice. 3. Classification by Severity ① Normal: shaking within ±10 RPM of the expected idle value. ② Moderate instability: shaking within ±20 RPM of the expected idle value. ③ Severe instability: shaking exceeding ±20 RPM of the expected idle value. ④ Violent shaking on one side of the expected idle value. 4. Classification by Cause ① Direct causes: mechanical components being dirty, worn, or improperly installed, leading to variations in individual cylinder power output, resulting in imbalance and unstable idle. ② Indirect causes: abnormalities in the engine's electronic control system causing poor combustion of the air-fuel mixture, making it difficult to balance cylinder power and leading to unstable idle. 5. Classification by Fault System ① Intake system; ② Fuel system; ③ Ignition system; ④ Engine.