Reasons for Unstable Idle Speed During Cold Start of a Car
2 Answers
Reasons for unstable idle speed during cold start of a car are as follows: Throttle Carbon Buildup: Blockage in the intake tract can cause idle fluctuations, and carbon or oil deposits inside the throttle are the most common causes of unstable idle speed. Carbon buildup on the throttle affects its opening degree, which in turn impacts the air intake volume, causing the engine speed to fluctuate erratically after a cold start, accompanied by strong vibrations. Severe oil or carbon deposits in the throttle, including dirty intake tracts, can make it difficult to start the car in cold weather. Cleaning the throttle is necessary, and throttle cleaning itself is considered a routine maintenance item. Weak Spark from Spark Plugs: Spark plugs can also accumulate carbon deposits, which can be cleaned to restore their activity. However, some spark plugs may experience performance degradation as they near the end of their service life. Weak sparks from spark plugs in individual cylinders can affect the car's performance. Faulty spark plugs can cause "misfire" symptoms, but minor faults in individual cylinder spark plugs may lead to unstable idle speed. Therefore, when a car exhibits unstable idle speed, cleaning or replacing the spark plugs is also a viable troubleshooting option.
I've repaired many cars before, and there are quite a few reasons for rough idling during cold starts. The most common one is a dirty throttle body with carbon buildup, which restricts airflow and causes shaking. Worn spark plugs or faulty ignition coils can also lead to untimely ignition. If the mass airflow sensor gets contaminated with oil, the ECU may misjudge the air intake, which is another frequent cause. Insufficient fuel pump pressure or clogged fuel injectors can result in incorrect air-fuel mixture. Remember, in winter when temperatures are low, poor oil fluidity can exacerbate these issues. From my experience, regular throttle body cleaning and using high-quality spark plugs can prevent this—I recommend checking every 5,000 kilometers. For older cars, the idle air control valve might also fail, so it's best to visit a professional shop to scan the ECU data with a diagnostic tool to ensure safety.
