If the engine idles at 1500 RPM after a cold start and gradually returns to 600-800 RPM as the coolant temperature rises to operating levels, this is a normal phenomenon. The engine is designed this way to serve the following purposes: 1. During cold starts, the coordination between various engine components has not yet reached optimal conditions. The ideal operating temperature for an engine after prolonged use is between 80-110°C. Therefore, during design and manufacturing, each component is optimized to achieve peak coordination at this temperature range. Increasing idle speed during cold starts helps the engine reach operating temperature more quickly. 2. When the engine is cold, intake air temperature is lower, leading to poor fuel atomization. Some fuel injected from the injectors fails to atomize properly and instead adheres to the intake passages, potentially forming carbon deposits. This also makes it harder to control the air-fuel mixture concentration entering the engine. Raising idle speed during cold starts helps increase engine coolant temperature. 3. The catalytic converter typically operates within a temperature range of 400-800°C, at which point it can effectively purify incompletely burned exhaust gases. Therefore, the engine not only retards ignition timing but also utilizes higher cold-start RPM to rapidly elevate exhaust gas temperature to the catalytic converter's operating range. In summary, higher idle speed during cold starts facilitates faster warming of engine components to achieve optimal operating conditions.
