400 RPM is an extremely low speed for a car engine, typically occurring only during the initial cold start phase or under abnormal idling conditions. The normal speed during cold start should be between 800 and 1200 RPM; if it remains below 800 RPM, it may indicate a fault in the ignition system or a fuel injection problem. For daily driving, it is recommended to maintain the speed above 2000 RPM to ensure power output and fuel economy. Long-term operation below 1500 RPM will lead to insufficient power and increased carbon deposits, while speeds above 4000 RPM may cause a sharp increase in fuel consumption and mechanical wear. For specific scenarios such as the break-in period of a new car, it is recommended to keep the speed below 3000 RPM to protect the engine. Overall, 400 RPM does not meet the requirements of normal operating conditions, and it is necessary to check whether the throttle, spark plugs, or fuel system are abnormal. The reasonable speed range should be adjusted according to the driving conditions: maintain 2000-2500 RPM on urban roads, and briefly increase to 3000-4000 RPM when overtaking at high speeds, but continuous operation in the redline area should be avoided.
