What Causes Unstable Idle and Stalling When the Engine is Cold?
2 Answers
Reasons for unstable idle and stalling when the engine is cold: Low car battery voltage: Use a multimeter to test the battery voltage. If it is below 10V, replacement is recommended. If it is around 11V, it can be used temporarily, but prepare for replacement. Above 12V is acceptable. Caused by carbon buildup: Check if it is due to carbon buildup. If the vehicle has traveled around 20,000 km, focus on inspecting and cleaning the throttle body. Spark plug issues: Inspect, test, and replace the spark plugs if necessary. Fuel system: Test and inspect components related to the starting and fuel system, such as the starter, fuel pump, and sensors.
Unstable idle and stalling during cold starts are mainly related to sensor issues. From my work experience, if the coolant temperature sensor fails, the ECU won't know how to adjust the air-fuel mixture. During cold starts when the mixture should be enriched, it becomes too lean instead, leading to unstable combustion which causes rough idle or even stalling. Dirty throttle bodies are also common - carbon buildup obstructs smooth airflow. If the idle air control valve or stepper motor malfunctions, it directly affects RPM control. Worn spark plugs or weak ignition coils make cold-weather starting difficult, increasing the chance of engine stall. Sometimes the ECU's self-learning function fails to update properly. I recommend first checking for loose sensor connections. If that doesn't help, visit a professional shop for diagnostic scanning. Don't delay as prolonged issues may damage the engine, making repairs more expensive.
