What are the reasons for unstable idle speed when the car is cold?
3 Answers
The reasons for unstable idle speed when the car is cold include: 1. The auxiliary air valve does not open or opens too little when the car is cold, causing the engine idle speed to be too low and resulting in unstable idle; 2. The idle control valve malfunctions when the engine is cold, opening too little, leading to too low idle speed and unstable idle; 3. The water temperature sensor fails, causing the engine computer not to increase the idle speed and enrich the air-fuel mixture according to the engine temperature when the car is cold, resulting in unstable idle; 4. Poor atomization or partial blockage of the fuel injector, which degrades the quality of the air-fuel mixture or provides too little fuel during cold idle conditions, causing the mixture to be too lean and leading to unstable idle.
I've been driving for over a decade, and unstable idle during cold starts is quite common, mainly because the engine hasn't warmed up yet, and various sensors might malfunction. For instance, if the coolant temperature sensor is inaccurate, the ECU will issue incorrect fuel commands, resulting in a mixture that's too lean or too rich, causing the idle to fluctuate. It could also be a carbon buildup issue—dirty throttle bodies affecting air intake or aging spark plugs causing unstable ignition. I remember last winter my car had this issue, shaking violently. After a check at the repair shop, it turned out to be a faulty mass airflow sensor, and cleaning it fixed the problem. I recommend using high-quality engine oil, performing regular maintenance to prevent carbon buildup, and waiting a few extra seconds during cold starts to let the engine warm up.
As someone who frequently deals with vehicles, I've summarized several reasons for unstable idle speed when the engine is cold: 1. Sensor malfunctions, such as inaccurate coolant temperature or intake air temperature sensors, misleading the ECU in adjusting fuel; 2. Ignition system issues, where worn spark plugs or aging ignition coils cause inconsistent ignition; 3. Fuel system failures, such as clogged fuel injectors or low fuel pump pressure affecting fuel supply; 4. Vacuum leaks, like cracks in the intake hose allowing air to seep in; 5. Excessive carbon buildup in the throttle body or intake manifold. During diagnosis, start by reading fault codes with an OBD scanner, then proceed step by step, beginning with simple tasks like cleaning the throttle body before checking the electrical circuits.
