Why Does the Car Have Unstable Idle Speed During Cold Start?
2 Answers
Unstable idle speed during cold start occurs because, in cold conditions, the car needs to bring the engine into its normal operating environment as quickly as possible. Therefore, during a cold start, the engine control unit automatically increases fuel injection, burning more fuel than usual at idle to rapidly raise the engine's coolant temperature and help the engine reach its optimal operating temperature quickly. Idle speed refers to an operating condition of the car where the engine runs without load, meaning the transmission is in neutral—a state often described as the engine working without delivering power. The engine's power comes from fuel combustion, which is facilitated by the fuel injection system, ignition system, and mechanical structure. Thus, unstable idle speed is often caused by abnormalities in these systems.
Last time I fixed my old car, I encountered this issue - the tachometer danced up and down during cold starts. It's actually caused by low temperatures thickening the engine oil, leading to insufficient lubrication of parts. A common culprit is excessive carbon buildup on the throttle valve sticking it shut, just like limescale blocking the spout of a kettle. It could also be a faulty coolant temperature sensor causing the ECU to misjudge temperatures and misfire fuel injection. The worst scenario would be clogged fuel injectors or aged spark plugs causing cylinder misfires, making the engine cough. I remember replacing the oxygen sensor once fixed it - that thing controls the air-fuel ratio. Recommend reading trouble codes first then checking spark plugs. If accompanied by white exhaust smoke, beware of head gasket issues. Using fuel additives to clean the fuel system before winter can prevent this, and older cars especially need frequent air filter changes.
