Why does the engine idle roughly when cold but smooth out when warm?
2 Answers
The reason for rough idling when cold that improves when warm is: Under cold operating conditions, the engine oil viscosity is relatively high, resulting in increased engine running resistance and insufficient cylinder pressure, making it difficult for individual cylinders to ignite and perform work, leading to misfires. As the engine runs and warms up, the operating temperature gradually rises, improving oil fluidity. At this point, the misfiring cylinders can resume normal ignition, naturally reducing the vibration. Cold idle roughness that disappears when warm may also be caused by inaccurate air-fuel mixture ratios, occurring in both open-loop and closed-loop control systems. If the vehicle's computer makes incorrect judgments, errors in controlling the air-fuel mixture and ignition timing through actuators may occur, reducing power output and causing vibration.
I have an old car that shakes violently when starting in cold weather, with noticeable vibrations on the steering wheel, but it runs completely normal after driving for a few minutes to warm up. This issue is usually caused by unstable ignition system when the engine is cold, poor ignition due to spark plug carbon buildup or electrode wear; it could also be clogged fuel injectors causing uneven fuel spray and rough combustion. Carbon buildup in the idle control valve can also make the RPM fluctuate. After warming up, the temperature rises, lubrication improves, and components fit tightly, making it stable. I've encountered a similar situation before—replacing the spark plugs and cleaning the throttle body immediately improved it. I recommend not delaying maintenance, performing regular inspections, especially changing to appropriate engine oil before winter to avoid severe carbon buildup damaging the engine.
