* Fuel Injectors: Cold weather can thicken fuel, making it harder for injectors to atomize properly. Worn or dirty injectors are particularly susceptible. This is a very likely candidate.
* Fuel Pressure Regulator: A failing fuel pressure regulator might not maintain consistent fuel pressure when cold, leading to a rough idle. This improves as the engine warms and parts expand.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the air entering the engine. A dirty or faulty MAF sensor can provide inaccurate readings, especially when cold, affecting the air/fuel mixture.
* Throttle Position Sensor (TPS): Similar to the MAF sensor, a malfunctioning TPS can lead to incorrect fuel delivery, particularly noticeable when cold.
* Ignition System: Worn spark plug wires, a weak coil, or failing distributor cap and rotor (if equipped) can cause misfires more pronounced when the engine is cold and components aren't fully warmed up. Cracked distributor cap is a classic "cold start" problem.
* PCV Valve (Positive Crankcase Ventilation): A clogged PCV valve can cause excessive crankcase pressure, affecting the air/fuel mixture and leading to rough running, especially when cold.
Less Likely (but possible):
* Coolant Temperature Sensor: While it's more likely to affect the engine's overall temperature control, a faulty sensor could provide incorrect information to the computer, influencing the fuel mixture.
* Engine vacuum leak: A vacuum leak can be more pronounced when cold as seals shrink. This is less specific to "cold running" but still a possibility.
Troubleshooting Steps:
It's best to start with the simpler and more common issues:
1. Check the basics: Ensure the battery is strong, all fluids (oil, coolant) are at the correct levels, and there are no obvious vacuum leaks.
2. Check spark plugs and wires: Inspect them visually for wear or damage. Consider replacing them if necessary.
3. Clean the MAF sensor: Use MAF sensor cleaner (follow instructions carefully).
4. Check fuel pressure (requires a gauge): This is more involved but helps rule out fuel delivery issues.
If these steps don't resolve the issue, a mechanic with OBD-II diagnostic tools can check for error codes stored in the car's computer. These codes can pinpoint the specific problem more accurately.
