* Dirty or Faulty Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A dirty or malfunctioning sensor can lead to a rough idle, especially when cold, as the computer gets inaccurate information about airflow.
* Dirty or Faulty Mass Airflow Sensor (MAF): Similar to the TPS, the MAF sensor measures the amount of air entering the engine. A dirty MAF sensor will provide inaccurate readings, causing a lean or rich fuel mixture leading to rough running, especially when the engine is cold and the sensor is less responsive.
* Vacuum Leaks: Small cracks or holes in vacuum hoses can cause erratic engine operation, especially noticeable during cold starts when the engine is working harder.
* Ignition System Problems: Worn spark plugs, faulty ignition wires, a failing ignition coil, or a distributor cap/rotor (if equipped) can all contribute to misfires, leading to shaking. This is particularly likely to be worse when the engine is cold because the components are less efficient.
* Low Compression: Worn piston rings or valve problems can lead to low compression, resulting in a rough running engine, often more pronounced on cold starts. This is a more serious issue.
* Faulty Idle Air Control (IAC) Valve: This valve regulates airflow at idle. A malfunctioning IAC valve can cause erratic idle speeds, including shaking, especially when cold.
* Engine Mounts: While less likely to *only* cause shaking on cold start, worn engine mounts can amplify vibrations, making a pre-existing problem seem worse.
* Fuel Delivery Issues: Problems with the fuel pump, fuel filter, or fuel injectors can lead to inconsistent fuel delivery, resulting in a rough idle. A cold engine may exacerbate these issues.
Troubleshooting Steps:
1. Check for Obvious Issues: Start by visually inspecting vacuum hoses for cracks or leaks. Look at the spark plugs; if they're very worn or fouled, that's a likely culprit.
2. Basic Maintenance: Change the air filter. A clogged filter will restrict airflow and can contribute to a rough idle, especially on a cold start.
3. Professional Diagnostics: The most efficient way to diagnose the problem is to take your car to a mechanic. They can use a scan tool to check for diagnostic trouble codes (DTCs) that may point towards faulty sensors or other components. A compression test will rule out major internal engine issues.
Don't attempt extensive repairs yourself without the proper knowledge and tools, as you risk further damage. The relatively simple checks (vacuum hoses, air filter, spark plugs) are good starting points, but professional help is usually recommended for this type of problem.
