* Vacuum Leaks: A significant vacuum leak can disrupt the air/fuel mixture at idle, leading to rough running. Under load, the engine's intake manifold vacuum is much higher, often masking the leak's effect.
* Dirty or Faulty Idle Air Control (IAC) Valve: The IAC valve regulates air intake at idle. If it's dirty, sticking, or malfunctioning, it won't provide the correct amount of air for a smooth idle.
* Faulty Throttle Position Sensor (TPS): The TPS tells the engine's computer (ECU) the throttle's position. A faulty TPS can provide inaccurate information, leading to poor idle control. At higher RPMs under load, the TPS signal might fall within an acceptable range, masking the issue.
* Dirty or Blocked Throttle Body: A dirty throttle body restricts airflow, negatively affecting idle. Cleaning it often resolves this.
* Spark Plug Issues (Less Likely): While a misfire usually causes rough running at all speeds, a weak spark plug *might* only cause problems at low RPM (idle) where the combustion pressure is lower. If it was a significant spark plug issue it would likely be rough while driving too.
* Engine Mounts: Worn engine mounts can amplify vibrations, making a slightly rough idle feel much worse. While this doesn't fix the underlying problem causing the rough idle, it's worth considering as a contributing factor to the *feeling* of shaking.
* Low Compression in one cylinder (Less Likely): Similar to spark plugs, it's less likely that only low compression in one cylinder would only appear at idle. Usually it would affect performance across the RPM range.
Diagnosing the Problem:
A proper diagnosis requires a mechanic to check the above components and systems using diagnostic tools. Simply checking for vacuum leaks with a spray bottle of carb cleaner around suspected leak points can be a useful starting point for a DIYer. However, diagnosing an electronic issue like a faulty IAC valve or TPS generally requires a scan tool that can read live data from the engine's computer.
