* Spark Plugs/Ignition System: A failing spark plug or issue in the ignition system (e.g., worn distributor cap, bad ignition coil) might intermittently misfire. This is more likely to occur under certain conditions like cold starts, high humidity, or when the engine is under load. If the misfire is only happening occasionally, the vibration would be intermittent.
* Fuel Delivery Problems: A partially clogged fuel filter, a failing fuel pump delivering inconsistent pressure, or a problem with the fuel injectors could lead to inconsistent fuel delivery. This would affect the engine's smooth running and could cause shaking, especially under load or at certain RPMs. The problem might be more noticeable on hotter days (fuel evaporates more easily) or when the tank is low.
* Vacuum Leaks: Small vacuum leaks in the intake system can create inconsistent engine performance. These leaks might be temperature-sensitive, worsening when the engine components expand in the heat.
* Engine Mounts: While less likely to be intermittent, worn engine mounts could cause increased vibration if they're already deteriorated. The vibration might be more pronounced on certain road surfaces or driving conditions, giving the impression of intermittency.
* Loose or Damaged Components: A loose belt, a slightly loose bolt on an engine accessory, or a component that's slightly out of alignment might only cause vibration under specific load or speed conditions, leading to an apparent intermittent problem. Heat expansion and contraction could influence this.
* Sensor Issues: Various sensors (e.g., mass airflow sensor, crankshaft position sensor) send information to the engine control unit (ECU). A faulty sensor providing intermittent incorrect data can cause the engine to run roughly.
* Moisture: In humid conditions, moisture can sometimes affect ignition systems and electrical connections, causing temporary issues that resolve as the moisture evaporates.
To diagnose the problem:
You need a proper diagnosis, and that likely means taking it to a mechanic. However, some initial checks you can make (if you're mechanically inclined) include:
* Check Engine Light: If the check engine light is on, get the codes read. This will often pinpoint a specific problem area.
* Inspect visually: Look for any obvious loose parts, leaks, or damaged components around the engine bay.
* Listen carefully: Listen for unusual noises or changes in engine sound that correlate with the vibration.
Without more information about the specific circumstances (when it happens, what conditions are present, any associated noises or warning lights), it's impossible to give a definitive answer. An intermittent issue requires careful observation and professional diagnosis.
