Ignition System:
* Worn spark plugs and/or wires: These are the most common culprits for rough idling. Old spark plugs misfire, leading to uneven combustion and a rough idle. Worn wires can cause similar issues by failing to deliver a strong spark. Check for wear, cracks, and corrosion.
* Distributor cap and rotor: Cracks or corrosion in these components can disrupt the spark delivery to the cylinders, leading to a rough idle. Inspect for wear and tear.
* Ignition coil: A failing ignition coil might not provide sufficient voltage to all cylinders consistently, resulting in a rough idle. This often requires testing with a multimeter.
* Ignition module (or control module): This component controls the ignition system. A faulty module can cause misfires and a rough idle. Requires specialized testing.
Fuel System:
* Dirty fuel injectors: Clogged injectors prevent proper fuel delivery to the cylinders. A fuel injector cleaner might help, but often professional cleaning or replacement is necessary.
* Low fuel pressure: A faulty fuel pump or pressure regulator can deliver insufficient fuel pressure, leading to a rough idle. Requires a fuel pressure gauge to test.
* Vacuum leaks: Leaks in the vacuum lines can disrupt the air/fuel mixture, causing a rough idle. Inspect all vacuum lines for cracks and proper connections.
* Dirty air filter: A severely clogged air filter restricts airflow, affecting the air/fuel mixture. This is an easy and cheap check; replace if dirty.
* Throttle body: A dirty throttle body can restrict airflow and cause a rough idle. Cleaning it with throttle body cleaner is often effective.
Other Potential Causes:
* EGR valve: The Exhaust Gas Recirculation valve can stick open or closed, disrupting the engine's operation.
* PCV valve: A faulty Positive Crankcase Ventilation valve can allow excessive crankcase pressure, affecting the engine's operation.
* Mass Airflow Sensor (MAF) sensor: A faulty MAF sensor provides incorrect air flow readings to the engine computer, leading to improper fuel mixture.
* Engine Sensors: Various other sensors (oxygen sensor, temperature sensors, etc.) can send incorrect data to the engine computer, resulting in poor idle.
* Vacuum leaks: These are common and can affect various engine systems.
* Failing engine mounts: While less likely to directly cause a rough idle, excessively worn engine mounts can cause vibrations that *feel* like a rougher idle.
Troubleshooting Steps:
1. Start with the easy checks: Inspect spark plugs, wires, distributor cap, rotor, and air filter. Replace any visibly worn or damaged parts.
2. Check for vacuum leaks: Listen carefully for hissing sounds around vacuum lines and the intake manifold. Use a propane torch (carefully!) to check for changes in idle speed near potential leak points (a rising RPM indicates a leak).
3. Clean the throttle body: This is a simple procedure that can often resolve idle issues.
4. Consider a professional diagnostic scan: A mechanic with a scan tool can read diagnostic trouble codes (DTCs) from the engine computer, which can pinpoint specific problems.
A rough idle is a symptom, not a diagnosis. Systematic checking of the above areas, starting with the simplest and most likely culprits, is the best approach. If you're not mechanically inclined, taking your car to a qualified mechanic is recommended.
