1. Gas Cap: The gas cap light is the simplest issue to address first. A loose, damaged, or improperly sealed gas cap can allow fuel vapors to escape. This triggers the light and can affect the car's ability to accurately read the oxygen levels in the exhaust, leading to a rough idle and the check engine light. Try tightening the gas cap securely. If the light goes out after driving for a while and the rough idle improves, you likely solved the main problem, though it might be wise to replace the cap for peace of mind.
2. EVAP System Issue (If Gas Cap Tightening Doesn't Help): Even with a tight gas cap, the Evaporative Emission (EVAP) system, which controls fuel vapor, could be faulty. A leak in the system (hoses, canister, purge valve) would cause similar symptoms. This requires more diagnostic work; you'll likely need to have the EVAP system checked with a scanner that can perform specific EVAP tests.
3. Vacuum Leak: A vacuum leak can disrupt the engine's air/fuel mixture, leading to a rough idle. Common areas to check (though this requires some mechanical knowledge):
* Intake Manifold Gaskets: These can crack or deteriorate over time, causing air leaks.
* Vacuum Hoses: Inspect all vacuum hoses for cracks, holes, or loose connections. They become brittle with age.
* PCV Valve (Positive Crankcase Ventilation): A faulty PCV valve can cause a vacuum leak and contribute to rough idling.
4. Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): These sensors measure air intake and throttle position. A dirty or faulty MAF sensor can send inaccurate readings, leading to a poor air/fuel mixture and rough idling. A failing TPS can also cause similar problems. These are relatively easy to check/clean (MAF sensor should be cleaned with MAF sensor cleaner ONLY, not brake cleaner).
5. Ignition System: While less likely if it's only a recent issue, worn spark plugs, wires, or a failing ignition coil could cause misfires, leading to rough idling and a check engine light.
6. Other Engine-Related Problems: Less probable given the symptoms, but a problem with fuel injectors, fuel pump, or other engine components could contribute.
What to Do:
1. Tighten the gas cap: This is the easiest and cheapest fix.
2. Check the engine light codes: Use a code reader (OBD-II scanner) to retrieve the diagnostic trouble codes (DTCs) stored by the car's computer. These codes provide clues to the problem's source. Auto parts stores often offer free code reading services.
3. Visual Inspection: Carefully inspect the vacuum hoses and the areas mentioned above. Look for anything that seems out of place or damaged.
4. Professional Diagnosis: If you're not comfortable doing this yourself, take your Focus to a mechanic. They have the tools and expertise to properly diagnose the issue. Mention the gas cap light and the rough idle – this will help them focus their diagnostics.
Don't continue to drive the car excessively with a rough idle, as it could cause further damage. The combination of symptoms points to a problem that needs attention to prevent more serious (and costly) mechanical issues.
