Here's a breakdown of possibilities and troubleshooting steps:
* Check the Check Engine Light: Get the diagnostic trouble codes (DTCs) read using an OBD-II scanner. This is the *most important* step. The codes will point you towards the specific problem. Common codes related to overheating and rough idle could include issues with the coolant temperature sensor, mass airflow sensor (MAF), or oxygen sensor (O2).
* Coolant System Integrity: Even though you changed the thermostat, double-check the entire cooling system:
* Thermostat Installation: Ensure the thermostat was installed correctly. A faulty installation can cause overheating and erratic idle.
* Coolant Level: Check the coolant level in the radiator and overflow reservoir. Top it off if necessary with the correct 50/50 mix of coolant and distilled water.
* Hoses and Clamps: Inspect all hoses for cracks, leaks, or looseness. Replace any damaged hoses or clamps.
* Radiator Cap: Make sure the radiator cap is in good condition and seals properly. A faulty cap can cause pressure buildup, leading to overheating.
* Water Pump: While less likely, a failing water pump could have contributed to the overheating and could still be a problem. Check for leaks and proper operation.
* Ignition System: Overheating can damage components in the ignition system.
* Spark Plugs and Wires: Inspect spark plugs for fouling, damage, or wear. Check spark plug wires for cracks or damage.
* Ignition Coil: A failing ignition coil can cause misfires and rough idle.
* Air Intake System:
* Air Filter: A clogged air filter can restrict airflow and cause rough idling. Replace it with a new one.
* MAF Sensor: The mass airflow sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause a rough idle. It might need cleaning (with MAF sensor cleaner only) or replacement.
* Other Sensors: Several sensors can contribute to a rough idle when malfunctioning. As mentioned above, the OBD-II codes are crucial for diagnosing sensor issues.
* Head Gasket: While a blown head gasket is a possibility, it's *less likely* if there's no milky oil or white smoke from the exhaust. However, a head gasket leak *might* not show these symptoms immediately. A compression test is the best way to check for head gasket failure. A leak-down test is even more definitive.
In summary: Don't jump to conclusions about the head gasket. Start by getting the DTCs read, then systematically check the cooling system and other potential causes listed above. If the OBD-II scan doesn't reveal the problem, a compression test might be the next step to rule out a head gasket issue. If you're not comfortable with these checks, it's best to take the vehicle to a qualified mechanic.
