* Coolant Temperature Sensor (CTS): While a faulty CTS can cause problems both hot and cold, a failing one *can* manifest as rough running *only* when warm. The engine relies on the CTS reading to adjust fuel delivery and ignition timing. An inaccurate reading when hot could lead to a lean or rich condition. This is a relatively inexpensive part to replace.
* Intake Manifold Gasket Leak: A leak in the intake manifold gasket will often become more pronounced as the engine warms up and components expand. This can cause vacuum leaks, leading to inconsistent air/fuel mixture and rough running. It's more common on higher mileage vehicles. This is more expensive to repair.
* Ignition System Issues (Worn Spark Plugs, Wires, Coil Packs): Worn spark plugs or ignition wires might work adequately when cold but fail to consistently fire when hot. Coil packs can also crack or degrade over time, particularly under heat stress. Check for cracked insulators on the coil packs. While possible, it's less likely to be solely a hot issue.
* EGR Valve Problems: The Exhaust Gas Recirculation valve can stick or become clogged, affecting the air-fuel mixture. This is more likely to cause problems at idle and lower RPMs, but it's certainly a possibility. A faulty EGR could cause issues at varied temperatures.
* Fuel Injectors: A failing fuel injector might start leaking or deliver inconsistent fuel as it heats up. This is less common but possible. Testing fuel pressure is needed for proper diagnosis.
* Mass Airflow Sensor (MAF Sensor): A MAF sensor issue is more likely to cause problems consistently, rather than just when the engine is warm. However, a malfunctioning sensor could read incorrectly due to heat.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any stored trouble codes. This is the first and most important step. The codes will narrow down the possibilities significantly.
2. Inspect visually: Look for any obvious signs of leaks, cracks, or damage to the intake manifold, vacuum hoses, and wiring.
3. Check Spark Plugs and Wires: Inspect the spark plugs for wear, fouling, or damage. Visually inspect the ignition wires for cracks or damage. Consider replacing them as a set if they're showing significant wear.
4. Test the Coolant Temperature Sensor: A relatively straightforward test can be done with a multimeter to check if the CTS resistance is within specification.
Important Considerations:
* Professional Diagnosis: If you are not comfortable working on your car yourself, take it to a trusted mechanic for diagnosis and repair.
* Heat Soak: Let the engine completely cool down before attempting any diagnostic work. Working on a hot engine is dangerous.
The "only when warm" symptom strongly suggests a heat-related issue. Focusing on the CTS and intake manifold gasket is a good place to start, especially after checking for DTCs. Good luck!
