Here are some possibilities for a rough idle on a 2006 Cobalt (especially one with low mileage) that could be manifesting only when hot:
* Vacuum Leak: A small vacuum leak might not be noticeable when the engine is cold, but as it warms up and components expand, the leak could become significant enough to cause a rough idle. Inspect all vacuum hoses and connections thoroughly for cracks, splits, or loose fittings. Pay close attention to the intake manifold and PCV system.
* Ignition System Issues: Worn spark plugs or ignition coils (especially if only one or two are failing) can cause a misfire that's more pronounced when the engine is hot. The heat could exacerbate any existing weakness in the ignition system components.
* Fuel System Problems: While less likely with low mileage, there's a possibility of issues within the fuel injectors. A clogged or sticking injector could cause a misfire at higher temperatures. Fuel pressure regulator problems could also be a factor.
* PCV System: The Positive Crankcase Ventilation system helps vent gases from the crankcase. A clogged or faulty PCV valve can cause a buildup of pressure, affecting the engine's operation, and is more likely to show up when the engine is hot and under pressure.
* Coolant Temperature Sensor: A faulty coolant temperature sensor can provide inaccurate readings to the engine control module (ECM), leading to incorrect fuel delivery and timing adjustments. This could cause rough idling, especially when the engine reaches operating temperature.
* ECM/PCM Issues: Although less common, a problem with the Engine Control Module (ECM) or Powertrain Control Module (PCM) – the computer controlling the engine – could be causing the rough idle. This is a more difficult diagnosis.
What to do:
1. Get a proper diagnostic scan: A mechanic with a professional-grade OBD-II scanner should perform a complete diagnostic scan. This will pull any trouble codes stored in the ECM, even if the check engine light isn't on. Many codes are triggered only under certain conditions (like when hot) and might not set the CEL immediately.
2. Thorough visual inspection: Have a mechanic (or carefully do it yourself) inspect all vacuum hoses, connections, ignition components, and the PCV system. Look for anything unusual.
3. Data logging: A good mechanic will likely utilize data logging to monitor engine parameters in real-time as the engine warms up and experiences the rough idle. This helps pinpoint exactly what's happening when the problem arises.
Ignoring the problem will likely make it worse and potentially more expensive to fix. Ignoring the dealership's advice is a good step in that direction. Find a reputable mechanic who can properly diagnose the issue. The low mileage on the vehicle suggests it shouldn't be experiencing these issues, making a thorough diagnosis even more critical.
