Possible Causes:
* Head Gasket Leak: This is a serious problem. A blown head gasket allows coolant to leak into the combustion chamber. This can cause the sweet smell of coolant in the exhaust, white smoke from the exhaust, and a rough running engine due to the coolant interfering with combustion. Low coolant level will also be observed.
* Cracked Cylinder Head or Engine Block: Similar to a head gasket leak, a crack in the cylinder head or engine block allows coolant to escape. This can also cause the sweet smell and rough running.
* Coolant Leak in the Intake Manifold: A leak in the intake manifold can introduce coolant into the intake system, affecting combustion and leading to a rough idle and coolant smell.
* Vacuum Leak: While less directly related to the coolant smell, a vacuum leak can cause a rough idle. It might be coincidental, but it's worth investigating.
* Ignition System Problem (Spark Plugs, Ignition Coils, etc.): A misfire due to a faulty ignition system will cause a rough idle, and is independent of the coolant smell. This needs to be checked separately.
* Other Engine Issues: Problems with sensors (MAF, IAT, etc.) or other engine components could contribute to a rough idle, though less likely to be directly connected to the coolant smell.
What to Do:
1. Don't drive the car extensively. Continuing to drive with a potential head gasket leak or other serious coolant leak could cause catastrophic engine damage.
2. Check the coolant level. A low coolant level is a strong indicator of a leak.
3. Inspect the engine bay visually. Look for any visible leaks around the engine, hoses, and radiator.
4. Check the exhaust for white smoke. White smoke, especially when the engine is cold, is a strong sign of coolant leaking into the combustion chamber.
5. Get it to a mechanic ASAP. This is not something you want to diagnose and repair yourself unless you have significant mechanical experience. The potential for serious engine damage is high. A mechanic can properly diagnose the root cause using diagnostic tools and pressure testing.
Ignoring these symptoms could lead to very expensive repairs. The sooner you get it checked, the better.
