Most Likely Culprits:
* Head Gasket Failure: This is the most common cause of overheating and white smoke (which is actually steam from coolant leaking into the combustion chambers). Head gasket failure can lead to coolant loss, low compression, and potentially mixing coolant with engine oil (making it milky).
* Cracked Cylinder Head: Similar to a head gasket failure, a cracked cylinder head allows coolant to leak into the cylinders. This is often caused by overheating itself.
* Cracked Engine Block: Less common, but a cracked engine block can also lead to coolant leaking into the cylinders, resulting in overheating and white smoke.
* Coolant Leak: While a leak *alone* might not cause white smoke *immediately*, if the coolant level drops severely enough, the engine can overheat and cause the steam you're seeing. Check for visible leaks around the radiator, hoses, water pump, thermostat housing, and engine block.
What to Do Immediately:
1. Stop driving the car immediately. Continuing to drive will likely cause catastrophic engine damage.
2. Turn off the engine. Let it cool down completely before attempting any further diagnosis. Trying to work on a hot engine is dangerous.
3. Do NOT add coolant. If you add coolant while the engine is still hot and there's a leak, the coolant could spray out under pressure and burn you. Wait until it's completely cool.
Troubleshooting Steps (After the Engine is Cool):
1. Check the coolant level: Carefully inspect the radiator and coolant overflow tank. Is the coolant level low?
2. Check for external leaks: Look closely around the entire engine for any signs of coolant leaks (wet spots, white residue).
3. Inspect the coolant: If you can safely remove some coolant, examine its appearance. Is it milky or oily? This suggests a mix with engine oil, pointing to a head gasket or block failure.
4. Check the engine oil: Check the oil dipstick. Is the oil milky or frothy? This is another strong indicator of a head gasket failure.
5. Pressure Test the Cooling System: A pressure test can help identify leaks in the cooling system that aren't readily visible. This is best done by a mechanic.
Next Steps:
* Take it to a mechanic immediately. Without professional diagnosis, further damage is almost guaranteed. The problem is likely serious and requires professional repair. Trying to fix this yourself could lead to irreversible engine damage and costly repairs.
Important Note: White smoke isn't always coolant. In rare cases, it could be related to burning oil (usually bluish smoke) if oil is leaking into the combustion chambers or from a severely worn engine. However, given the overheating, coolant leak is the primary suspect.
