Here's a breakdown of why:
* Head Gasket Failure: A blown head gasket allows coolant to leak into the cylinders. This coolant turns to steam when the engine heats up, causing the white smoke. The leak can also cause the engine to lose compression, leading to a knocking sound. This is the most likely culprit given your description.
* Cracked Cylinder Head: A crack in the cylinder head could also allow coolant to enter the combustion chamber, producing the same symptoms. However, this is generally less common than a head gasket failure.
* Other Possibilities (Less Likely Given the Symptoms): While less probable considering the white smoke, a knock *could* also be caused by:
* Low oil: Check your oil level immediately. Running low on oil will damage the engine quickly.
* Worn bearings: This would likely be accompanied by other symptoms like a rough idle and metal shavings in the oil.
* Connecting rod issues: Similar to worn bearings, this would usually involve more severe knocking and potential engine seizing.
How much will it cost?
This is impossible to say without a proper inspection. Repair costs vary wildly depending on location, labor rates, and whether you opt for a professional repair shop or attempt it yourself (not recommended for a head gasket).
* Head Gasket Replacement: This is a significant repair. Expect to pay anywhere from $1000 to $3000 or more depending on the factors above. This includes parts (head gasket, possibly other gaskets and seals) and labor.
* Cracked Cylinder Head: This is even more expensive. A replacement cylinder head can cost hundreds of dollars, plus the labor to remove, replace, and resurface the head.
* Other issues: Repairs for other causes mentioned (low oil, worn bearings etc.) could range from inexpensive (adding oil) to very expensive (engine rebuild or replacement).
What to do:
1. Do NOT drive the car any further: Continued driving could cause catastrophic engine damage.
2. Check the coolant level: Is it low?
3. Check the oil level and condition: Is it low? Does it look milky or have a strange color? (Milky oil is a strong indicator of coolant mixing with oil.)
4. Have a mechanic inspect the vehicle: This is crucial to determine the exact cause and get an accurate cost estimate for repair.
Don't delay getting it checked. The longer you wait, the more damage you risk, potentially leading to a much more expensive repair or even a need for engine replacement.
