Possible Causes:
* Seized Bearings: This is a common cause. Lack of lubrication, low-quality oil, or using the wrong type of oil can cause bearings to seize. Overheating can also contribute.
* Broken Connecting Rod: A broken connecting rod will completely lock the engine. This often occurs due to engine damage (like a sudden impact) or severe wear.
* Worn Piston Rings/Scored Cylinder: Worn piston rings can cause excessive friction, leading to a locked engine. A scored cylinder (scratches or gouges in the cylinder wall) will also prevent movement.
* Valvetrain Problems: A bent pushrod, sticking valve, or other valvetrain issue *might* interfere with crankshaft rotation, although this is less likely to completely lock it.
* Foreign Object: A piece of debris, such as a bolt or nut, could have entered the engine and become lodged, preventing movement.
* Frozen Engine (Extreme Cold): In extremely cold climates, the oil may thicken enough to prevent the engine from turning.
Troubleshooting Steps (Proceed with Caution):
1. Safety First: Disconnect the spark plug to prevent accidental starting.
2. Check for Obvious Obstructions: Carefully inspect the engine for any visible damage or foreign objects that might be obstructing the engine's movement.
3. Attempt to Turn the Crankshaft: Use a wrench on the crankshaft nut (located at the end of the crankshaft) to try and turn the engine. If it doesn't budge, it's definitely locked up. Do not force it.
4. Remove the Spark Plug: With the spark plug removed, try turning the crankshaft again. This reduces compression and might reveal if the problem is compression-related.
5. Check the Oil: Inspect the oil level and condition. Is it low? Is it dirty or milky (indicating water contamination)? Low oil is a major contributor to seized bearings.
6. Drain the Oil: Drain the oil to check for metal shavings or other debris, which could indicate internal damage.
Next Steps:
If the engine remains locked up after these steps, you'll likely need professional help or a significant amount of mechanical knowledge. The extent of the damage will dictate the repair path:
* Minor Issue (e.g., slightly seized bearing): With some mechanical expertise, it might be possible to free up the engine with penetrating oil and careful turning. However, this is risky and may worsen the damage.
* Significant Damage (e.g., broken rod, scored cylinder): This will likely require engine rebuild or replacement. The cost of repair might exceed the value of the engine, especially on older models.
Important Note: Attempting to force a locked-up engine can cause significant and costly damage. It's best to seek assistance from a qualified small engine repair shop or a knowledgeable mechanic. They have the tools and expertise to diagnose the problem accurately and perform the necessary repairs.
