* Seized Bearing: This is a very likely culprit. A connecting rod bearing, main bearing, or camshaft bearing could have failed due to lack of lubrication, debris in the oil, or bearing fatigue. This often results in a sudden and complete engine lock-up.
* Broken Connecting Rod: A connecting rod could have fractured, jamming the piston and causing the engine to seize. This often happens due to excessive wear, fatigue, or a pre-existing crack exacerbated by stress.
* Oil Pump Failure: If the oil pump failed, the engine would quickly lose lubrication, leading to rapid bearing failure and seizing. A blocked oil pickup tube could also cause this.
* Camshaft Failure: While less common, a camshaft could fail catastrophically, especially if it's worn or damaged. This can result in valve train interference and engine seizure.
* Internal Component Failure (less likely): A less probable but still possible cause is a piece of a broken internal component, such as a piston fragment or a piece of a broken valve, that wedged itself and locked the engine.
* Overheating (less likely given the timeframe): While overheating usually leads to gradual problems, a very rapid overheat *could* cause the engine to seize, though the timeframe makes this less likely. However, if the engine ran hot for a significant period even before the lockup, it's a possibility.
Crucial Steps for Diagnosis:
1. Do NOT attempt to crank the engine. Trying to force it will likely cause significant further damage.
2. Check the oil: Inspect the oil level and condition. Look for metal shavings or unusual discoloration. This provides initial clues about the nature of the failure.
3. Professional Inspection: Have a qualified marine mechanic inspect the engine. They will need to assess the damage, determine the cause, and recommend the appropriate repairs. This is absolutely necessary to avoid further, potentially expensive damage.
It's impossible to definitively diagnose the problem without a proper inspection. The timeframe suggests a sudden, catastrophic failure rather than something gradual. A professional marine mechanic with experience in MerCruiser engines is the best resource to identify and resolve the issue.
