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Any methane left in the engine crankcase and fuel lines is automatically flushed into the atmosphere - and CalMac has yet to establish how much will be released.

The air here smells like crankcase oil — as it has for decades — but there is far less activity now than there was just three years ago, and local communities are feeling the pinch.

National Highway Traffic Safety Administration say there could be an electrical short in the positive crankcase ventilation valve heater.

The report states the mechanic cleaned the oil filter, sent metal fragment samples to a laboratory for analysis and added clean oil to the engine crankcase.

One, the Polar Sea, has been idle since a 2010 crankcase explosion.