Auxiliary Diesel Develops Severe Mechanical Noise

Piston from a Yanmar 3GM30 diesel

Piston from a Yanmar 3GM30 diesel

The photo is the number one piston from a Yanmar 3GM30 diesel as removed from a C&C LF38. The engine is the original, is over 20 years old and has thousands of hours (no hour meter) of use around the Maritimes, Eastern USA and the Caribbean. After launch this year the engine became harder to start and on occasion a mechanical noise would occur. Basic checks did not indicate any malfunction.

On the second day of an extended trip the engine noise became continuous with black smoke and the engine was shut down immediately. Disassembly was done in the boat including removal of the oil pan by lifting the block. Number one piston, the aft most, as this engine is a “V” drive, was found to have stuck rings and marks on the cylinder wall which indicated the piston and ring set had started “binding or vibrating” during operation, hence the mechanical noise. The cylinder walls were honed and three new pistons with new rings were installed and this auxiliary diesel is running in top form again.

Part of the required operating regime of this engine is many hours at 1200 rpm to maintain battery charge. No doubt this would have much to do with carbon buildup, stuck ring or rings and the manifestations that required this expensive repair. As the primary cause may be the carbon buildup the possibility of over cooling, particularly at the number one cylinder during extended low load, low rpm battery charging, should be investigated.

by Ed Sulis

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