Need help ASAP with cracked Volvo penta 55

[DougR]

I think that pooling of the sea water is only a small part of the corrosion equation. When the engine is running and a sea water leak is spraying into the exhaust stream there is a cloud of water mist/spray oscillating back and forth in the exhaust passage, particularly at low RPM. Valve overlap and exhaust system design keep positive and negative pulses bouncing around in the pipe, and these pulses uniformly wet the inside of the pipe causing what we are seeing. There is the possibility of a gasket material issue, but it's probably minor or this problem would be much more prevelant than it is.

A similar problem occurred some years ago on GM 454 big block gasoline engines that were used by VolvoPenta, Mercruiser and others. There was so much valve overlap in the camshaft that at low engine RPM the pressure pulses in the exhaust pipes and center outlet risers would actually cause water to flow counter to the exhaust gas flow on the inside surfaces of the pipe. This could be seen quite clearly with clear plastic exhaust pipes used for testing. Water penetration into the engines was a problem for quite awhile. The problem was solved by using a small crossover pipe in the exhaust system between left and right engine banks. This disrupted the pulse strength enough to stop the counter flow.

DougR

[hayford]

My boat is also fitted with a D2-55A from 2003, and has just suffered a similar problem. Hole in the cooler where it connects to the elbow. We are going for a replacement and hope to sell the old cooler for repair.

It has also only done about 1000hrs, having spent most of its time sat in a marina in Spain.

Interestingly I had anticipated there may be a problem and fitted a flushing system as described last year. Obviously too late to stop the damage.

Waiting for the yard to do the repairs, will report back if we can tell what caused the problem when the old unit is out.