We have an adp41 stern drive diesel either 1999 or 2000. When anchored or stopped in a following sea and boat loaded with extra ice and coolers on board, saltwater is coming in through the outdrive exhaust and ends up in the oil pan. Can we prevent this? It is happening on the port engine. That side of our boat has more weight and sits a few inches lower in the water. We hear there used to be a butterfly valve in the exhaust elbow but they are not available that we know of. Are there any fixes for this problem.