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Another one in the series "What is that SMELL?" Mark I, three cabin. There are two odors associated with our heads. The stale intake water which occurs when the boat sits, especially in these dog days of summer, the water is 86 degrees right outside the boat. We'll call it odor # 1. The other is the smell of older components. I have replaced as many hoses as possible, but the tanks are old, and permiated. They smell, somewhat, it's not too bad, especially when we are on the boat with air moving. Specifically the aft holding tank, under the starboard aft bunk, has been cleaned best as possible, and I have sealed the lid with tape. We'll call this odor # 2. Several times, lately, there has been a new, very bad, odor. It is one of the family of sceptic odors, but it is is not #1 or #2....it's worse. It occurs in the SB aft cabin, not the head, when under sail, and heeling to SB.... I then ventilate the heck out of the cabin and it goes away. Today, with the lid off the tank compartment, I plugged the vent on the aft holding tank, and pumped it for a while. There were no odors eminating from tank or it's assiociated hoses.... After a while I took the plug out of the vent, and there was a notable decompression, in other words, I believe the system is air tight where it needs to be.... Any suggestions?