P42 Heads
Hi Gil Guy, I always leave my head through hulls closed until it is time to use them. Since the head and holding tank are both below the waterline the siphon break for one or both of the heads is not functioning properly, allowing sea water to slowly flow through the head and into the holding tank. The aft head solenoid and siphon break are located in the main propulsion room, starboard bulkhead. The forward head is located behind a small panel on the forward side of the most forward starboard cabinet in the main salon.
Depending upon whether the siphon break is SS or plastic, it may just need to be cleaned. You can check the solenoid by using a volt meter and to observe whether it closes the break when the head is on operation and open when it stops.
Terry Cox