I'd agree with the idea that I'm suffering from an air leak in the deck pump out connection, except that it still does not add up.
When the pump works, it takes about 2 to 3 seconds to prime and it turns really fast, then slows somewhat as the pumping load builds. When it has its troubles, it starts slowly then goes slower and slower until it stops - exactly as if it were completely clogged. Furthermore, the pump mysteriously began working again and I was able to empty my tank in the yard. I then opened the inspection
port in the tank to run in fresh water and clean out the tank. I was in the process of draining this water when the pump began acting up again. The deck port had not been touched in the whole process. Why the change and why would the pump act as if the impeller were stuck when seconds before it was wet and running freely?
Going forward I have a couple of ideas I'm going to work on.
I'm getting a new HDPE tank and the drains are going to be on top with pipes down to the bottom. This won't help in keeping the pump lubricated but it does mean that I can work on the pump without getting sewage all over the boat. There will be two drains - one for the pump and one for the deck access. Furthermore, I'm going to move the pump over onto the top of the tank, where I have good access to it, and it is a very short run to the drain outlet. (Right now the pump is under a cabinet and it is a dark and difficult place to work.)
Since all this plumbing will be above the water line, and above the holding tank
for that matter, I'm not going to use marine
grade hoses. I'm going to use clear reinforced vinyl tubing so I can see what is going on.
Lastly, and I'd like some feedback on this idea, what about drilling and tapping a small hole in the impeller pump body and plugging it with a short stainless or nylon screw. (Short enough not to extend past the inner edge of the impeller body.) With the pump in an easily accessible place, it would be a simple job to remove the screw from time to time and squirt in a shot of rubber lubricant. Crazy or brilliant idea?