Thanks for all of the helpful input.
The old impeller was fine, no broken vanes. Just changed it "in case"!
The PO may have had impeller troubles / broken vanes in previous years, not sure!
There is no antisiphon loop in the line. I shut the seacock when the engine is not running, so I don't feel I require a loop.
re: obstuctions to the intake - I agree a plastic bag, etc could account for an occasional blockage. However, I just started the engine for the 1st time this year (on the hard, running from a clean bucket of water), and it did it again. Started with good water flow, then stopped!!!! After revving it up, and putting the hose back on, it ran for 10 minutes with lots of water out the exhaust
All of the hose / clamps seem OK / tight on the intake side, and I had the exhaust elbow
off late last season - very clean/ no obstuctions. I also soaked it in muritatic acid (weakened), and circulated the acid through the block.
Tested the thermostat last year - opened and closed fine.
Re: the thermostat - As I understand it, I should have raw water pumping the whole time, whether it circulates through the block, or bypasses it, to then go out the wet exhaust.
I do have a service
manual, and am getting pretty familiar with this engine!
What bothers me is the intermittent nature of this loss of water. It may not be an airlock, as i suggested - but whatever it is only happens once in a while, and revving the engine seems to get the pump to overpower the problem. If I have excessive salt
buildup, or other restrictions, I would think I wouyld see reduced water flow / overheating
all of the time.
Any other thoughts???