Some what similar problem, but I get no flow:
Did some maintenace work on the engine, everything was running fine before I did the following items on Yanmar 3GMD (direct salt water cooled):
1. Adjusted valves.
2. Replaced three engine head/block zincs (more than 50% so cleaned and replaced).
3. Replaced raw water pump
Went to start engine, started up fine. About a gallon of water came out of exhaust
and then nothing- higher or lower rpm on engine still no cooling water. Thought maybe the cooling water seacock was clogged so I attached a bucket (that I use to flush the system with- it puts a head of water to the pump
inlet- so it primes the inlet) of freshwater and started the engine. Still no water. I ran out of time, will try to trouble shoot a little later today. Never had a problem with air lock or pump
priming (after impeller change) so wonder if when I pulled out the zinc, debris clogged the outlet water passages? Where the thermostat is is very small areas for water passage. Or maybe the debris clogged my mix elblow. Still suprised what ever happened stopped all the flow. The pump
belt is good.
Any ideas? I can trace the system, it just takes time.
Will post what I find.