Originally Posted by RainDog
Sorry for the delay in responding. Indeed the problem was cavitation in the pump. Bleeding the coolant while the engine was running fixed the issue. The mechanic
said these Yanmar are notoriously difficult to bleed.
So you had an air bubble, that became a real problem when engines started "reverse flow cooling"
is back in the day engines were cooled by thermal siphon, cool water in the bottom, heat rises out to the top and into the radiator, only flow was due to thermal siphon. Power increased and a pump was added to assist the thermal siphon. I'm pretty sure at least all our older boat motors are pump assisted thermal siphon.
Then some racer
who understood things and knew the head
was the hottest part of the motor
, that it should get the coolest water started pumping the coolant in reverse, I believe the Chevy LT1 motor
was the first mass produced motor to do so. Problems soon arose as these reverse flow motors had to be bled of trapped air when the coolant was changed, and old mechanics didn't know to do that.
I'm going out on a limb here, but I'd bet it's the water heater hose that is the cause of the trapped air in our boat motors, and we have to bleed our cooling
systems just like they were reverse flow.
How did you bleed it? Reverse flow engines usually have a bleeder screw on the thermostat neck that makes bleeding easy