Bukh Overheating and the unexpected solve

Long story short, but still long, motoring home after Oz day fireworks in Syd, 20min into a 40min motor and over temp alarm... went to idle, alarm went off, waited and increased rpm, overheating again... motor off floating with a incoming tide back to mooring and short motor runs... after 1.5hrs, back on mooring

Public holiday spent googling like a warrior and in the engine bay around the raw water cooled Bukh 24, process of elimination.... still pumping water yes, check the steam tube( what a bastard) a couple little bits of calcium but not much, remove thermostat and test in a pot of hot water.... not perfect but operational after cleaning up, never fully closed or fully opened but moved, removed engine driven fridge cooling from circuit as maybe restricted flow... running out of options now..
Put a new hose in from V drive cooling to water pump, only to notice the suction would collapse hose...give up and order new hose.
Bit the bullet and took a second mortgage and ordered a bunch of parts from Bukh, and a new clear metal helix vacuum hose. Fitted new impeller(last one only 4 weeks old), new thermostat, steam tube gaskets, new hoses throughout from V drive to pump to block, took for test run and the temp was heading north way to quick.

What next???
Plenty of water coming out exhaust, brand new cooling circuit???? Do I need to flush circuit???
Then I noticed slight air bubbles in the new clear hose, shake it and a few more bubbles????
Search further, and after numerous test runs, found the slightest of air leaks in the first hose after strainer, not enough to leak water in bilge, however under suction, I guess air would enter and either:
a) Super heat and expand in block creating steam which Bukhs are known for, but reduce cooling
b) Slowly building up in blocks highest point and form a big air pocket, and water not cooling block, would of thought the thermostat is the highest point and all air went there but I'm not a air bubble inside the block to tell..

Either way, problem now solved, and probably caused from fitting a new auto bilge a few weeks back and disturbed the hose...however unless I had clear pipe I would never of seen the slightest amount of air bubbles, and would be chasing my tail still I would never consider the tiny air bubbles a issue...but I guess over time they build up to a big air bubble somewhere inside the block.

Anyway, motor is back to 60-65 degrees under full revs into river current, and a new cooling circuit which in hindsight didn't need, but now I know the system inside out and backwards, and learnt something new... hopefully this helps someone else quickly and cheaply diagnose their overheating issue.

It never appeared in any overheating search across multiple forums, manuals, or mine or mates knowledge banks. I guess water out exhaust, doesn't actually mean it's cooling!!

Short long story over, and the Bukh powers on!!

Goodwinds,

Cabron

Thanks Cabron, very useful to know.