Volvo 2030 overheating.

[Ammonite]

+1 Kinsale373
If you have mixed IAT (which is what I think is specified for this engine or at least the earlier versions) with OAT or HOAT antifreeze this 'can' cause the sludge you describe

 

[wully1]

So,

After soaking the cooling system with central heating cleaner for a few days then flushing through with a hose I got some sandy, gritty scale out along with grotty water. Pulled off the water pump and tube stack for a double check along with the (stainless steel) exhaust elbow. I did get a few more bits of scale but nothing much. Buttoned everything up carefully then waited for the new Kiwi prop to arrive.
Launched the boat today in the wind and rain and ran the engine alongside for 4 1/2 hours in total under load at tick over, 1000 and 1500 RPM. Before shutting down for lunch I increased to max revs with just under 2000 RPM and a bit of black smoke. Looks like the prop needs a bit of tuning.. (it‘ll Rev past 3000 out of gear, 1500 in astern)
Anyway the engine didn’t overheat although it did dump about 1/2 litre of coolant at max revs - I found this after returning from lunch. The coolant level was just at the top of the tube stack so I left it there and ran the engine for the final 1 1/2 hours.
The coolant from the bilge looked a bit brown and dirty so I’m going to drain and replace again. Fingers crossed it was a blockage in the engine cooling galleries..